THE PARTICLE GUIDE TO AIR FILTRATION

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INDEX

1. INTRODUCTION

This manual has been produced in an attempt to assist the end user in understanding the principles of air filtration. Furthermore, it aims at arming the end user with enough information and technical knowledge to enable them to evaluate air filters in a fair and objective manner. More often than not, ignorance by end users, and in many cases incompetence of personnel in the Filtration Industry, leads to the incorrect specification and use of air filters. Ultimately the onus and responsibility of correct filter selection rests with the end user and it is - therefore essential for the end user to acquire a general understanding of filtration products used in air-conditioning systems. Filters form a fraction of the costs of an air-conditioning system, yet the significance of air filters in terms of preserving air-conditioning plant and equipment as well as the protection of specific environments and operations is often overlooked. More often than not, the purchase and installation of air filters takes place just prior to commissioning and handover of an air-conditioning plant. It is therefore often an afterthought and for this reason little or no effort goes into the correct filter selection, or as is more often the case, the correct purchase of the filters. Once the guarantee period of a new installation ends, responsibility reverts back to the end user. Unfortunately damage or inefficiency is not always evident during the guarantee period. More often than not difficulties only become evident at such times when suppliers and contractors are off site and free of any obligation. It is therefore hoped that this manual will assist in the evaluation and selection of air filters as well as assisting in providing a general understanding of the fundamentals of air filtration.

2. PRINCIPLES OF AIR FILTRATION

2.1 THE NEED FOR AIR FILTRATION

Air-conditioning is defined as the simultaneous control of the temperature, humidity, air movement, and the cleanliness of the air. This section deals with the filtration of air in order to control cleanliness. There are basically five reasons for filtering the air i.e.

  • (a) health,
  • (b) protection of air-conditioning equipment,
  • (c) protection of internal surfaces of buildings,
  • (d) to protect the manufacturing process and improve the quality of the manufactured product, and
  • (e) protection of equipment and machinery.

Health

Proper filtration will remove bacteria, viruses and allergies from the air. This will improve employee efficiency by reducing absenteeism through sickness. Filtration also prevents the spread of bacteria in hospitals which reduces infection. It has been proven that approximately 25% of infection occurring after operations is caused by airborne bacteria. It is also a known fact that Legionnaires Disease is caused by the spread of bacteria. Although filtration for health reasons is most important in hospitals, it is also applicable in normal air-conditioning systems. People who suffer from hay fever, for instance, benefit greatly by working in an air-conditioned building with a proper filtration system. Filtration is also applied for health reasons where a process causes toxic materials which, if allowed to be blown into the general atmosphere, could cause a health hazard. An example of this is the exhaust air from a pharmaceutical manufacturing company.

Protection of Air-conditioning Equipment

One of the most fundamental reasons for filtering air is to protect equipment. Cooling or heating coils become coated with a layer of dirt which reduces temperature transfer and increases air resistance. As coils, especially multi-row coils with very small fin spacing, are difficult, if not impossible, to clean, it is always better to prevent the accumulation of dust instead of trying to remove it. Because cooling coils often operate wet, they are more susceptible to being blocked by dirt than are heating coils. Less efficient temperature transfer also increases energy consumption and/or fuel costs.

Protection of Internal Surfaces

in Buildings Much of the staining of internal surfaces is due to the carbon element in the air. These particles, which are very small in size, cause staining of ceilings, walls, curtains and furniture and will often lead to the necessity of repainting walls and cleaning of furniture, curtains and carpets. Unfortunately, the smaller components of atmospheric dust are the worst offenders in smudging and discolouring building interiors and high efficiency filters are required to remove the small particles. Staining of surfaces close to diffusers especially ceiling mounted, can also be caused by entrainment of air which is carrying carbon particles.

Improvement of Product Quality

The miniaturisation of integrated circuits is continuing at a faster and faster pace. Even the tiniest speck: of dust in the air during the manufacturing process of a "chip" could ruin the circuitry. Painting of cars and other high quality products is another example where the high standards demanded today cannot be maintained if there is dust in the air used to ventilate the spray booths. A further example is the pharmaceutical industry where the air has to be virtually sterile to prevent the contamination of drugs.

Protection of Equipment and Machinery

Dust in the air also affects equipment and rotating machinery- With grease, dust forms a type of grinding paste which damages bearings and rotating machinery components. The larger dust particles in the air are mainly responsible for this type of damage to equipment and machinery.

2.2 CHARACTERISTICS OF PARTICLES

  • (a) Size
  • (b) Size distribution
  • (c) Shape
  • (d) Density (Mass - Solid/Gaseous)
  • (e) Surface (Texture - Wet/Dry)

2.3 PARTICLE SIZE DISTRIBUTION

All the particles we have been considering are suspended in air. The smallest particles are far less than 1 urn in diameter, the largest over 200 um. A micrometer (um) is one thousandth of a millimetre. That means the full stop at the end of this sentence is about 500 um in diameter and could hold over 2 million smoke particles (diameter 0.3 um) . In good light conditions it is possible to see particles of down to 10 um diameter. The following diagrams show the different types of particles and the respective particle size ranges as well as the 'particle size/distribution' of a typical dust sample.

2.4 TYPES AND SOURCES OF CONTAMINATION

Atmospheric dust is a complex mixture of smokes, mists, fumes, dry granular particles and fibres. A sample of atmospheric dust may contain quantities of soot and smoke, minerals such as rock, metal or sand, organic materials such as grain, flour, wool, hair, lint, and plant fibres and perhaps also mould spores, bacteria and pollen. Particles are not generally called dust unless they are smaller than 80 microns (1 micron is l/l 000th of a millimetre). The air contains millions of different size dust particles, most of them so small that they cannot be seen by the naked eye. Even the few larger particles are normally no larger in diameter than human hair. Even though the ratio of small particles to large particles in the air is one million to one, the large particles account for nearly all the weight of dust in the air, but the small particles are responsible for the staining of building surfaces. The amount of particulate matter suspended in the air is called "load" or "concentration". An excellent opportunity to see a small fraction of the dust concentration present in the air is in a beam of sunlight streaming through a window. The bright light reflects off the airborne particles enabling one to see the larger ones. One method of measurement of "load" is by weight. It is measured in milligrams per cubic metre (mg/m3) . Some typical dust loads are given below:

  • Rural and suburban areas: 0,05 - 0,5 mg/m3 (total mass of solids)
  • Metropolitan areas: 0,1 - 1,0 mg/m3 (total mass of solids)
  • Industrial areas: 0,2 - 5,0 mg/m3 (total mass of solids)
  • Factories or work rooms areas: 0,5 - 10,0 mg/m3 (total mass of solids)

2.4.1 TYPES

  • a) Solid Particulates

    Dust, fumes and smoke as well as siliceous and other sands; naturals synthetic lints; and carbon make up this category. Free carbon appears as soot, fly ash and smoke. These elements have been identified as the most staining and damaging in normal atmospheric air. Combined carbon is found as decayed animal and vegetable matter, germs and pollens and many other forms.

  • b) Mist Contamination

    Filtration contaminants can take the form of mists and fogs which are basically liquid particles and are as damaging as solid particulates.

  • c) Gaseous Contaminants

    Gaseous or non-particulate contaminants prove to be equally difficult to control and can influence systems with disastrous effects due to corrosive qualities. What makes this category worse is the fact that detection is extremely difficult.

2.4.2 SOURCES OF CONTAMINANTS

Some sources of airborne contaminants are obvious and come to mind instantly:- Industry churning out countless varieties of noxious fumes; free carbon as a result of incomplete combustion and exhaust emissions as well as deposits from brake lining wear. However, there are other less apparent sources of man-made contamination which are generated during our everyday lives:- Lint is shed from our clothing and fibre particles are released from carpeting as we walk about. Bodily detritus, such as dead skin and hair is constantly lost into the atmosphere. Similar phenomena also occur in nature:- Smog, fog and free carbon are all produced naturally while combined carbon (the result of decaying animal and vegetable waste) , constantly litter the atmosphere to be dispersed by the wind. A light breeze can carry airborne contaminants for vast geographical distances. All of the above particles are suspended in normal atmospheric air. There are enormous variations in particle sizes which range from less than 1 micron in diameter to over 200 micron in diameter. The severity of the problem is proportionate to the surrounding areas. The contaminant problem becomes far more severe when plant rooms or equipment are situated within or close to:-

  • a) Steel Works
  • b) Foundries
  • c) Industrial Sites
  • d) Power Stations
  • e) Chemical Industries
  • f) Mine dumps

SECTION 3

AIR FILTRATION TECHNIQUES

Extreme variations in the size and nature of atmospheric particles, together with the different environments in which they are encountered, generate a need for several filtering methods. One method for large particles, another for small, some methods for solid particles, other for liquid and so on. These filtering methods, however, are all based on one, or a combination, of five filtering principles:

  • settling
  • electrostatic precipitation
  • inertial separation
  • viscous impingement
  • sieving or straining diffusion and
  • interception

Settling Principle

The settling principle works for massive particles only and relies on the force of gravity acting on particles carried along in an airflow. Due to this force the particles gradually settle and are thus removed from the airflow.

Electrostatic Precipitation

With electrostatic precipitation the principles become slightly more involved. Electrical forces are used to influence the particles. An ionization source electrically charges the particles, which are then carried, by an airflow, over paralleled pairs of electrically charged plates. Each particle is then attracted to a plate having opposite charge. In some types of electrostatic filters, large particles attract smaller ones to form an agglomerate that is finally blown off the plates into an impingement or diffusion type filter. Other units employ an adhesive on the plates, which must then be cleaned at regular intervals.

Inertial separation

This system again relies on the particles being of a massive nature an on the principle that an object will continue in a straight line unless acted upon by an external unbalanced force. By imparting a radical change in the direction of the airflow the heavier particles continue in the original direction of the airflow and are thereafter discharged into a collector bin. Two types of filters using the inertial seperation principle are used: The first is the classic Inertial Seperator and the second is the Cyclone Seperator.

Viscous Impingement.

In the title viscous impingement principle, the term 'impingement' refers to the manner in which particles are removed from the airflow while 'viscous' indicates how once removed, they are prevented from returning. Consider a particle in an airflow as it approaches an obstacle. As the airflow diverts around the obstacle, the particle, due to its mass and consequent inertia, tends to continue in the original direction. It therefore collides (impinges) with the obstacle (filter target fibre) and is separated from the airstream. Molecular forces alone are insufficient to adhere the particle to the obstacle. That is why an adhesive (viscous substance) is applied to the obstacles in order to prevent a particle's return to the airflow. This principle obviously works most effectively for heavy particles having large inertias and so it is mostly used in primary, or roughing, filters. To ensure maximum efficiency the following conditions must be met:- as many large obstacles (filter target fibres) as possible to produce sufficient deviations of the airstream. a high velocity (1.5 to 3.0 m/s) to reduce the chance of a particle deviating around the obstacle together with the airflow. an efficient adhesive to retain these heavy particles.

Sieving Principle

The name 'sieving principle' is almost self-explanatory. Basically, to remove particles of a certain size, say 10 micron and larger from an airflow, an obstacle with openings small than 10 micron is placed in the airflow which simply restricts the particles from passing through. Such an obstacle could take the form of a synthetic filter media.

Interception and Diffusion

Small, very light particles follow the airstream but are, under certain conditions, intercepted by molecular attraction forces between the particle and the obstacle. This is called the interception principle. A relatively low velocity (0.1 to 0.2 m/s) is required. All extremely small (sub-micrometer sized) particles are subject to random motion. This is known as Brownian motion and is caused by internal molecular forces. (In large particles the resulting net molecular force is insufficient to produce appreciable movement) . It is possible to trap these small randomly moving particles by placing densely packed obstacles in their path. As the particles diffuse through the obstacles, hence diffusion principle, they are trapped. Because we are dealing with minute particles, the molecular adhesion between obstacle and particle is sufficient for the particle to be retained without adhesive.An important condition for successful filtering using this principle is a very low velocity (0.02 m/s) through the medium. This increases the time needed by the particle to diffuse through the medium and so increases the chances of interception. As most atmospheric particles are of sub-micrometer size, this principle is of major importance in air filtration techniques and is mostly used in medium to high efficiency extended surface filters.

Combination of Filtration Principles

To clean air effectively, viscous impingement, sieving, and interception/diffusion principles need to be put into effect. To provide effective all round particle removal, a viscous impingement filter and the sieving filter first remove particles in the range of approximately 3 to over 100 um, followed by an extended surface filter, based on the interception/diffusion principle that deals with the particles of 0.3 to 3 um size. Finally an absolute filter working solely on the diffusion principle cleans up the particles of 0.3 um and under. As you see, even though the face velocity (air velocity through the duct) remains the same at each point in the duct, the media velocity (air velocity through the different media) is reduced. This is accomplished at manufacture by increasing the media surface area in filters which employ the interception/diffusion principle. For absolute filters (diffusion principle) a duct extension reduces the face velocity.

An example will demonstrate this effect. If the duct is small enough for one standard size filter (0.37m2) and the required airflow is 3400 m3/h then the face velocity is 3400/3600 x 0.37 = 2.5 m/s. This in turn is the media velocity of the first impingement filter which is placed vertically in the duct and works most efficiently at high air velocity. Next consider an extended surface pocket filter which might have a surface area of 8.5m2. Its media velocity is thus 0.11 m/s, slow enough for efficient use of the interception/diffusion principle. The final HEPA filter has approximately 20m2 of media surface. The media velocity is then 0.02 m/s, and the filter captures sub-micrometer particles in its dense media structure. It is important to remember that combination of impingement, sieving, interception and diffusion principles in one filter always results in reduced efficiency, both for collection of large and small particles. Best performance is only achieved when separate filter types employing different filtration techniques are used in tandem.

SECTION 4

AIR FILTER TYPES

TYPES OF FILTERS AND THEIR PERFORMANCE

There are three broad categories of air cleaners. The first, the fibrous media unit filter, is characterised by a gradual increase in accumulated dust load that increase pressure drop up to some maximum permissible value. During this time, the efficiency also increases. If allowed to load too heavily with dust, however, the efficiency will again fall off. The second category, the renewable media filter commonly called roll filter, uses a media usually of a fibrous type. In this case, new media is continually introduced into the airstream and loaded media is removed so that efficiency, resistance and loading remain essentially constant. The third classification is referred to as electronic air cleaners. These units have essentially constant pressure drop and efficiency unless their elements become severely dust loaded. Various combinations of the above types may be used. For example, a fibrous media filter may be placed both upstream and downstream of an electronic filter. The upstream unit to remove the large particles and the downstream unit to catch any particles that may blow off the plates of the electronic precipitator. Another attractive and frequently used combination is to use a lower efficiency (high arrestance) filter upstream of a very high efficiency filter to extend the life of the more expensive high efficiency filter.

Panel Filters

These are flat panel filters made up of coarse fibres. The filter media are coated with a viscous substance such as oil, which acts as an adhesive on particles that impinge on the fibres; hence the name viscous impingement filters. This type of filter is available in a permanent or throwaway style as well as with a replaceable medium or cleanable medium. They are characterised by low pressure drop/ low cost, reasonable arrestance, but low efficiency. The most common media are glass fibres, metallic wools, expanded metals, foils and crimped screens. Permanent-type panel filters have a metal frame and metal media that will withstand repeated washing. They are used for the collection of grease and oil mists in kitchens and similar applications. When they are wetted with oil, they can be used as a pre-filter in air conditioning installations. Metal filters are made in thickness of up to 100 mm, their rated face velocity is approximately 2,5 m/s. Initial resistance is in the region of 20 Pa (50 mm thick) , recommended final resistance is approximately 130 Pa. Arrestance is approximately 75 to 80% when a good type of viscous material is used. When dry, their arrestance is very low. This type of filter generally does not have a recommended direction of air flow. Disposable filters are constructed of inexpensive materials and are designed to be discarded after one period of use. The frame is usually a combination of cardboard and metal stiffeners. The filter media usually consists of fibreglass which, in the better gualities, has a progressive density construction which allows dirt to penetrate deep into the media utilising its full depth. The air entering side has an open weave fibre pattern with larger diameter fibres. The fibre diameters become smaller and the weave progressively tighter from the air entering to the air leaving side of the media. When the media is clean, dirt particles penetrate deeper before being trapped. As the back of the media loads up, particles are caught progressively closer to the air entering surface. This media design eliminates face loading and increases both arrestance and dust holding capacity.

Disposable panel filters are used in small air handling units and are very often included as original filters on new units. They are suitable for installations where dust loading conditions are particularly heavy. They are also excellent pre-filters to higher efficiency filters. Rated face velocity is again approximately 2,5 m/s. Initial resistance about 40 Pa and recommended final resistance 125 Pa. Average arrestance ranges from 65% to 85% for filter pads of 25 and 50mm thickness respectively. Disposable pad filters are similar to the disposable filters described above, except that the frame is made in steel or aluminium. Each time the filter has collected a full dust load, the hinged grid is swung open, the inexpensive pad is discarded and replaced with a new pad. The cost of replacing the media will be less than the cost of replacing the entire filter. When replacing pads, the media must be tucked in behind the flanges to ensure proper air seal. Washable filters have a special type of filter media which can be cleaned many times (up to 15 times). The frame is usually made more substantial and very often the filter media is moulded into the frame to ensure that air will not bypass the media. The number of times the filter can be washed depends on the severity of the operating conditions. This type of filter should not be used when the air being filtered is damp or greasy. The washing of filters must be done strictly according to the manufacturer's recommendations. If filters, after being washed, are thin in sections or have holes, they must be discarded.

Extended Surface Filters

Extended surface filters are of the dry, replaceable medium type. The filter media is made of cellulose, asbestos, glass fibre, wool, felt or synthetic material. The air passages through the medium are smaller than those of the viscous impingement type filter and, therefore, lower air velocities to avoid excessive resistances are necessary. In order to obtain a large surface area relative to the cross sectional area, the medium is usually pleated in accordion-form or formed in pockets.The pleated panel-type filter is made in thicknesses varying from 25 mm to 150 mm and filter areas with a filter medium area to face area between 1,5 to 1 and 10 to 1. These filters can be used at nominal face velocities in which case the dust holding capacity is greatly increased or at higher face velocities in order to save space as a smaller total filter face area is reguired. The filter material used in the pleated panel filters varies from a dry, washable non-woven material which has an arrestance of up to 85% and an efficiency of up to 20%, to progressive density fibreglass with an arrestance of up to 95% and an efficiency of up to 30%. Initial resistance of pleated filter depends on the material and the face velocity selected and can vary from 35 to 80 pascals. Recommended final resistance is approximately 250 Pa. The pocket-type filters have a filtering to face area ratio up to 30 to 1 and are made in a variety of filter media. These filters can be selected to give a variety of advantages such as

  • (a) longer life,
  • (b) lowest energy cost, and
  • (c) lowest maintenance cost.

The pockets on these filters must be very accurately made to very strict standards. Some of the important properties are as follows:

  • (a) Pockets must be made to strict sizes to ensure a balanced pocket design which provides maximum pocket opening, but prevents pockets from touching each other when "blown up" which would greatly reduce filter area.
  • (b) If pockets are sewn together, all stitching must be sealed to prevent leakage.
  • (c) Header frames must be properly and accurately made to fit into the holding frames.
  • (d) Pockets must be properly fixed to header frames.

Pocket filters are made in a variety of face areas and a variety of depths (or length of pocket) ranging from 300 to 900 mm. Efficiencies of this type of filter vary from below 20% up to 95% (arrestance 85 to 99%+).Initial pressure drop varies from 40 pascals at low face velocities for the low efficiency materials, to approximately 175 pascals at the higher face velocities and with the higher efficiency filter media. The recommended final resistance for the low efficiency pocket filters is approximately 200 pascals and the higher efficiency ones, approximately 400 pascals. Pocket-type filters cost much more than the filters discussed previously and in order to make them last longer, it is recommended that pre-filters be installed. These pre-filters remove lint and other large particles that would otherwise quickly load up the pocket filters. These lower efficiency dust pre-filters can be economically replaced several times during the life of the pocket filters. Hepa-type filters are supported pleat-type filters with high efficiency. They are also called absolute filters. Filter media area to face area ratio can be as high as 100 to 1. This type of filter can remove ultra-fine particles from the air used for ventilation of clean rooms and other areas where contamination control is critical. Examples are, operating theatres, computer manufacturing, space laboratories, photo and film manufacturing, nuclear power stations and food processing. Hepa filters are manufactured under exacting conditions to meet the most rigid specifications. Each hepa filter is tested to guarantee the specified efficiency. The minimum initial efficiency guaranteed is 99.97% on 0,3 micron particles (penetration 0.03%). Efficiencies can go as high as 99,9995% on 0,12 micron particles (penetration 0,0005%). Hepa filters are very costly and, because of their very high efficiency and even higher arrestance, would be blocked up very quickly by the larger particles of dust. For this reason, pre-filters are absolutely necessary.

Roll Filters

Roll filters, or renewable media filters, are filters in which clean media is introduced into the airstream as needed to maintain essentially constant resistance. This also maintains constant efficiency. The filter medium used in this type of filter consists of a progressive density fibreglass mat which is coated with a viscous adhesive.The clean medium is rolled up in rolls with a width of between 600 mm and 1,8 metres and approximately 20 metres long. The dirty media is rolled up on a roll. Advancing of the media is automatic by means of a pressure sensor or a photoelectric sensor which actuates the media advance motor. When the roll is exhausted, the entire roll is thrown away and a new one fitted. A failsafe control indicates when the roll is finished at the same time stopping the media advance motor. Accumulated dirt will not break away, but will cling to the fibres. The thickness of the media varies between 15 and 50 mm and must have sufficient strength to maintain its full width under the tension created as the roll advances. If the media "necks" (becomes narrower) and pulls out of the side channels, unfiltered air bypasses the media. All good media include some method to give a high tensile strength. Arrestance of the roll filter media varies between 60 and 95%, whilst the initial resistance is between 40 and 75 pascals. These initial resistances are, however, not important as the pressure drop across the filter media is always maintained at a final pressure drop or operating resistance of approximately 125 pascals. Dust holding capacity depends on the quality of the filter media. Progressive density materials have a good dust holding capacity and, although slightly more expensive, are more economical. Proper operation of the automatic roll filter unit is the key ingredient in achieving maximum results with the lowest possible operating costs. The two most important factors are

  • (a) advancement of the media when it reaches the pre- determined upper operating resistance level, and
  • (b) the incremental advance of the media after it reaches this level.

For best results, an automatic media advance control device supplied by the manufacturer, should be applied. At each advancement, only 100 to 200 mm of new media should be exposed.ASHRAE tests comparing the dust holding capacity of clean sheets of media versus the same media advances in small increments, have proven that incremental advance resulted in nearly double the dust holding capacity.

Electrostatic Filters

Positive ions generated at the high potential ionising wires flow across the airstream, strike and adhere to the dust particles in the air. Those particles then pass into the system of positively and negatively charged plates. The charged dust particles will be attracted to the plates and are, thus, removed. A potential of approximately 12 000 volts is used to create the ionising field and about 6 000 volts is maintained between the plates on which the dust collects. These voltages can be dangerous and an arrangement by which the electricity supply to the filter is switched off when the door is opened, must be incorporated. Air cleaners of this type, unless incorporating pre-filters or after-filters, offer negligible resistance to air flow. Care must, however, be exercised to ensure that the air flow/ through the filter is uniformly distributed. In many filters, perforated plates or some other arrangement are added to ensure even distribution of the air. The efficiency of the filter is sensitive to air velocity changes. For this reason, the velocity must be kept constant. The resistance of electronic filters, including the baffles or after-filters, is usually about 35 to 65 pascals at velocities between 1,5 and 2,5 m/s. These filters are efficient, low pressure drop devices for removing fine dust and smoke particles. Cleaning is generally accomplished by washing the plates in place with hot water from a water hose or by using a fixed or moving nozzle system. Once the dust particles have been collected/ they are held on the plates by adhesion forces which, for some types of dust, is insufficient. Covering of the plates with adhesive may be necessary. When the plates are used dry, the dust particles may form agglomerates on the plates which eventually blow off. For situations like this, the units are followed downstream by a secondary filter or storage section. The dry agglomerates produced in the electronic filter are allowed to blow off and be caught by the downstream filter. Roll filters are very often used for the after-filter. This arrangement results in an overall combination which provides both a high degree of cleaning efficiency and the convenient maintenance associated with automatic filters of this design.Dust which passes through an ioniser and is charged but not removed, carries the electrical charge into the space. If this continues on a large scale, as can happen when maintenance is bad, a space charge will be built up which tends to drive this charged dust to the walls. Thus, an electronic air cleaner which, for any reason, charges but does not remove the dust, can blacken walls faster than if no cleaning device were used.

Odour and Corrosion Control Systems

Air can contain unpleasant odours or corrosive gases. Odours can be dangerous and unpleasant to humans, whilst corrosive gases and fumes can eat away on relay contacts, contact surfaces at plug-in points, commutators, etc. Metal salts emigrating across insulators can cause shunting or failure. One way to overcome the problems, is to introduce plenty of outside air (if clean). However, energy is needed to cool or heat, humidify or dehumidify this air, and, at times, it can be more economical to filter the return air instead of introducing outside air or, if the outside air is contaminated, to filter the ventilation air. To remove odours and corrosive fumes, the air is drawn through a bed of media which chemically absorbs them. One such absorbent consists of a compound of activated alumina pellets impregnated with potassium permanganate which attracts gaseous contaminants, absorbs and finally destroys them by oxidation or chemical reaction and modification. Another method of purifying air is by drawing it through a bed of activated carbon. The carbon, in this case, absorbs the impurities. When the carbon is saturated and does not absorb any more gaseous or vaporised impurities, it must be replaced, or it can be regenerated.

SECTION 5

AIR FILTER TESTING METHODS

AIR FILTER EVALUATION

5. AIR FILTER TESTING METHODS / AIR FILTER EVALUATION

5.1 AIR FILTER PERFORMANCE CHARACTERISTICS

There is no one test method which can be universally applied to all types of air cleaners, nor can the results from different methods of testing filters be directly compared or interpolated one with the other. Valid comparisons of the performance of different air filters can only be made when the efficiencies are based on the same test method and the same type of test dust. The internationally recognised procedure for measuring filter performance is the ASHRAE test standard developed by the American Society of Heating, Refrigerating and Airconditioning Engineers. The ASHRAE tests are used on all types of filters except the very highest efficiency filters called "HEPA" filters. There are four key filter performance characteristics that are important. They are:

  • RESISTANCE TO AIR FLOW (Pascals)
  • DUST HOLDING CAPACITY (Grams)
  • ARRESTANCE (%s) [Average Synthetic Dust Mass Arrestance]
  • EFFICIENCY (%) [Atmospheric Dust Spot Efficiency]

The ASHRAE test provides the four characteristics. Resistance to airflow is measured in pascals, dust holding capacity is measured in grams, and arrestance and efficiency are expressed as a percent.

5.1.1. Resistance to Airflow (Pascals)

Every filter presents resistance to the airflow in which it is placed. This is usually measured as the pressure drop across the filter and expressed in Pascals (N/m2) or inches water gauge. Energy, supplied by the ventilator in the air handling system/ is therefore required to drive air through the filter. Generally speaking viscous impingement filters have a lower resistance to airflow than those using the interception/diffusion principle.In use a filter's resistance increases as it collects particles. For this reason, not only the initial resistance but also the final resistance, at which the filter needs replacing, is specified. Thus it is possible to determine the varying energy requirements of the ventilation system throughout the filter's life.

5.1.2 Dust Holding Capacity (Grams)

Dust holding capacity indicates the amount of contaminant a filter holds at its rated final resistance having determined the extent to which the air is to be cleaned, and specified a filter efficiency accordingly, dust holding capacity determines the filter life. It is thus a major factor in defining filter quality. And since we are considering replaceable filters, dust holding capacity is effectively one of the measures of the cost per cubic metre of air filtered. (The other measure is resistance to airflow i.e. energy expended in driving air through the filter) .

5.1.3 Arrestance (%) [Average Synthetic Dust Mass Arrestance]

Arrestance is a measure of'the mass of dust a filter can remove from the airstream. It is stated as a percentage.

Arrestance = mass of dust caught/mass of dust fed x100 = %

The arrestance of a filter is tested by feeding a carefully weighed amount of specially formulated synthetic ASHRAE test dust into the airstream going into a filter. The filter is weighed before and after the test so that the amount of dust caught can be determined. For example, if 500 grams of dust were fed and the filter caught 400 grams, the arrestance would be: 400/500 x 100 = 80%.Arrestance provides a good indication of a filter's ability to remove the larger, heavier particles present in the air. It is used primarily as a measurement of the air cleaning ability of lower cost filters such as panel filters, media pads or automatic roll filters that use fibreglass or synthetic media.

5.1.4 Efficiency (%) [Atmospheric Dust Spot Efficiency]

In air conditioning practice, one is normally concerned with the effect the dust has on the equipment and the staining power of the dust in the atmosphere. The larger particles clog up coils and damage bearings, but the smaller particles have the staining power. Thus, to measure a filter's ability to remove the small, stain-causing particles of carbon and other substances normally contained in the atmospheric air, a different test is required - the efficiency test. The /ASHRAE/efficiency test is carried out as follows: Untreated atmospheric dust is drawn through the filter to be tested. Equal samples of air are drawn upstream and downstream of the filter through identical discs made of glass fibre filter paper. The efficiency of the filter in preventing wall discolouration is measured by comparing the light transmission of the upstream and downstream test papers. (The more the paper stains, the less light will be transmitted through the test paper). The efficiency test provides a very realistic indication of a filter's ability to remove the same kind of particles that cause staining of walls and ceilings in buildings.For HEPA filters of the type used in clean rooms, operating theatres and nuclear applications, the normal test is the Thermal DOP method. In this method, a smoke cloud of essentially uniform droplets of Di-Octyl- Phthalate (DOP) is fed to the filter. The particles of DOP are 0,3 micron in diameter. Any particles that penetrate the body of the filter or leak through the gasket cracks pass into the downstream region where they are thoroughly mixed. The concentration of particles upstream and downstream is measured by a light scattering photometer from which the DOP efficiency is calculated. Because HEPA type filters have efficiencies so near to 100%, the efficiency or the penetration of the filter can be given. A filter having a DOP efficiency of 99,97% has a penetration of 0,03%. It is extremely important in the installation of hepa filters that no air bypasses the filter as this seriously affects the efficiency and the penetration.

5.1.5 Comparison of Arrestance and Efficiency

Unfortunately, no one test method has ever been devised to satisfactorily measure the air cleaning effectiveness over the entire range of filter performance. To illustrate the usefulness of the two methods, let us compare a panel filter with an arrestance of 85% with one of 75%. The difference in arrestance (10%) is significant and in practice, the arrestance procedure allows one to detect a noticeable difference in air cleaning capability between the two filters. If the same two panel filters were tested by the efficiency procedure, they would appear essentially the same. Both have less than 20% efficiency and the ASHRAE tests provide somewhat erratic values below 20%. The bar chart shown in 5.3 gives a comparison between the various methods of testing filters.

5.2 AIR FILTER TEST METHODS

5.2.1 ASHRAE 52-76 5.2.1.1

Synthetic Dust Mass Arrestance Test

Procedure: A controlled quantity of artificial dust is fed into the air (1) , drawn through the test dust, through the filter under test (2), and a high efficiency filter (3) that has been carefully weighed prior to the test. A pressure gauge provides readings of the pressure drop across the filter. The test is repeated until a selected pressure drop is reached.

Conclusions: This method is useful for comparing low efficiency impingement type filters. Since large particles account for most of the weight of the test dust, this test will not indicate how effect a filter is in removing the small stain-producing dust particles. Actually, about 70% of the soiling capacity of normal atmospheric air is represented by only about 7% (by weight) of the particles. Moreover, this 7% consists of very small particles in the 0.1 to 3.0 micron range.

5.2.1.2 Dust Holding Capacity Test

The capacity of the filter to hold dust is an important characteristic which determines the effective field life of the filter. To be meaningful for comparing filters, the air velocity, dust held, the arrestance values and the pressure drop of the filter must all be related. The ASHRAE Standard 52-76 outlines the detailed procedure. The dust holding capacity of a filter is the dust held (in grams) when the manufacturer's stated final pressure drop is reached. This test is usually run in conjunction with the ASHRAE Weight Arrestance Test. This type of test does not always relate well to actual performance of a filter with atmospheric dust and must be used with caution when evaluating filter performance.

5.2.1.3 Atmospheric Dust Spot Efficiency Test

Procedure: A controlled quantity of atmospheric air is drawn into the test duct and intermittently through an upstream sampler (1) containing a clean filter paper whose ability to transmit light has previously been measured. The air passes through the filter under test (2) and then continues through a downstream sampler (3) containing a filter paper whose ability to transmit light has previously been measured. The amount of air sampled through the upstream filter paper is adjusted so that the upstream and downstream filter papers are able to transmit light nearly equally at the end of the test. The percentage difference in the total quantity of air drawn intermittently through the upstream filter paper and continuously through the downstream filter paper gives the dust spot efficiency of the test filter.

Conclusions: This test is more indicative of the ability of a filter to remove the small stain-producing particles in the air, but it is subject to variations in results because of the variations in composition and concentration of atmospheric contaminant from area to area and time to time in the same location. Note: When considering Atmospheric Dust Spot Efficiency as a filter selection criteria it is important to consider the INITIAL Dust Spot value. This will indicate the Dust Spot at the start of the test or installation. It is desirable to have the INITIAL value as close to the AVERAGE value as possible. The Average Atmospheric Dust Spot Efficiency is what the filter is specified/rated as.

5.2.2 PARTICLE EFFICIENCY TEST

This test method provides a correlation of particle sizes and the respective counts both upstream and downstream of the filter. Simple calculation provides a particle efficiency rating expressed as a percentage per particle size. pressure gauge

Procedure: Atmospheric air is drawn through the test duct and through the test filter (1). Particle sensors (2) collect _a representative sample of dust particles in the air before and after the test filter. An electronic discriminator (3) measures^the number of particles, of a pre-selected size, ranging from 0.3 microns to 6.0 microns. These constitute 95% of the total number of particles in average air. The comparative particle count readings on the digital display indicate the filter's ability to trap these sizes of airborne particles. This information, combined with pressure drop data, provides a true prediction of^the filter's efficiency throughout its life, regardless of geographical location.

Conclusions: This filter rating method measures the number of particles of a specific size a filter stops without regard to the stain producing gualities of the particles.

For this reason the test can be reproduced at any time and in any part of the country. This makes it the most reliable and accurate filter test available. The following examples of particle counts show the effect of HEPA filtration. The count on the left was taken on the air entry side of the HEPA filter and that on the right on the clean air side. It can therefore be calculated for example that the Particle Efficiency on 0.5 micron particles is (100-(12/8235*100) = 99.85%.

5.2.3 DOP EFFICIENCY TEST

The ASHRAE Arrestance test and the ASHRAE efficiency test are destructive, and therefore, only a sample of the units manufactured can be tested. The DOP test on the other hand is a non-destructive test and can therefore be performed on filters manufactured. DOP stands for di-octyl phthalate, a plasticizer and paint component that atomizes into uniform 0.3 micron-diameter droplets. The DOP test measures actual concentration of the test aerosol penetrating the filter.

Procedure: This test is run in the same manner as the Particle Efficiency test except the test aerosol is a di-octyl phthalate (DOP) mist consisting of particles 0.3 microns in diameter. The concentration of parti.cJ.es (by count) measured upstream and downstream of the filter determines the efficiency.

Conclusions: This method is generally used to rate very high efficiency or "absolute" filters because of the small particle size of the contaminant and the very high efficiencies usually demanded (99 + %). The DOP test is usually applied only to filters for "clean rooms" or other special applications requiring very clean nearly sterile air.

5.2.4 SODIUM FLAME TEST

In the sodium flame test (British Standard 3928) a measured concentration of salt particles is introduced as a fine cloud into an airflow which passes through the filter under test. The concentration of salt remaining in the airflow on the clean air side of the filter is then measured using a flame photometer. Penetration of the filter is thus determined. Research establishments in many countries consider this the standard method for evaluation of HEPA filters. Extremely accurate test equipment is obviously required. The equipment used by AAF in Europe was constructed under the supervision of the United Kingdom Atomic Energy Research Establishment of Harwell, which is also responsible for carrying out regular checks and calibrations. Capable of testing filters up to 914 x 1824 mm, it is one of the largest sodium flame test rigs in Europe. The paricle cloud is formed by atomizing a 2% solution of sodium chloride in water and is injected into a drying tube upstream of the filter under test. When the water has evaporated, the remaining solid matter consists of salt particles ranging from 0.01 to 2.0 microns with an average diameter of 0.6 microns. These are carried through the test filter by the airflow.

A sample of the filtered airflow is bled off to the flame photometer. Here it is passed through a controlled gas flame, where any salt particles will burn with a yellow colour. The yellow light emitted is in proportion to the mass of salt passing through the flame. A photocell views the gas flame and its response to the sodium light emissions is measured. This response is immediately converted, by previous calibration, to give the quantity of particles escaping through the filter.

Leak Testing

Despite a high overall efficiency and low particle penetration/ a filter might still possibly contain a pin-hole allowing particles larger than those specified to pass through. A special scanning test for leaks i^ therefore implemented. This involves spraying paraffin oil upstream of the filter and then monitoring, with a canning probe, the concentration of paraffin passed by the filter. If a leak is detected it can sometimes by repaired by glueing. The test is then repeated to ensure the repair is effective.

5.2.5 "SALT TEST"

Although this is not a definitive test it is an indicator of the 'openess' or 'porosity' of a low efficiency 'strainer type' filter media. By using normal Cerebos salt and pouring it through the filter media the porosity of the media can be observed. In carrying out the test it should be remembered that salt particles are between 10,000 and 100,000 times the size of airborn particles and contaminants.

5.4 FILTER EVALUATION

In order to evaluate Air Filters it is essential to have a good understanding of the filter test methods. Once this is achieved filter evaluation can be carried out with confidence.
Test method
Test Contaminant
Method of Evaluating Inlet Concentration
Method of Evaluating Outlet Concentration
Method of Evaluating Filter Efficiency
Class of Filters for Which Test Method is Most Applicable
ASHRAE Atmospheric Dust Spot
Atmospheric Dust
Discoloration ot a White Filter Paper
Same as Inlet
Percent Increase in Discoloration Properties with Increasing Dust Load.
Electrostatic Filters and Fiber Air Filters Capable of Substantial Reduction in Staining Properties of Atmospheric Dust.
ASHRAE Weight Arrestance
Synthetic Dust Composed of 72% A.C. Test Dust-Fine, 23% Molocco Black, 5% No. 7 Cotton Linters
Pre-Weighed Charge of Dust Fed to Test Filter
All Air Passed Through a Final Filter to Retain Dust Passing Thru Test Filter
Percent Retention by Weight
Viscous-Impingement Type Filters and Other Low and Medium Efficiency Filters
DOP
Particles of Dioctylphlhalate 0.3 Micron in Size
Optical Measurement of Amount of Light Reflected by the Particles
Same as Inlet
Percent by Number of Particles Stopped by Filter
Absolute" Filters and Thobe of Similar Type
Particle Efficiency
Atmospheric Dust
Same ad DOPTest
Same as DOP Test
Same as DOP Test
All Types of Filters
 
The following example of two filters 'A' and 'B' illustrates how with a simple understanding of ASHRAE 52/76 one can easily evaluate their true performance. Filter 'A' that appeared exeptional based on 'Dust Holding Capacity versus Rands' in fact performs extremely badly when one simply divides the Arrestance figure into the Dust Holding Capacity.
COST (Rands)
DUST HOLDING CAPACITY (grams)
AVERAGE DUST SYNTHETIC MASS ARRESTANCE (%)
DUST FED (grams)
DUST PENETRATED (grams)
FILTER "A"
R25
300
40
750
400
FILTER "B"
R50
100
90
111
11

5.5 SABS 1424 1987

An understanding of the SABS 1424 - 1987 Specification which covers "Filters for use in Airconditioning and General Ventilation" is essential. A copy is available on request.

SECTION 6

AIR FILTER SELECTION

6.1 AIR FILTRATION LEVELS

6.1.1 Filter Classification (as per ASHRAE)

CLASS
EFFICIENCY (%}
ARRESTANCE (%)
1
20
90
2
25-30
92
3
30-35
94
4
35-40
95
5
90-95
     
ROOM
FUNCTION
FILTER CLASS
1
Technical rooms
1.1
Battery rooms rooms
1
1.2
Mechanical plant
1
1.3
PABX rooms
3
1.4
Lift motor rooms
3
1.5
UPS rooms
3
2.
Rooms for general use
2.1
Stores
1
2.2
Archives
3
2.3
Passages, Entrances
1
2.4
Waiting rooms
1
3.
Offices
3.1
General offices
3
3.2
Board rooms
3
3.3
Reception areas
3
3.4
Computer rooms
4
3.5
Lecture rooms
3
4.
Public rooms
4.1
Libraries
1
4.2
Airport halls
1
4.3
Cinemas, concert halls
3
5.
Shopping centres
3
6.
Hotels
6.1
Guest rooms
2
6.2
Conference rooms
3
6.3
Kitchens
2
6.4
Laundry , clean
3
6.5
Laundry, dirty
1
6.6
Dining rooms, cafeteria
3
7.
Hospitals
7.1
Operating theaters
5
8.
Laminar flow rooms
Section 10
8.1
Operating theater
8.2
Laminar flow benches
8.3
Clean rooms

 

6.1.2 SELECTING THE LEVEL OF FILTER EFFICIENCY

This is often the most difficult part of filter selection. The answer lies largely in the reason for filtering the air. If the main reason is to protect the coils and ducts from becoming coated with a layer or dirt, then good disposable panel filters, media pads, pleated filters, or automatic roll filters will usually do the job, under normal operating conditions. If you want to prevent staining of interior surfaces, such as diffusers, ceilings, walls, drapes, and light fixtures, then higher efficiency extended surface filters should be used. What level of efficiency depends partly on the conditions of the incoming air. Low level air intakes generally have dirtier air than intakes at higher levels. Building located near heavy industrial plants or in the downtown area of large cities usually have particularly dirty air. Nearby construction or seasonal factors may also effect the level of efficiency required to provide the desired degree of clean air in the occupied areas. If very critical activities are being conducted, such as hospitals, pharmaceutical or food processing, or electronics assembly operations, very high efficiency filtration is required, very likely HEPA filters.

6.2 SELECTION OF FILTERS

No single air filter can clean the air under all conditions. Therefore, a wide variety of products is available to provide the desired level of clean air at the roost economical cost. Table 12.2 gives some typical applications of filter based on efficiency and arrestance. Selecting the best type or combinations of filters requires an investigation of the options available. Each air filter system must virtually be custom designed in order to accommodate the many variables, such as:

a) Environment

  • Operating conditions
  • Dust concentrations
  • Required quantity of air/filtration level
  • Accessability and space

b) Air Filter - Purchasing/Performance Specifications

  • Filter face area (L x W)
  • Filter depth (D)
  • Number of pockets/pleats
  • Air velocity at filter face (M/S) / air volume (M3/S)
  • Resistance - initial/final (proportional to energy consumption)
  • Average dust weight arrestance
  • Initial dust spot efficiency
  • Average dust spot efficiency

c) Maintenance

Four options are available:

  • Replace complete- filter
  • Replace filter media only
  • Recondition by washing or cleaning
  • Automatic self-cleaning

The economics of each method - including both labour and equipment replacement costs - should be considered in air filter selection. d) Budget Initial installation or replacement costs Operating costs Compare "Rands for Grams"

The air volume to be handled will be decided by the design, whilst the available space depends on site conditions. The space availability must be considered at the design stage. Space for service and maintenance is absolutely essential. It must be remembered that an installation that is difficult to service will most probably be inadequately serviced or not serviced at all. Fan coil units and self contained air conditioners often limit the selection to panel filters. However, central station air handling equipment should not necessarily be limited to the use of panel filters. The operating conditions will depend on the area where the plant is to be installed. The dust concentration at a particular building location will be the first factor which will determine the type and efficiency of the primary filters to be used in the system. If the objective is to protect the coils and other mechanical filters, panel filters, media pads, pleated filters or roll filters would be sufficient. These filters do an excellent job of removing the large, heavier particles and will generally do a satisfactory job of protecting the coils under normal dust loading conditions. If, however, the objective is to prevent staining of internal surfaces or to protect a product, more efficient extended surface filters will be required. These filters remove the microscopic particles of carbon and other substances that cause staining and contamination. The available fan pressure is important in the smaller packaged plants because, if the fan pressure is not available, it means that the filters must be replaced long before they are saturated or be cleaned much more frequently. On bigger plants, extra resistance affects the fan power required to force the air through the filter. For each filter, the initial pressure drop, as well as the recommended maximum pressure drop, is given in the catalogues. If the system is required to deliver a specified minimum air volume, the fan must be able to deliver that specified volume when the filters have their final operating resistance.The fan pressure provided for comfort applications is usually the system resistance, including the average resistance of the filters (i.e. initial + final pressure drop divided by 2). This will mean that, with clean filters, the plant will deliver slightly more than design volume and when the filters are dirty, the volume of air supplied will be slightly less than design. If a constant volume of air delivery is desired, it will be necessary to maintain a constant system resistance by means of dampers or by choosing a constant resistance filter such as a roll filter or an electronic air cleaner.

6.2.1 CORRECT SELECTION OF PRE-FILTERS

The primary purpose of pre-filters is to extend the life of higher efficiency filters. This is done by removing the larger, heavier particles from the air. What I mean by "large" is particles five microns or so and larger. Particles of this size can be effectively removed by low efficiency filters, such as disposable panel filters, media pads, or automatic roll filters. These are far less expensive than extended surface filters. Without pre- filters, the larger particles would plug up the extended surface filters more quickly, causing their resistance to rise faster, resulting in shorter life. The diagram illustrates the life extention of a HEPA filter by the correct use pre-filters.

6.2.2 DIRT BUILD-UP AROUND AIR DIFFUSERS IN HIGH EFFICIENCY FILTER SYSTEMS

This is caused by air currents that are set up by the air coming out of the diffusers and also by the movement of people and objects within the room. Dirt is picked up by the air turbulence and circulated about the room. A percentage of the dirt is eventually deposited on the air diffusers. The dirt may be coming into the room through the air handling system but it is more likely to come in through open doors or windows, or be generated within the room itself. This situation is most likely to occur in high traffic areas, especially if they are carpeted, or where there is equipment likely to generate dirt such as copying machines. In area such as these, the filters cannot prevent this condition no matter how efficient they are.

6.2.3 Four Step Approach to Filter Selection

  • Step One: is to determine the type of air filtration systems installed in your buildings. The factors you need to consider are frame size, space between the frames and coils or fan, filter face velocity, efficiency, pressure drop and air flow capacity. We recommend that you have an experienced air filtration specialist take what we call a "Filter Evaluation Survey" to identify exactly what your filter needs are in each system. FIBATRON will work with you to conduct a survey and make- recommendations to improve performance and reduce cost
  • Step Two:is to determine the level of clean air required for each filter system. If protecting the coils from becoming coated with dirt is all that is necessary, disposable panel filters, media pads or automatic roll filters are adequate under normal operating conditions. On the other hand, if a particular system supplies air to a furnished area where a clean environment is important, higher efficiency extended surface filters should be considered. Of course, where highly critical activities are being conducted, such as surgery or pharmaceutical processing, HEPA filters should be used
  • Step Three: is to determine the amount of dust holding capacity you would like to obtain from the filters. This varies by type of filter, so it is important that you consult with someone who is knowledgeable about your systems and applications to give you proper guidance.Again, FIBATRON will work with you in establishing the proper levels of both efficiency and dust holding capacity.
  • Step Four:. is to develop purchasing specifications that identify the e f ficiency ( or arrestance) and dust holding capacity levels you desire. Any reputable filter manufacturer should be able to help you in writing performance oriented specifications.

Establishing specifications that clearly spell out the minimum levels of efficiency and dust holding capacity that are acceptable to you is a very important step in getting the right filters for your applications at the lowest total annual operating cost.

6.2.4 Do's and Don'ts in Air Filter Selection

  • a) Do have airflow as uniform across the filter face as possible.
  • b) Do consider the use of a prefilter with high efficiency units for longer service life.
  • c) Do provide weather lovers with trash screens on intakes.
  • d) Do select filters carefully when variable air volumes are involved. (Volume less than 20% or more than 130% or normal rating may be encountered which required careful selection for optimum performance).
  • e) Don't overrate air filters beyond manufacturer's recommendations.
  • f) Don' t exceed manufacturer ' s recommended final resistance values for any given filter.
  • g) Don't forget horsepower cost requirements for high-resistance filters
  • h) Don't consider only "first-cost" when selecting an air filter. Life-cycle costing fives you factual comparisons.
  • i) Don't install high efficiency filters without adequate prefliters.

6.3 FILTER SPECIFICATION

Once the appropriate level of performance has been determined for the filters, it is essential to make up purchasing specifications that set minimum acceptable performance ratings for efficiency and/or arrestance, dust holding capacity, initial and final resistance and air handling capacity. These specifications must be presented to all the suppliers who want to tender. All suppliers should be requested to verify the performance of their products by submitting ASHRAE test reports from an independent testing laboratory. Only by following the procedure above will one be able to select the right filter for the system and application at the roost economical operating cost. After you have established the proper performance levels for each system, you can now prepare purchasing specifications. Clearly spell out the desired levels of efficiency, or arrestance, and dust holding capacity for each type of filter required. Any reputable filter manufacturer should be able to assist you in writing performance oriented specifications. Present your specifications to all suppliers and request bids on filters that meet your performance requirements.

6.4 SAMPLE SPECIFICATIONS - to be used as a guideline only

[ NOTE : OPTIONS ARE INDICATED WITH AN ASTERIX (*) ]

6.4.1 FILTER SIDE ACCESS HOUSINGS, HOLDING FRAMES GASKETING. FILTER CLIPS and PRESSURE GAUGES

  • Primary/Secondary Filter Side Access Housing

    The Primary/Secondary Filter Side Access Housing will be the FIBATRON SLIDE PACK SIDE ACCESS HOUSING and will conform to the following:

    HOUSING

    The housing will be factory fabricated and assembled. The housing will be constructed of 1.6mm mild steel and will be finished with one layer of epoxy powder coated paint. The housing will be of welded construction and be equipped with up-stand flanges to facilitate field installation.

    FILTER HOLDING FRAMES, GASKETING AND CLIPS

    The housing will contain standard FIBATRON FILTER HOLDING FRAMES which will be mounted on slide rails to allow for the complete removal of the filter holding frames. The filter holding frames will have gasketing sealer material applied to both sides (primary and secondary). A minimum of two FIBATRON FILTER CLIPS will be used on the primary filter side and four on the secondary filter side. The filter holding frame slide rails are to have a replaceable woven pile, polypropylene finned sealing strip so as not to allow air by-pass. The filter holding frames will have gasketing applied to the vertical sides so as to effectively seal between the frames.

    ACCESS DOOR

    The housing will have a single side access door large enough to allow access by an average size person. The side access door will be air tight and have a quick release device which will allow easy removable of the door.

    PRESSURE MEASUREMENT

    The housing will- have pressure measurement gauges across both PRIMARY and SECONDARY filters (see FILTER HOLDING FRAMES).

    FILTERS

    The filters will be as per the FILTER SPECIFICATION.

  • HEPA Filter Side Access Housing

    The HEPA Filter Side Access Housing will be the FIBATRON HEPA SIDE ACCESS HOUSING and will conform to the following:

    HOUSING

    The housing will be factory fabricated and assembled. The housing will be constructed of 1.6mm mild steel and standard square tubing and will be finished with one layer of epoxy powder coated paint. The housing will be of welded construction and be equipped with up-stand flanges to facilitate field installation. The complete housing will be capable of withstanding 2000 Pa pressure without leakage.

    FILTER MOUNTING GRID, SEALING AND FILTER HOLDING BOLTS

    The housing will contain a square tubing arrangement onto which the HEPA filters will mount. The method of sealing will be NEOPRENE GASKETING/SILICONE (*). Filter holding bolts will be provided and will be of such a length so as to either accommodate a 148mm or 292mm HEPA filter (refer to FILTER SPECIFICATION). The design and construction of the mounting grid and the filter holding bolts will be such that it provides for sealing of individual filters.

    ACCESS DOOR

    The housing will have a single side access door large enough to allow access by an average size person. The side access door will be air tight and have a quick release device which will allow easy removable of the door.

    PRESSURE MEASUREMENT

    The housing will have a pressure measurement gauge across the filter bank.

    HEPA FILTERS

    The filters will-be as per the FILTER SPECIFICATION. D.O.P NOZZLE (* -> Optional) The housing will be provided with a D.O.P nozzle on the upstream side of the filter bank so as to allow for D.O.P. entrainment when testing.

  • Filter Holding Frames

    The filter holding frames will be FIBATRON DUAL PRESSURE MEASUREMENT FILTER HOLDING FRAMES and will conform to the following:

  • The filter holding frame will be manufactured of 1mm galvanised mild steel. The frame will have pre-punched holes to allow for easy field installation as well as having self centering dimples. The four sealing flange corners will be flush mitred and secured in order to form a uniform sealing surface. FIBATRON CORNER RISERS will be used when more than one filter is mounted on one side of the filter holding frame. The filter holding frame will be constructed in such a way to allow for filters to mount from both sides (primary filter and secondary filter separated to allow for dual pressure measurement). The filter holding frames will have gasketing sealer material applied to either one side or both sides of the frame depending on whether a secondary filter is installed at the time of initial installation. The gasketing material will be applied over the filter clip connection to the frame. The filter retainer clips will be capable of being installed without the requirement of tools, nuts or bolts. The holding frame shall be designed to accommodate standard size filters with the application of the appropriate type fastener. Filter banks having more than three filters high or wide will be strengthened with 3mm thick flat plate stiffeners bolted either vertically or horizontally between every second row of filter holding frames. Large filter banks will be supported by means of rectangular, tubular uprights and horizontal 3mm flat plate or tubular sections. The construction will depend on the size of the filter bank and the pressure differential.

  • Gasketing and Filter Clips

    The gasketing shall be FIBATRON FILTER GASKETING MATERIAL - NEOPRENE or GREY'ESTER (*).

    NEOPRENE

    NEOPRENE gasketing will be used on all filter holding frames in which high efficiency filters are to be installed or on the filters themselves. Care will be taken in applying the gasketing material to the filter or the filter holding frame. A suitable contact adhesive will be used on the surface to which the gasketing is to be applied. The surface onto which the gasketing is to be applied will be thoroughly cleaned before applying the adhesive or isketing. Corners are to be carefully mitred to prevent air by-pass and end-to-end connections are to be made using a suitable contact adhesive. When applying the gasketing care will be taken to ensure that an elastic force is not imparted into the gasketing material.

    GREY ESTER

    GREY ESTER gasketing will be used on all filter holding frames in which low to medium efficiency filters are to be installed. Care will be taken in applying the gasketing material to the filter or the filter holding frame. The surface onto which the gasketing is to be applied will be thoroughly cleaned before applying the gasketing. Corners are to be carefully mitred to prevent air by-pass and the ends are to be overlapped to ensure a suitable seal. When applying the gasketing care will be taken to ensure that an elastic force is not imparted into the gasketing material.

    The filter retainer clips will be the FIBATRON FILTER RETAINER CLIP - TYPE 'C' or "S" (*).

    TYPE 'C' RETAINER CLIP (*)

    The type 'C' clip will be a galvanised spring steel clip for use on pre-primary, primary and secondary filters and will fit securely on the filter holding frame. The 'C' clips will be used in pairs on the upstream side of a filter holding frame. Four clips will be used on the downstream side of a filter holding frame. The filter retainer clips will be capable of being installed without the requirement of tools, nuts or bolts.

    TYPE 'S' RETAINER CLIP (.*)

    The type 'S' clip will be a spring clip for use on secondary and tertiary filters of 150mm and 300mm depth. The clips will fit securely on the filter holding frame. The 'S' clips will be used in pairs on the upstream side of a filter holding frame. Four clips will be used on the downstream side of a filter holding frame. The filter retainer clips will be capable of being installed without the requirement of tools, nuts or bolts.

  • Pressure Measurement Gauges

    Pressure measurement gauges shall be FIBATRON PRESSURE GAUGES INCLINED MANOMETER or MAGNAHELIC INCLINED MANOMETER The inclined manometer will be capable of measuring the maximum pressure difference when the filters are dirty (see FILTER SPECIFICATION). MAGNAHELIC PRESSURE GAUGE The magnahelic pressure will be capable of measuring the maximum pressure difference when the filters are dirty (see FILTER SPECIFICATION).

    6.4.2 PRE-PRIMARY FILTERS

  • Filter Media Holding Frames

    The filter media holding frames will be FIBATRON TYPE AC-15, AC- 25 or AC-50 (*) and will conform to the following:

  • The U-channel frame will be constructed of 0.5mm galvanised mild steel. The U-channel edges will be double returned by at least 4mm in order to give added strength to the frame section and to prevent injury to personnel handling the frame. The frame will have 3mm diameter wire grids on both sides of the frame to restrain the filter media. The grid on the air entry side will be constructed in such a way to allow for easy removal of the grid for the changing/cleaning of filter media. The grids will be spot welded and finished with epoxy powder coated paint.

    15mm FILTER MEDIA HOLDING FRAME (FIBATRON AC 15) The filter media holding frame will have a depth of 15mm. The frame will contain one pad of filter media as specified in the FILTER SPECIFICATION

    25mm FILTER MEDIA HOLDING FRAME (FIBATRON AC 25) The filter media holding frame will have a depth of 25mm. The frame will contain one pad of filter media as specified in the FILTER SPECIFICATION

    50mm FILTER MEDIA HOLDING FRAME (FIBATRON AC 50) The filter media.holding frame will have a depth of 50mm. The frame will contain one pad of filter media as specified in the FILTER SPECIFICATION

  • Synthetic Media (cleanable)

    The synthetic filter media will be FIBATRON SYNTHETIC CLEANABLE FILTER MEDIA.

    The filter media will be of one of the following arrestance levels (*) :

  1. LOW : 5mm (FIBATRON RL 22)
  2. MEDIUM : 10-15irun (FIBATRON RL 44 or RL 66)
  3. HIGH : 20mm (FIBATRON RL 88)

The synthetic cleanable filter media will be constructed of synthetic fibres' and will be thermally bonded. The media will have a progressive density media fibre construction to allow the larger dust particles to penetrate the media to the more dense exit face allowing for high dust holding capacity. The filter media will be able to resist degradation due to repeated cleaning. The media will be colour coded on the air entry side. The media will compress nominally under an air approach velocity of 1.5 - 2.5 m/sec and maximum dust loads.

PERFORMANCE

(*) LOW ARRESTANCE (FIBATRON RL 22 FILTER MEDIA)

The filter media will be tested to ASHRAE Standard 52/76 and have the following properties:

  • Media Thickness : 5mm Filter
  • Air Approach Velocity : 1.5 m/sec
  • Air Flow Rate (Filter Area = 0.37 m2) : 0.558 m3/sec
  • Initial Resistance (@ 1.5 m/sec) : 20 Pa
  • Final Resistance (@ 1.5 m/sec) : 125 Pa
  • Average Synthetic Dust Mass Arrestance : 65 % (@ 125 Pa)
  • Dust Holding Capacity (@ 125 Pa) : 50 grams/m2
  • Initial Dust Spot Efficiency : N/A
  • Average Dust Spot Efficiency . : < 20 %

(*) MEDIUM ARRESTANCE (FIBATRON RL 44 FILTER MEDIA)

  • The filter media will be tested to ASHRAE Standard 52/76 and have the following properties:
  • Media Thickness : 10mm
  • Filter Air Approach Velocity : 1.5 in/sec
  • Air Flow Rate (Filter Area =0.37 m2) : 0.558 m3/sec
  • Initial Resistance (@ 1.5 in/sec) : 25 Pa
  • Final Resistance (@ 1.5 in/sec) : 125 Pa
  • Average Synthetic Dust Mass Arrestance : 70 % (@ 125 Pa)
  • Dust Holding Capacity (@ 125 Pa) : 75 grams/m2
  • Initial Dust Spot Efficiency : N/A
  • Average Dust Spot Efficiency : < 20 %

    (*) MEDIUM ARRESTANCE (FIBATRON RL 66 FILTER MEDIA)

    The filter media will be tested to ASHRAE Standard 52/76 and have the following properties:

  • Media Thickness : 15mm
  • Filter Air Approach Velocity : 1.5 m/sec
  • Air Flow Rate (Filter Area = 0.37 m2) : 0.558 m3/sec
  • Initial Resistance (@ 1.5 m/sec) : 35 Pa
  • Final Resistance (@ 1.5 m/sec) : 200 Pa
  • Average Synthetic Dust Mass Arrestance : 80 % (@ 200 Pa)
  • Dust Holding Capacity (@ 200 Pa) : 180 grams/m2
  • Initial Dust Spot Efficiency : N/A
  • Average Dust Spot Efficiency : < 20 %

    (*) HIGH ARRESTANCE (FIBATRON RL 88 FILTER MEDIA)

    The filter media will be tested to ASHRAE Standard 52/76 and have the following properties:

  • Media Thickness : 20mm
  • Filter Air Approach Velocity : 2.5 m/sec
  • Air Flow Rate (Filter Area = 0.37 m2): 0.944 m3/sec
  • Initial Resistance (@ 2.5 in/sec) : 85 Pa
  • Final Resistance (@ 2.5 in/sec) : 200 Pa
  • Average Synthetic Dust Mass Arrestance (@ 200 Pa): 85-90 %
  • Dust Holding Capacity (@ 200 Pa): 300 grams/m2
  • Initial Dust Spot Efficiency : N/A
  • Average Dust Spot Efficiency: < 20 %

    Synthetic Media (disposable)

  • The synthetic filter media will be FIBATRON SYNTHETIC HIGH ARRESTANCE DISPOSABLE FILTER MEDIA. The synthetic disposable filter media will be constructed of synthetic fibres" and will be thermally bonded. The media will have a progressive density media fibre construction to allow the larger dust particles to penetrate the media to the more dense exit face allowing for high dust holding capacity. The media will a scrim backing on the air exit side to give added arrestance properties. The media will compress nominally under an air approach velocity i 1.5-2.5 in/sec and maximum dust loads.

    PERFORMANCE

    (*) HIGH ARRESTANCE - BACKED (FIBATRON CAP 0300 FILTER MEDIA)

    The filter media will be tested to ASHRAE Standard 52/76 and have the following properties:

  • Media Thickness : 20mm
  • Filter Air Approach Velocity : 0.75 m/sec
  • Air Flow Rate (Filter Area =0.37 m2) : 0.278 m3/sec
  • Initial Resistance (@ 0.75 m/sec) : 75 Pa
  • Final Resistance (@ 0.75 m/sec) : 250 Pa
  • Average Synthetic Dust Mass Arrestance : 96 % (@ 200 Pa)
  • Dust Holding Capacity (@ 200 Pa) : 350 grams/m2
  • Initial Dust Spot Efficiency : N/A
  • Average Dust Spot Efficiency : < 20 %

    (*) HIGH ARRESTANCE - BACKED (FIBATRON PSB 5060 FILTER MEDIA)

    The filter media will be tested to ASHRAE Standard 52/76 and have the following properties:

  • Media Thickness: 20mm
  • Filter Air Approach: 0.75 m/sec
  • Velocity Air Flow Rate (Filter Area =0.37 m/2): 0.278 m3/sec
  • Initial Resistance (@ 0.75 in/sec) : 90 Pa
  • Final Resistance (@ 0.75 in/sec) : 250 Pa
  • Average Synthetic Dust Mass Arrestance (@ 200 Pa): 98 %
  • Dust Holding Capacity (@ 200 Pa) : 400 grams/m2
  • Initial Dust Spot Efficiency: N/A
  • Average Dust Spot Efficiency: < 20 %

    Fibre-Glass Media (oil-wet/dry)

The fibre-glass filter media will be FIBATRON FIBRE-GLASS FILTER MEDIA. OIL-WET FIBRE-GLASS FILTER MEDIA - ROLLS/PADS (FIBATRON OIL-WET FIBRE-GLASS : RED)

The oil-wet fibre-glass filter media will be constructed of continuous filament glass fibres. The media will be at least 50mm thick and have a progressive density media fibre construction to allow the larger dust particles to penetrate the media to the more dense exit face allowing for high dust holding capacity. 'he media will be colour coded on the air entry side. The media will have a laminated backing as an integral part of the media to prevent 'necking' when used in the automatic roll- clean application. The media will be treated with a viscous oil containing a tackifier having a drip free viscosity of 2100 C.P.S. at 25 degrees, to allow for effective impingement of dust particles. The media will compress not more than 12mm (in 50mm media depth) under an air approach velocity of 1.5 m/sec and loaded with dust to 125 Pa.

PERFORMANCE

The fibre-glass filter media will be tested to ASHRAE Standard 52/76 and have the following properties:

  • Filter Air Approach Velocity : 2.5 m/sec
  • Air Flow Rate (Filter Area = 0.37 m2) : 0.944 m3/sec
  • Initial Resistance (@ 2.5 m/sec) : 50 Pa
  • Final Resistance (@ 2.5 m/sec) : 125 Pa
  • Average Synthetic Dust Mass Arrestance : 86 % (@ 125 Pa)
  • Dust Holding Capacity (@ 125 Pa) : 850 grams/m2
  • Initial Dust Spot Efficiency : N/A
  • Average Dust Spot Efficiency : < 20 %

    DRY FIBRE-GLASS FILTER MEDIA - ROLLS/PADS (FIBATRON DRY FIBRE-GLASS : GREEN)

    The dry fibre-glass filter media will be constructed of continuous filament glass fibres. The media will be at least 50mm thick and have a progressive density media fibre construction to allow the larger particles to penetrate the media to the more dense exit face allowing for high dust holding capacity. The media will be colour coded on the air entry side. The media will have a laminated backing as an integral part of the media to prevent 'necking' when used in the automatic roll- clean application. The media will compress not more than 12mm (in 50mm media depth) under an air approach velocity of 1.5 m/sec and loaded with particles to 125'Pa.

    PERFORMANCE

    The 'test-oiled' fibre-glass filter media when tested with ASHRAE dust to ASHRAE Standard 52/76 will have the following properties:

  • Filter Air Approach Velocity : 2.5 m/sec
  • Air Flow Rate (Filter Area = 0.37 m2) : 0.944 m3/sec
  • Initial Resistance (@ 2.5 m/sec) : 50 Pa
  • Final Resistance (@ 2.5 m/sec) : 125 Pa
  • Average Synthetic Dust Mass Arrestance : 86 % (@ 125 Pa)
  • Dust Holding Capacity (@ 125 Pa) : 850 grams/m2
  • Initial Dust Spot Efficiency : N/A
  • Average Dust Spot Efficiency : < 20 %

    Grease Eliminators

  • The grease eliminator will be the FIBATRON GE-25 or GE-50 (*) GREASE ELIMINATOR filter and conform to the following:

    The filters will be the permanent, impingement, washable type and be constructed of galvanised mild steel/stainless steel (*) mesh rigidly enclosed in a stamped galvanised mild steel/stainless steel (*) frame 25mm/50mm (*) thick. The mesh will be protected with expanded metal for light or heavy duty applications.

    PERFORMANCE

    (*) 25mm GREASE ELIMINATOR (FIBATRON GE-25)

    The 'test-oiled' grease eliminator filter when tested with ASHRAE dust to ASHRAE Standard 52/76 will have the following properties:

  • Filter Air Approach Velocity: 2.5 m/sec
  • Air Flow Rate (Filter Area =0.25 m2): 0.625 m3/sec
  • Initial Resistance (@ 2.5 m/sec): 40 Pa
  • Final Resistance (@ 2.5 m/sec): 150 Pa
  • Average Synthetic Dust Mass Arrestance (@ 150 Pa): 85 %
  • Dust Holding Capacity (@ 150 Pa): 400 grams/m2
  • Initial Dust Spot Efficiency: N/A
  • Average Dust Spot Efficiency: < 20 %

    (*) 50mm GREASE ELIMINATOR (FIBATRON GE-50)

    The 'test-oiled' grease eliminator filter when tested with ASHRAE dust to ASHRAE Standard 52/76 will have the following properties:

  • Filter Air Approach Velocity : 2.5 m/sec
  • Air Flow Rate (Filter Area = 0.25 m2) : 0.625 m3/sec
  • Initial Resistance (@ 2.5 m/sec) : 75 Pa
  • Final Resistance (@ 2.5 m/sec) : 200 Pa
  • Average Synthetic Dust Mass Arrestance : 90 % (@ 200 Pa)
  • Dust Holding Capacity (@ 200 Pa) : 650 grams/m2
  • Initial Dust Spot Efficiency : N/A
  • Average Dust Spot Efficiency : < 20 %

6.4.3 PRIMARY FILTERS

Disposable Pleated Panels

PRIMARY FILTER - METAL FRAME DISPOSABLE The primary disposable panel filter will be the FIBATRON PM-33 50mm/100mm (*) METAL FRAME DISPOSABLE PRIMARY PANEL FILTER and will conform to the following:

FILTER FRAME

The filter frame will be of U-channel form and will be constructed of 0.5mm galvanised mild steel. The U-channel edges will be double returned by at least 4mm in order to give added strength to the frame section and to prevent injury to personnel handling the frame.

FILTER MEDIA

The filter media will be synthetic non-woven polyester staple fibres. The fibres will be mechanically bonded by needle punching and consolidated by an acrylic emulsion binder. The filter media must have a mass not less than 125 grams per square meter. The filter media will be pleated into a pack so as to have at least 20 pleats per meter. The pleats will be supported on one side by 0.65mm diameter drawn galvanised mild steel wire mesh with grid spacings of 25mm x 25mm. The pleats will be evenly spaced and the pleat profile will be of a 'V shape so as to allow air to penetrate to the trough of the pleat thereby allowing maximum-air flow. The filter media must have an effective area of not less than 2.0/3.8 (*) times the face area of the filter.

SEALING

The complete filter will be firmly bonded into the frame by means of a suitable contact adhesive which will be sufficiently thick enough to prevent any air leakages.

PERFORMANCE

(*) 50mm Disposable Primary Filter (FIBATRON PM-33)

The disposable primary filter will be tested to ASHRAE Standard 52/76 and have the following properties:

  • Filter Air Approach Velocity : 2.5 m/sec
  • Air Flow Rate (Filter Area = 0.37 m2) : 0.944 m3/sec
  • Initial Resistance (@ 2.5 m/sec) : 65 Pa
  • Final Resistance (@ 2.5 m/sec) : 250 Pa
  • Average Synthetic Dust Mass Arrestance : 80 % (@ 250 Pa)
  • Dust Holding Capacity (@ 250 Pa) : 340 grams/m2
  • Initial Dust Spot Efficiency : N/A
  • Average Dust Spot Efficiency : < 20 %

(*) 100mm Disposable Primary Filter (FIBATRON PM-33)

The disposable primary filter will be tested to ASHRAE Standard 52/76 and have the following properties:

  • Filter Air Approach Velocity : 2.5 m/sec
  • Air Flow Rate (Filter Area =0.37 m2) : 0.944 m3/sec
  • Initial Resistance (@ 2.5 m/sec) : 50 Pa
  • Final Resistance (@ 2.5 m/sec) : 250 Pa
  • Average Synthetic Dust Mass Arrestance : 80 % (@ 250 Pa)
  • Dust Holding Capacity (@ 250 Pa) : 500 grams/m2
  • Initial Dust Spot Efficiency : N/A
  • Average Dust Spot Efficiency : < 20 %

    Cleanable Pleated Panels

    PRIMARY FILTER - CLEANABLE

    The primary cleanable panel filter will be the FIBATRON WP-88/77 25mm/50mm/10Omm

    (*) PRIMARY CLEANABLE PANEL FILTER and will conform to the following:

    FILTER FRAME

    The filter frame will be of U-channel form and will be constructed of 0.5mm galvanised mild steel. The U-channel edges will be double returned by at least 4mm in order to give added strength to the frame section and to prevent injury to personnel handling the frame.

    FILTER MEDIA

    The filter media will be synthetic non-woven polyester staple fibres. The fibres will be mechanically bonded by needle punching and consolidated by an aery lie emulsion binder. The filter media must have a mass not less than 250 grams per square meter. The filter media will be able to resist degradation due to cleaning. The filter media will be pleated into a pack so as to have at least 40 pleats per meter. The pleats will be supported on both sides by 0.65mm diameter drawn galvanised mild steel wire mesh with grid spacings of 25mm x 25mm. The pleats will be evenly spaced and the pleat profile will be of a 'V shape so as to allow air to penetrate to the trough of the pleat thereby allowing maximum air flow. The filter media must have an effective area of not less than 2.0/4.2/8.6 (*) times the face area of the filter.

    SEALING The complete filter will be firmly bonded into the frame by means of a suitable contact adhesive which will be sufficiently thick enough to prevent any air leakages as well as maintaining the filter media in place during cleaning processes.

    PERFORMANCE

    (*) 50mm Cleanable Primary Filter (FIBATRON WP-77)

    The cleanable primary filter will be tested to ASHRAE Standard 52/76 and have the following properties:

  • Filter Air Approach Velocity : 2.5 m/sec
  • Air Flow Rate (Filter Area =0.37 m2) : 0.944 m3/sec
  • Initial Resistance (@ 2.5 in/sec) : 80 Pa
  • Final Resistance (@ 2.5 m/sec) : 250 Pa
  • Average Synthetic Dust Mass Arrestance : 86 % (@ 250 Pa)
  • Dust Holding Capacity (@ 250 Pa) : 380 grams/m2
  • Initial Dust Spot Efficiency : N/A
  • Average Dust Spot Efficiency : < 20 % (*)

100mm Cleanable Primary Filter (FIBATRON WP-77)

The cleanable primary filter will be tested to ASHRAE Standard 52/76 and have the following properties:

  • Filter Air Approach Velocity : 2.5 m/sec
  • Air Flow Rate (Filter Area = 0.37 m2) : 0.944 in3/sec
  • Initial Resistance (@ 2.5 m/sec) : 70 Pa
  • Final Resistance (@ 2.5 m/sec) : 250 Pa
  • Average Synthetic Dust Mass Arrestance : 86 % (@ 250 Pa)
  • Dust Holding Capacity (@ 250 Pa) : 490 grains/m2
  • Initial Dust Spot Efficiency : N/A
  • Average Dust Spot Efficiency : < 20 %

    Cassette Filters

  • Please contact us for the latest cassette filters specifications

6.4.4 TWO-STAGE PRIMARY FILTERS

  • Pre-Primary / Primary Filter

    TWO STAGE PRIMARY FILTER - DISPOSABLE/CLEANABLE

    The two stage primary panel filter will be the FIBATRON AAP-110 or AAP-160 (*) TWO STAGE PRIMARY FILTER and will conform to the following:

    FILTER FRAME

    The filter frame will be of U-channel form and will be constructed of 0.5mm galvanised mild steel 110mm/160mm (*) deep. Two angle brackets will be spot welded to the inside of the U-- channel frame which will support a separator grid. The separator grid will be 10mm thick 'barrel pleated' 25mm x 25mm x 0.65mm drawn galvanised mild steel wire mesh which will separate the oil-wet fibre-glass pre-filter from the dry cleanable pleated pack.

    FILTER MEDIA

    Pre-Filter : Disposable Oil-Wet Fibre-Glass Pad

    The oil-wet fibre-glass filter media will be constructed of continuous filament glass fibres. The media will be at least 50mm thick and have a progressive density media fibre construction to allow the larger dust particles to penetrate the media to the more dense exit face allowing for high dust holding capacity. The media will be colour coded on the air entry side. The media will have a laminated backing as an integral part of the media to prevent 'necking'. The media will be treated with a viscous oil containing a tackifier having a drip free viscosity of 2100 C.P.S. at 25 degrees, to allow for effective impingement of dust particles. The media will compress not more than 12mm (in 50mm media depth) under an air approach velocity of 1.5 m/sec and loaded with dust to 125 Pa.

    Primary Filter : 50mm/100mm (*) WP-77/88 Primary Cleanable Filter

    The filter media will be synthetic non-woven polyester staple fibres. The fibres will be mechanically bonded by needle punching and consolidated by an acrylic emulsion binder. The filter media must have a mass not less than 250 grams per square meter. The filter media will be able to resist degradation due to cleaning.

    The filter media will be pleated into a pack so as to have at least 40 pleats per meter. The pleats will be supported on both sides by 0.65mm diameter drawn galvanised mild steel wire mesh with grid spacings of .25mm x 25mm. The pleats will be evenly spaced and the pleat profile will be of a 'V shape so as to allow air to penetrate to the trough of the pleat thereby allowing maximum air flow. The filter media must have an effective area of not less than 4.2/8.0 (*) times the face area of the filter.

    SEALING

    The primary filter pack will be firmly bonded into the frame by means of a suitable contact adhesive which will be sufficiently thick enough to prevent any air leakages as well as maintaining the filter media in place during cleaning processes.

    PERFORMANCE

    Pre-Filter : Disposable Oil-Wet Fibre-Glass Pad

    The fibre-glass filter media will be tested to ASHRAE Standard 52/76 and have the following properties:

  • Filter Air Approach Velocity : 2.5 m/sec
  • Air Flow Rate (Filter Area =0.37 m2) : 0.944 m3/sec
  • Initial Resistance (@ 2.5 m/sec) : 50 Pa
  • Final Resistance (@ 2.5 m/sec) : 125 Pa
  • Average Synthetic Dust Mass Arrestance : 86 % (@ 125 Pa)
  • Dust Holding Capacity (@ 125 Pa) : 850 grams/m2
  • Initial Dust Spot Efficiency : N/A
  • Average Dust Spot Efficiency : < 20 %

    (*) Primary Filter :(50nnn WP-77/88 Primary Cleanable Filter

    The primary filter will be tested to ASHRAE Standard 52/76 and have the following properties:

  • Filter Air Approach Velocity : 2.5 in/sec
  • Air Flow Rate (Filter Area =0.37 m2) : 0.944 m3/sec
  • Initial Resistance (@ 2.5 in/sec) : 80 Pa
  • Final Resistance (@ 2.5 in/sec) : .250 pa
  • Average Synthetic Dust Mass Arrestance : 86 % (@ 250 Pa)
  • Dust Holding Capacity (@ 250 Pa) : 380 grams/m2
  • Initial Dust Spot Efficiency : N/A
  • Average Dust Spot Efficiency : < 20 %

    NOTE: COMBINE RESISTANCES FOR FILTER TOTAL RESISTANCE

    (*) Primary Filter : 100mm WP-77/88 Primary Cleanable Filter

    The primary filter will be tested to ASHRAE Standard 52/76 and have the following properties:

  • Filter Air Approach Velocity : 2.5 m/sec
  • Air Flow Rate (Filter Area =0.37 m2) : 0.944 rn3/sec .
  • Initial Resistance (@ 2.5 m/sec) : 50 Pa
  • Final Resistance (@ 2.5 m/sec) : 250 Pa
  • Average Synthetic Dust Mass Arrestance : 86 % (@ 250 Pa)
  • Dust Holding Capacity (@ 250 Pa) : 460 grams/m2
  • Initial Dust Spot Efficiency : N/A
  • Average Dust Spot Efficiency : < 20 %

NOTE: COMBINE RESISTANCES FOR FILTER TOTAL RESISTANCE

6.4.5 SECONDARY FILTERS

Pocket Filters

The pocket filter will be the FIBATRON SP-50, SP-85, SP-95 or SP99 (*) POCKET FILTER and will conform to the following:

The pocket filter will be constructed of high dust spot efficiency fibre-glass media pockets attached to a galvanised metal frame. Cardboard or 'basket' type frames are unacceptable.

FILTER FRAME

The filter header frame will be of a return U-channel and will be constructed of 0.5mm galvanised mild steel. The pocket surround frames will be constructed of 0.5mm galvanised mild steel. The pocket surround frames will be connected to the filter header frame in such a way so as not to allow air by-pass.

FILTER MEDIA

The filter media will be made of high density fibre-glass micro fibres constructed in such a manner so as that the filter media conforms with the spot efficiency specified when tested to ASHRAE standard 52/76. The filter media will be at least 6mm thick. The media shall be supported on the air exit side by a fibre- glass scrim backing.

POCKETS

The pocket filter will have 3/6/7 (*) pockets. The pockets will be constructed by means of stitching. Individual pockets will have rows of tapered stitching every 75- 100mm internally to restrain the pockets from over-inflating and touching one-another. The pockets must be supported by a minimum of 45 stitch points per 0.1m2 of pocket surface area. The pockets will be shaped so as to allow uniform proportional tunnel velocities in the passages of the air-entering and air- exit side of the filter thereby allowing maximum air flow. All stitch holes in the filter media will be sealed by means of applying a suitable adhesive. The pocket design will allow a minimum of 90% open face area for minimum entrance loss. Adjacent pockets will be mechanically fastened to the pocket surround frames using a non-piercing clinch method of construction.

PERFORMANCE

The pocket filter will be tested to ASHRAE Standard 52/76 and have the following properties:

Note: Due to the multitude of pocket filter sizes and combinations it is impossible to generalise test results.

The following figures serve as a guideline only. In compiling an actual specification a detailed study must-be made of exact requirements after which a pocket filter specification can be written.

GUIDELINE ONLY :

  • Length of Pockets : ??? mm
  • Number of Pockets : ? No.
  • Minimum Gross Media Area : ?? m2
  • Filter Air-Approach Velocity ; 2.5 m/sec
  • Air Flow Rate (Filter Area = 0.37 m2) : 0.944 m3/sec
  • Initial Resistance (@ 2.5 m/sec) : ?? Pa
  • Final Resistance (@ 2.5 m/sec) : 250 Pa
  • Average Synthetic Dust Mass Arrestance : ?? % (@ 250 Pa)
  • Dust Holding Capacity (@ 250 Pa) : ??? grams
  • Initial Dust Spot Efficiency: ?? %
  • Average Dust Spot Efficiency: ?? %

Guideline figures for arrestance and dust spot efficiency are as follows:

DUST SPOT EFFICIENCY (%)

ARRESTANCE (%)
30 - 35 %
94 %
45 - 50 %
96 s
60 - 65 %
97 %
80 - 85 %
98 %
90 - 95 %
99 %

 

Disposable Panel Filter

HIGH EFFICIENCY PANEL FILTER - DISPOSABLE

The primary cleanable panel filter will be the FIBATRON AVM-100 SECONDARY DISPOSABLE PANEL FILTER and will conform to the following:

FILTER FRAME

The filter frame will be of U-channel form and will be constructed of 0.5mm galvanised mild steel. The U-channel edges will be double returned by at least 4mm in order to give added strength to the frame section and to prevent injury to personnel handling the frame.

FILTER MEDIA

The filter media will be synthetic non-woven polyester staple fibres. The fibres will be mechanically bonded by needle punching and consolidated by an acrylic emulsion binder. The filter media will have a scrim backing which will provide the dust spot efficiency specified. The filter media must have a mass not less than 250 grams per square meter. The filter media will be pleated into a pack so as to have at least 40 pleats per meter. The pleats will be supported on both sides by 0.65mm diameter drawn galvanised mild steel wire mesh with grid spacings of 25mm x 25mm. The pleats will be evenly spaced and the pleat profile will be of a 'V shape so as to allow air to penetrate to the trough of the pleat thereby allowing maximum air flow. The filter media must have an effective area of not less than 4.2 times the face area of the filter

SEALING

The complete filter will be firmly bonded into the frame by means of a suitable contact adhesive which will be sufficiently thick enough to prevent any air leakages.

PERFORMANCE

(*) l00mrn Disposable Secondary Filter (FIBATRON AVM-100)

The cleanable primary filter will be tested to ASHRAE Standard 52/76 and have the following properties:

  • Filter Air Approach Velocity : 2.5 in/sec
  • Air Flow Rate (Filter Area =0.37 m2) : 0.944 m3/sec
  • Initial Resistance (@ 2.5 in/sec) : 120 Pa
  • Final Resistance (@ 2.5 in/sec) : 250-300 Pa
  • Average Synthetic Dust Mass Arrestance : 95 % (@ 250 Pa)
  • Dust Holding Capacity (@ 250 Pa) : 450 grams/m2
  • Initial Dust Spot Efficiency : 45 %
  • Average Dust Spot' Efficiency : 60 %

Cassettes Filters

  • Please contact us for the latest cassette filters specifications

6.4.6 TERTIARY FILTERS

'HEPA' Configuration

The tertiary filter will be the FIBATRON TURBOCELL-90 and will conform to the following:

The filter will be of 'HEPA type' construction. The element will be made from a continuous length of high efficiency glass paper media, folded in deep pleats, each pleat separated from the next by a corrugated spacer.

FILTER SURROUND FRAME

The filter surround frame will be constructed of 0.5mm galvanised mild steel and pop-riveted together to form a rigid cartridge. The construction and glueing of the media into the surround frame will be of such a nature that there is no air by-pass.

FILTER MEDIA

The filter media will be a glass fibre paper with an average dust spot efficiency of 95 %. The filter media will be pleated into a pack with corrugated separators to allow the flow of air down the full length of the pleat. There will be at least 100 pleats per meter. The pleats will be evenly spaced and the pleat profile will be uniform to allow air to penetrate effectively thereby allowing maximum air flow. The filter media must have an effective area of not less than 50 times the face area of the filter.

SEALING

The complete filter will be firmly bonded into the frame by means of a suitable contact adhesive which will be sufficiently thick enough to prevent any air leakages.

PERFORMANCE

The Tertiary filter will be tested to ASHRAE Standard 52/76 and e the following properties:

  • Filter Air Approach Velocity : 2.5 m/sec
  • Air Flow Rate (Filter Area = 0.37 m2) : 0.944 m3/sec
  • Initial Resistance (@ 2.5 in/sec) : 170 Pa
  • Final Resistance (@ 2.5 m/sec) : 600 Pa
  • Average Synthetic Dust Mass Arrestance : 96,% ( 600 Pa)
  • Dust Holding Capacity (@ 600 Pa) : 7.60 grams
  • Initial Dust Spot Efficiency : 81 %
  • Average Dust Spot Efficiency : 94 %

    6.4.7 HEPA FILTERS

    HEPA Ultra High Efficiency Filters

    HEPA - ULTRACELL 99.97 s (Ultra High Efficiency Filters)

    The tertiary filter will be the FIBATRON ULTRACELL 99.97 % HEPA FILTER and will conform to the following:

    The element will be made from a continuous length of high efficiency glass paper media, folded in deep pleats, each pleat separated from the next by a corrugated spacer.

    FILTER SURROUND FRAME ;

    The filter surround frame will be constructed of 1.2mm mild steel land pop-riveted together to form a rigid cartridge. The surround [frame will be finished by coating with epoxy powder. The construction and glueing of the media into the surround frame |wx be of such a nature that there is no air by-pass.

    FILTER MEDIA

    TChe filter media will be a glass fibre paper which will allow a aaximum of 0.03 % D.O.P. penetration. The filter media will be pleated into a pack with corrugated [Separators to allow the flow of air down the full length of the pleat. There will be at least 100 pleats per meter. The pleats pill be evenly spaced and the pleat profile will be uniform to |llow air to penetrate effectively thereby allowing maximum air llow. le filter media- must have an effective area of not less than 50 times the face area of the filter.

    SEALING

    The complete filter will be firmly bonded into the frame by means of a suitable contact adhesive which will be sufficiently thick aough to prevent any air leakages. Each filter will be factory tested for D.O.P. penetration and |ll be approved by the factory quality control officer.

    PERFORMANCE

HEPA filter will be tested to ASHRAE Standard 52/76 and have ie following properties:

  • Filter Air Approach Velocity : 1.3 in/sec
  • Air Flow Rate (Filter Area = 0.37 m2) : 0.480 m3/sec
  • Initial Resistance (@ 1.3 in/sec) : 250 Pa
  • Final Resistance (@ 1.3 in/sec) : 600 Pa
  • D.O.P. Penetration : 0.03 % (@ 600 Pa)

SECTION 7

COST SAVING METHODS OF AiR FILTRATION

7.1 EFFECTIVE MAINTENANCE AIR FILTER MAINTENANCE

The problem of removing dust from the airstream is a two-fold one. The first concern is the removal of the dust from the air, the second is the means of removing the collected material from the filter. The question of maintenance has taken on special significance in recent years because of the increased cost of labour. The four maintenance methods are:

  • a) replacing the complete filter,
  • b) replacing the filter media only (as in panel filters),
  • c) reconditioning by washing or cleaning, and
  • d) automatic self-cleaning (as in roll filters) .

The advantage of replacing the complete filter is that no washing plant is required and that a new filter, manufactured under controlled conditions, is installed and the plant is out of commission for a short time only. Disadvantages are disposal of the old filter and the cost of the new filters. This last item, however, must be carefully watched as disposable filters are cheaper than washable filter. To compare washables and disposables, the owning cost must be calculated. This is dealt with under life-cycle costing. Replacing the filter media only means that labour is required to remove the dirty filters and to replace the individual filter pads. This takes time which costs money and also means that the plant is stopped for the duration of the operation unless a complete set of spare frames is available. Washing or cleaning filters is a time-consuming operation which, if not properly and expertly done, will damage the filter causing loss of efficiency and dust holding capacity as well as air bypass through holes in the filter material and passed the often, through washing and handling, deformed frames.If the filters are washed or cleaned strictly according to the supplier's instructions (if available), filters could last as long as claimed. Regular checks by competent personnel must be made as to the suitability or re-installation of the washed or cleaned filters. Automatic self-cleaning roll filters handling 20 m3/s of air fitted with 20 m long 2 m wide roll of filter material with an arrestance of 88% will last approximately one year in a metropolitan area with a dust load of about 1 mg/???. This means that the roll would only have to be replaced once a year, an operating that would probably take approximately two hours. (The above is based on a dust holding capacity of 500 grams per m2 at a final resistance of 125 pascals).

When to Replace or Clean Filters.

Filters will . "look" dirty long before they are "dirty". Because of this, looking at filters to decide when they are to be replaced or cleaned is an extremely unreliable method. A filter is dirty when it has reached its recommended or selected final resistance. The selected final resistance is sometimes different from the recommended value because the fan cannot handle the recommended pressure or, at that pressure, would have a too much reduced air handling capacity. In order to find out what the pressure drop across the filter is, a filter gauge must be installed at each filter bank. This filter gauge must be reliable and give the correct reading at all times (some fluid filled gauges loose the fluid which will cause an inaccurate reading). An alternative is to provide properly installed pressure measuring points to which the serviceman can connect his pressure gauge. In both cases, the filter bank must be fitted with a notice giving both the initial and recommended final pressure drop. (An indication of the type of filter on which the pressures are based would be an advantage).

Holding Frames, Clips, Gaskets and Sealing

Air will follow the path or least resistance. In other words due care and attention must be provided to ensure that all open areas are completely sealed off to prevent dirty air bypass. The most common points to watch for:-

  • a) Holding Frames: All holding frames must be sealed from the clean air side with a suitable caulking material.
  • b) Every second row of holding frames, in multiple frame assemblies, must be re-inforced with stiffeners to prevent distortion due to airflow and differential pressure.
  • c) Access doors should carefully be sealed off with caulking material and closed-cell rubber gaskets installed to prevent dirty air bypass.
  • d) Only use closed cell gasketing on filter holding frames and ensure effective bonding to the metal surface.
  • e) A permanent and positive seal can only be achieved when four clips per holding frame are used.

More often than not, two clips are not adequate, especially when secondary filters are installed.

Correct Handling Methods

All too often expensive and often delicate filters are rendered useless due to poor handling procedures. Panel filters are re-moved and flung onto the floor for cleaning purposes. Secondary filters are dropped for disposal with all the collected dust now all over the plant room floor. As the system starts up, the dust is drawn into the clean air side. Bag filters are often torn and damaged during installation. This media will tear quite easily especially when hooked onto clips or pulled through the holding frame section. Hepa filters are punctured by probing fingers and other objects. All cassette type secondary filters are constructed from delicate fibres and contact even by hand can cause irreparable damage. Safe handling procedures and training must be provided to semi-skilled operators. Insist on training courses by your suppliers! Good after sales service must include correct implementation of the products and education of the operators.

Filter Cleaning Procedures

Operators must be trained and equipped to service filters with care. In doing so, filter performance and life will drastically improve with major cost savings in the long run. It is however the duty of the filter manufacturer and in turn the skilled supervisor to educate the operators to have more regard for the costs of filtration. It is a proven fact that some 70% of replacement costs are attributed to bad handling and cleaning methods. A well designed cleaning bay or cleaning system, which is not expensive, will pay dividends and ensure optimum life with peak performance.

Cleaning of Panel Filters

The correct cleaning techniques can dramatically extend the life span of cleanable panel filters.

a) Filter Media Used In Cleanable Panel Filters

The most common and effective type of media used in cleanable panel filters are synthetic non- woven medias. Synthetic non-woven medias have the following properties:

  • a) Synthetic (Polyester) fibres preferably of a virgin nature i.e not reprocessed.
  • b) Non-woven - needle punched.
  • c) Not less than 200 grams per square meter.
  • d) Fibres bonded by an acrylic emulsion binder.

The quality of the media is affected by three things:

  • i) the type and quality of virgin fibre used.
  • ii) the weight of the finished media (grammes per square meter).
  • iii) the number of times the media has been passed through the needling punching beds. This results in a 'thin, heavy and intertwined ' media.

It is therefore important to note that by lowering standards of the above three criteria one can dramatically affect the quality and life span of the finished product. "CHEAPER FILTERS MEAN A SHORTER LIFE SPAN"

b) Handling of Cleanable Panel Filters

In order to extend the life of cleanable panel filters and in order to preserve the original characteristics of the filter media, great care should be taken when handling the filters in the cleaning and transportation stages. The following should be noted:

  • i) Only use personnel trained in the maintenance of cleanable panel filters.
  • ii) Make sure personnel handling the filters understand the basic principles of filtration as well as when to identify when a filter needs to be replaced.
  • iii) Handle filters carefully in order to' protect the filter frame from being damaged. A dented frame can result in air bypass which defeats the purpose of the filter.
  • iv) Take care to protect the media pleat profile. Most reputable makes of panel filters make use of a wire mesh to support the pleats in a "V-Form". This is to allow effective airflow as well as maximising on the filter media used in the filter. By damaging the pleat profile one restricts the airflow as well as decreasing the amount of effective media area. This results in a shorter cleaning interval.

c) Cleaning Techniques

Cleanable panel filters were known as washable' filters. This description changed when it was proven that alternating air cleaning with water cleaning can extend the life span of a cleanable panel filter by 150-300%. It is widely recommended that filters should be 'air-cleaned' twice to every one 'water- clean' . The following paragraphs cover both techniques.

i) Air Cleaning

Air cleaning is a far less destructive means of cleaning and for this reason preserves the life of a cleanable filter. It is however less effective in removing dust particles and is therefore a method recommended for using in conjunction with washing. The following should be noted:

  • Keep the compressed air nozzle off the media as direct pressure will damage the media.
  • Apply main cleaning in the reverse direction to airflow (see airflow arrow on filter) although light frisking with compressed air in the reverse direction can assist in dislodging particles.

ii) Water Cleaning

This method is far more effective than air cleaning but does greater damage to both the filter media and the adhesives used in the filter manufacturing process.The following should be noted:

  • Filter cleaning detergent greatly assists in the removal of dust particles. For this one obviously requires a type of pressure cleaner in which the detergent can be mixed with the water prior to washing.
  • The use of warm water also assists in the washing of panel filters. Once again one would require a type of heated washing plant.
  • By using a controlled and gentle jet of water, wash the filter in the reverse direction to air flow (see airflow arrow on filter).
  • Do not apply spray directly onto the media as the damage caused to the non-woven media will drastically affect the performance of the filter media as well as it's life span.
  • Always wash and dry the filters with pleats vertical. The weight of the water in the media will damage the filter if allowed to lied horizontally during washing or drying (the media will pull away from the frame - allowing air bypass).
  • Further it is not practical or feasible to clean filters effectively and re-install them immediately thereafter. A spare set should be used to prevent operational interruptions. In doing so the contaminated filters could effectively be cleaned and dried prior to re-installation. In the case where filters are not dried, the dust solidifies within the media and results in almost immediate clogging. When this happens it is an almost impossible task to then try and clean effectively again.

d) Filter Life

A cleanable panel filter's life can be extended by many months and many washes if they are handled correctly in the cleaning processes. So don't ask the magical question: "How long can a cleanable filter last?" The answer is a simple one:

"MORE OFTEN THAN NOT IT DEPENDS ON YOU AND YOUR MAINTENANCE METHODS"

MANOMETERS AND DIFFERENTIAL PRESSURE INDICATORS

Almost every commodity has limits of operation. Such is the case with filters. When cleanable primary filters are not cleaned on time, it becomes almost impossible to achieve the desired results at a later stage. By the same token, increased possibilities of dirty air bypass and therefore premature blocking of expensive secondary filters will be the end result. Not to mention the cost factors which are not always tangible, such as increased power consumption. The correct use of instrumentation will certainly benefit the end user and pay dividends in the long run.

RECORDING OF HISTORY AND SERVICE INTERVALS

In order to compare filters, plan maintenance and budget it is essential to record and monitor filter performance. Each air handling unit or system should have a recording sheet to establish actual filter life, cleaning intervals and differential pressure information. Information of this nature will improve the planning schedules and assist with training.

7.2 LIFE CYCLE COSTING

Life Cycle Costs The total cost of owning a filtration system can only be calculated if the following important cost factors are considered.

  • a) The initial cost of the system.
  • b) The future replacement costs, both in terms of replacement material cost and labour installation cost.
  • c) The mechanical maintenance cost (of the filters, the equipment and the building internal surfaces).
  • d) The energy cost of the fan operation, in order to overcome the filter airflow resistance. With the increasing electricity costs, this factor becomes more and more important.
  • e) Reliability of supply.

Assuming a constant owning and operating cost, the higher the efficiency, the greater the benefits to the owner. Among the benefits of high efficiency air filtration are:

  • a) reduced building maintenance costs (housekeeping and decorating),
  • b) removal of health hazards,
  • c) improved lighting through cleaner fixtures,
  • d) reduced maintenance on other components on the airconditioning system,
  • e) protection of irreplaceable items such as those found in museums, art galleries and libraries, and
  • f) improved visibility and reduction of odours associated with smoking in higher occupancy spaces.

The complete calculation of the life-cycle cost is beyond the scope of these lectures, but a simple example is given below to illustrate the point.

Example One

An airconditioning plant for a building in a metropolitan area supplies 20 m3/s and operates for 10 hours per day and 5,5 days per week. Assuming a dust load of 0,1 mg per m3, the amount of dust to be removed per annum equals

Q x 3600 x T x C/1000 x 1000

= 20 x 3600 x 10 x 5.5 x 52 x 0.1/1000 x 1000 = 20.6 kg/annum

  • Q = volume of air to be filtered in m3/s
  • 3600 = number of seconds per hour
  • T = operating hours per annum
  • C = airborne dust concentration in mg/m3
  • 1000 x 1000 = conversion from mg to grams and grams to kg

Consider a selection between 2 filters 'A' and 'B':

  • Filter A can handle 1,2 m3/s, has a dust holding capacity of 2,2 kg and costs R150,00.
  • Filter B can handle 0,94 m3/s, has a dust holding capacity of 300 grams and costs R30,00.
  • The resistance of both filters is 88%; initial and final resistances are identical.

Answer: (Note: '/' = divide)

Filter A

  • No. of filters required =20/1,2 =17
  • Total dust holding capacity= 17 x 2,2 =37,4 kg
  • Dust caught per annum = 0,88 x 20,6 =18,1 kg
  • Filters will last 37,4 / 18,1 =2,06 yr
  • Cost of filters per year = 17 x 150 / 2,06=R1 238

Filter B

  • No. of filters required = 20 / 0,94 =21
  • Total dust holding capacity= 21 x 0,3 =6,3 kg
  • Dust caught per annum = 0,88 x 20,6 =18,1 kg
  • Filters will last = 6,3 / 18,1 =0,35 yr
  • Cost of filters per year = 21 x R30 / 0,35=R1 800

From the above, it can be seen that the more expensive filter 'A* costs much less to own. In addition, the more expensive filter only has to be changed every 2 years instead of the other filter's once every 4 months (0/35 year). This will make the difference even bigger.

Example Two

The same plant as in Example One has to be upgraded. The filters are to be replaced with high efficiency filters having an arrestance of 99% and a dust holding capacity of 510 grams. The air handling capacity is the same. The cost, however, is R225,00 per filter. Calculate the annual cost of the new filters under the following conditions

  • a) without pre-filters,
  • b) with permanent washable pre-filters with an arrestance of 80%, a dust holding capacity of 450 grams, a capacity of 1,2 m3/s and a cost of R60,00.

Answer:

  • a) Total dust holding capacity =17 x 0,51 = 8.67kg
  • Dust caught per annum =0,99 x 20,6 = 20.394kg
  • Filters will last 8,67 / 20,394 = 0/425 yr (5 months)
  • Cost per annum = 17 x R225 / 0,425=R9 000
  • b)Total dust holding capacity =17 x 0,45 = 7,65 kg
  • Dust caught in pre-filters per annum =0/80 x 20,6 =16,48 kg
  • Filters will last 7,65 / 16,48 =0,464 yr
  • Filters cleaned =1 / 0,464 =2,2 times per annum
  • High efficiency filters will be fed with 20,6 -16,48 =4,12kg/yr
  • Dust caught in these filters per annum =0/99 x 4,12 =4,08kg/yr
  • Dust holding capacity=8,67kg
  • Filters will last = 8,67/ 4,08 =2,13yrs
  • Cost per annum =17 x R225 /2,13 =R1 796
  • Cost of cleaning pre-filters per annum = 2,2 X R100 =R 220
  • Total = Rl 996

(Cost of cleaning filters estimated as one day for one man @ R100,00 per day per man). Note: the cost of the pre-filters has not been included as they are of the permanent washable type. This example clearly shows that the owning cost of the system with the washable pre-filters is much less - a saving of R7 000,00 is realised.

IMPORTANT NOTE:

The above examples take into account tangible costs only. It is however important to keep in mind the untangible costs such as the damage done by dust particles should these particles not be arrested by a filter. This damage could take the form of physical damage to equipment or processes or the cost in replacing higher efficiency filters at an earlier than budgeted stage.

SECTION 8

INSTALLATION OF AIR FILTERS

Most filters are identified with an arrow indicating the proper direction of air flow. Alternatively, the filter is marked with "clean air side". The two reasons why filters should always be installed with air flow in the direction indicated are (a) maximum service life, and (b) structural. The media in many filters is designed with "progressive density". This permits depth loading of particles through the entire thickness of the media. If the filter is installed backwards, dirt quickly builds up on the air entering side (now the denser material) . This is called "face loading" and causes resistance to rise quickly which shortens service life. Structural : most filters are design with supports on the air leaving side to prevent the media from blowing out as resistance increases. Improper installation greatly increases the risk of a blowout which can contaminate the air and damage equipment. Many air filters are available in units of convenient size for manual installation, cleaning and replacement. Some typical nominal filter sizes are 600 x 600; 600 x 500; 500 500; 500 x 400 and 400 x 400 mm and 15 fo 20 mm thick. In large systems, the frames in which the units are installed are bolted or rivetted together to form a filter bank. Several manufacturers provide side-loading filter section of various types of filters. Filters are changed from outside the duct making service areas in the - duct unnecessary, saving cost and space. The efficiency of any filter is, of course, sharply reduced if air leaks around it, through either poorly designed or installed frames or filters. The higher the efficiency of the filter, the more attention must be paid to the rigidity and sealing effectiveness of the frame. Air filters should be installed in the outdoor intake ducts as well as the recirculation and bypass duct of airconditioning and/or ventilation systems. Filters are logically placed ahead of heating and cooling coils and other airconditioning equipment in the system to protect them from dust. The dust captured in the outdoor air intake is likely to be mostly particularly matter of a greasy nature, while lint may be predominant in return air.Where high efficiency filters are used to protect critical areas, it is important that the filters be installed as close to the room as possible and in the positive pressure side of the system so that any leakage will be outward. Filters must be installed so that, wherever possible, the face area is at right-angles to the airflow. Eddy currents and dead air spaces should be avoided. Air must be distributed uniformly over the entire filter surface, using baffles or diffusers if necessary. Higher than normal air velocities impinging directly on the face of the filter must be avoided as these can damage the filters.

The following recommendations apply to filters installed with central fan systems.

  • a) Duct connections to and from the filter should changes size or shape gradually to assure even air distribution over the entire filter area.
  • b) Sufficient space should be provided in front of or behind the filter, or both, depending on its type, to make it accessible for inspection and service. A distance of 0,5 to 1,0 metre will be required, depending on the filter selected.
  • c) Access doors of convenient size must be provided to the filter service areas.
  • d) Doors on the clean air side must be gasketed to prevent ingress of unclean air. All joints and seams in ducts or plenuma must be air tight on the clean air side.
  • e) All filter banks must be caulked to prevent bypass of unfiltered air. The higher the efficiency o the filter, the more important this is.
  • f) Electric lights must be installed in the service areas.
  • g) Filters installed close to the inlet must be protected against the weather.
  • h) Filters must have permanent indicators to give warning when the filter resistance reaches too high a value or is exhausted/ as with roll filters.
  • i) Electronic filters must have an indicator or alarm system to indicate when the high voltage is off or shorted out.

SECTION 9

SIZING OF FILTER BANKS

The single most important thing to remember in the sizing of filter banks is the following equation:

Q = VA

Q) = QUANTITY

(V) = VELOCITY

(A) = AREA

N.B.

  • 1. Always work to frame size.
  • 2. Always ask the customer what is the square allowed for on the drawing or what maximum face velocity is called for in the consulting engineers specification.
  • 3. Pressure loss readings are always taken in front of the filters.
  • 4. The normal maximum face velocity should not exceed 2,5M/s. (l,3M/s for HEPA filters)

Example to work out Area (A)

  • (Q) 7,89M3/s
  • (V) maximum 2,5M/s
  • (Q) 7,89 / (V) 2,5 = 3/156M2 (A)
  • 0,610 0,610 = 0,37 (area of one filter 610mm x 610mm )
  • (A) 3,156 / 0,37 = 8,53 (therefore 9 filter panels are required).

Example to work out Velocity (V)

  • (Q) 120000M3/Hr
  • (A) 4 metres wide x 3 metres high
  • (Q) 120000 / 3600 - 33,33M3/S
  • (A) 4M X 3M = 12M2
  • (Q) 33,33 / (A) 12 - (V) 2,77M/s

Example to work out pressure loss

Always consult the resistance graph (curve) applicable to the filter.

(Q) 120000M3/Hr

(A) 4 metres wide x 3 metres high = 12M2

(V) 2,77m/s

At a face velocity of 2,77 M/s the pressure loss on a 50mm deep filter would be 75 pascals.

TERMINOLOGY

  • Quantity - (Q) Air Quantity or Air Volume
  • M3/s (metres cubed per second) terminology most commonly used
  • CFM (cubed feet per minute)
  • M3/Hr (metres cubed per hour)
  • M3/Min (metres cubed per minute)
  • L/s (litres per second)
  • Velocity - (V) Speed
  • M/s (metres per seond) terminology most commonly used
  • FPM (feet per minute)
  • Area (A)
  • M2 (metres squared) terminology most commonly used
  • FT2 (feet squared)
  • Resistance - Pressure Loss/Drop or Pressure Differential
  • Pascals
  • Kilo Pascals
  • Inches
  • Inches w.g. (inches water gauge)
  • mm w.g. (millimeter water gauge)

CONVERSIONS

Example for Quantity Q

  • CFM to M3/s (35,31 x 60)
  • 2000 CFM/60 x 35,31 = 2000/2119 = 0,94M3/s (Q)
  • M3/Hr to CFM
  • 60000 M3/Hr/60 x 35.31 - 35310 CFM (Q)

Example for Velocity (V)

  • FPM to M/s
  • 500 FPM/196.8 = 2,54M/s (V)

Example for Resistance

  • a) inches w.g. to pascals
  • 1"' w.g. = 254 pascals
  • 4" w.g. x 254 = 1016 pascals
  • b) inches w.g. to mm w.g.
  • 1" w.g. = 25,4 mm w.g.
  • 4" w.g. x 25,4 = 101,6 mm w.g.
  • c) m'm v.q. to pascals
  • 25,4 mm v.g. = 254 pascals

To convert:

  • Feet to metres multiply by 0.3048
  • Metres to feet multiply by 3.281
  • Cubic feet to cubic metres multiply by 0.02832
  • Cubic metres to cubic feet multiply by 35.31

SECTION 10

CLEAN ROOMS

INTRODUCTION

Nowadays with the help of suitbale airconditioning systems we can accurately control and regulate room air temperature, humidity, air movement and pressure difference. The desired degree of cleanness of ^e intake air can also be attained without difficulty by means of high efficiency filters. However, despite efficient flltration of inc intake air it can be difficult to lower the dust content in a room to the desired value. Dust particles are Generated by the working process taking place in a room. In addition, each person present emits particles depending on the activity, so contributing to the General level of contamination. Turbulence created mixing of conditioned air with room air, results in the Particles being quickly distributed all over the room. Even with increased rates of air change the purging effect in the room is insufficient, since only a small portion of the emitted dust particles will leave the room with exit air. The use of air locks in In conjunction with clean-rooms and constructional modifications to give greater air change rates have not brought the expected success. Further measures such as the provision of special clothing and the distribution of the working personnel over large floor area have brought about onlv marginal improvement. Experience from numerous plants and tests proves that in spite of highly efficient filtration of the air attained self-cleaning of the room cannot be attained

10.1 SYSTEMS WITH NON-TURBULENT LAMINAR FLOW

The fundamental realization that the self- cleaning of a room can be attained only with an improved flow has in the sixties led to the construction of the first "laminar flow clean- room" in the USA. The system comprises an even now of air through the whole room with absolute filtered air. The particles emitted in the room are thus carried away with the exit air within a few seconds.At an average air speed of 0,45 m/s with a horizontal air flow from a filter wall to the opposite suction wall or with a vertical air flow from the filter ceiling to a perforated floor system, the various working areas can be isolated against one another. This results in lower costs for clothes and preparations. Compared with conventional "clean-rooms", this conception requires also less severe working prescriptions. The savings in costs, the considerably lower space requirement per working place and its increased productivity due to lower rates of waste permit quick amortisation of the investment for clean-room systems.

10.2 CLASSIFICATION

"Clean-rooms" mean individual areas or enclosures which are provided with suitable arrangements for limiting the number of particles in the room air. In 1960, a first definition was laid down by US Federal Standard 209a. The figures of the three clean-room classes: 100, 10 000 and 100 000 correspond approximately with the maximum admissible number of particles of between 0,5 and 5 micrometers per cubic foot, or.per 28 dm3 of air. These standards also contain supplementary indications on the admissible number of particles of other sizes, designations, conditions, and also recommendations for the operation of "clean- rooms" . US Federal Standard 209d 1988 is the most recent revision. Class limits in particles per cubic foot of size equal to or greater than particle sizes shown (micrometers)

CLEAN ROOM CLASSIFICATION TABLE MEASURED PARTICLE SIZE)(MICROMETERS)

CLASS
0.1
0.2
0.3
0.5
5.0
1
35
7.5
3
1
N/A
10
350
75
30
10
N/A
100
N/A
750
300
100
N/A
1 000
N/A
N/A
N/A
1 000
7
10 000
N/A
N/A
N/A
10 000
70
100 000
N/A
N/A
N/A
100 000
700

A "laminar flow clean-room" is a "clean-room" of class 100 with laminar flow of 0/45 m/s 0,1 m/s. Such rooms having an even horizontal or vertical air flow are self-cleaning. Conventional "clean-rooms" with turbulent air flow of classes 10 000 and 100 000 are air conditioned rooms with a corresponding supply of filtered air. The definition of the air quantity and the choice of the air distributing and suction systems as well as the quality class of the installed filters is based partly on experience which specialised builders of "clean- rooms" have collected.

10.3 Basic Clean-Room Designs

Open Plenum System

Consists of confined space between roof and grid of HEPA filters. Supply air diffuses through filters into room.

Hood System

Supply air is fed to each individual HEPA filter through connecting ductwork. HEPA filters must always be prefiltered, preferably by two stages of filtration.

10.4 DESIGNING AND COMMISSIONING "CLEAN-ROOMS"

Only approved and tested high efficiency filters should be installed in "clean-rooms". The quality of the various filter fixtures and their arrangement must match the required type and class of clean-room together with its air system, bearing in mind the high quality of the installed filters.When commissioning clean-rooms, accurate tests on the overall system have to be carried out. Extensive measurements using suitable photometers and particle counters are imperative in the interests of both the client and the manufacturer. Where clean-room technique is involved, nothing must be left to chance; nothing should be taken for granted. Care and experience in filter manufacture and in the design, erection and operation of such plants are essential to secure good results.

GLOSSARY OF A!R FILTRATION TERMS

The following list is provided to enable the end user to look up terms he/she may come across. Only terms used in filtration for air conditioning plants are included.

Adhesive

Substance placed on the fibres of air filters to aid in the holding of dust particles.

Air, Outdoor

Air taken from outside the airconditioned space or system.

Air, Return

Air which has been returned to the airconditioning system or building for recirculation or exhaust. The return air duct will generally be found before the air filters where the return air is mixed with incoming outdoor air.

Anemometer

Instrument used to measure the velocity of air.

Apparatus House

Boxlike structure which houses the fan, coils, filter bank, outdoor air dampers and return air dampers. The chambers at each end are referred to as the plenum chambers.

Arrestance

A measure of the mass of synthetic ASHRAE dust a filter can remove from the airstream. It is stated as a percent - the mass of dust caught as a percent of the mass of dust fed. It is used on low efficiency filters and provides a measure of air cleaning capability only on larger, heavier particles.

ASHRAE Dust

A specially formulated dust used in the ASHRAE 52-76 test method for determining arrestance and dust holding capacity. It consists (by weight) of 5% cotton lint, 23% carbon black and 72% standardised air cleaner test dust.

Automatic Self-Renewable Media

Filter An air filter device that automatically feeds clean media across the face of the unit to expose it to the airstream. The media is supplied in roll form and is rewound onto a take-up roll as it is used. When the entire roll is used, it is discarded and replaced with a clean one.

Bypass

Unfiltered air escaping around a filter because it has not been properly sealed in place.

Capacity

Volume of air (CFM) which a filter unit is rated to handle by the manufacturer.

CFM (Cubic Feet Per Minute)

Measure of the volume of air being circulated by a system. An air handling system rated at 20,000 CFM has a volume of air equal to 20,000 cubic feet entering the plenum every minute.

Clean Rooms

A specially constructed enclosed area environmentally controlled with respect to airborne particulates, temperature, humidity, air pressure, airflow patterns, air motion and lightning.

Coils

A unit designed to heat or cool the air after it enters the building. Normally the coils, which are banked of finned tubes, are located after the air filters and before the duct work.

Contaminant

Airborne dirt, dust, spores, bacteria/ and allergens, sometimes referred to as "aerosols".

Diffuser

System of baffles used to distribute airflow evenly.

DOP

(Di-Octyl-Phthalate) An oil-like plasticiser which is readily atomised to form the test aerosol used in overall penetration tests of hepa filters.

Disposable

Refers to an expendable component or assembly which is discarded and replaced with a new unit when completely loaded.

Downstream

That portion of the system located after the filter. Also, the leaving air or the clean air side of a filter.

Dust Holding Capacity

The total mass of ASHRAE test dust a filter can hold before reaching a given final resistance. The amount will vary depending on the size and design of the filter and airflow rate. Reported in grams or grams per square metre. Provides a relative measure of filter service life.

Efficiency

In general terms, efficiency is the degree to which a filter will perform in removing solids. Specifically, it refers to any of three filter tests : ASHRAE 52-76 Arrestance, ASHRAE 52-76 Atmospheric Dust Spot or DOP Penetration. Extended Surface Filter A category of filter that is designed with pleats or pockets to increase the amount of media exposed to the airstream within a given face dimension. Greater filter surface area reduces media velocity, increases efficiency and dust holding capacity. Fibreglass A term used to describe a variety of filter media made with fine glass fibres. Fibre Breakoff Pieces of the filter fibres breaking off and entering the air stream, thereby becoming contaminants. Final Filter The last and most efficient filters in a multi-stage progressive filtration system.

Final Resistance

The maximum recommended pressure drop across a filter. Used as an indicator as to when a filter should be changed. Expressed in pascal. Synonymous with final pressure drop.

Filter

A term generally applied to a device used to remove airborne particulate from the air. Filter may be one of a number of types such as panel, automatic self-renewable, extended surface, hepa or electrostatic. The term "filter" is sometimes erroneously used to describe the media used inside the device.

FPM (Feet Per Minute)

The speed (velocity) of the air at a given point in the air handling system.

HEPA Filter

"High efficiency particulate air" capable of removing minimum of 99,97% of 0,3 micron DOP smoke particles (penetration = 0,03%).

Impingement

A method of filtration that is effective on particles with sufficient inertia to cause them to leave the air stream and collide with a fibre. Often referred to as "viscous impingement" when the fibres are coated with an adhesive.

Initial Resistance

Differential pressure across a clean filter. Expressed in Pascal Synonymous with initial pressure drop.

Interception

A special case of the impingement method of filtration that does not depend on the inertia of particles to bring them in contact with a fibre. Interception occurs when a particle follows the airstream, but touches a fibre as it attempts to flow around it. The particle is held by the inherent adhesive forces between the particle and the fibre.

Life Expectancy

The service life or change-out interval of a filter cartridge. Even with known dust holding capacities, useful life will vary according to contaminants entering the filter, particularly on make-up of air systems.

Loading, Centrifugal

Process by which contaminants tend to collect at the back of an extended area filter rather than evenly throughout the filter media. As air flows into a filter of this type it must bend, or change direction, to go through the sides of the filter. Because of their inertia, heavy particles resist change in direction, and will continue in their original path and collect at the back of the filter. This causes the filter resistance to rise less sharply than would be expected on the basis of extended area alone.

Loading, Depth

The collection of contaminants throughout the entire thickness of the filter media rather than on the surface.

Loading, Face

The collection of contaminants on the surface of the filter media rather than throughout the entire thickness. This causes a rapid rise in filter resistance.

Manometer

Gauge used to measure air pressure.

Medium

The porous material through with air is passed to remove particulates. Generally made of fibreglass, polyester or metal. Usually confined within a frame or cell sides, the assembly is referred to as a filter or filter cartridge.

Micron

A unit of length in the metric system equal to one millionth of a metre (1/1 000th of a mm) . Commonly used as a measure of particle -size or fibre size in filter media. The naked eye can see a particle approximately 10 microns or larger.

Migration

The process by which the adhesive on a filter releases itself from the filter fibres and enters the air stream, becoming a contaminant. Migration causes clogged coils and dirty ducts as the adhesive collects in the system.

Net Effective Media Area

The amount of media area in a filter that is exposed to airflow and usable for collecting airborne contaminants. Opposite of blind spots or dead area. Synonymous with net effective filtering areas.

Panel Filter

A low efficiency filter consisting of a flat sheet of media that is usually contained within a cardboard frame. An alternate design has an integral wire frame, normally made with fibreglass or synthetic media from 15 to 50 mm thick.

Penetration

The leak rate through the filter, penetration is expressed as a percentage based upon a specific particle size. % penetration is equal to 100% efficiency. Hepa filter, for example, have a 0,03% maximum penetration on 0,3 micron particles (99,97% efficiency).

Pre-Filter

A filter placed in front of another filter to remove the larger, heavier particles. Primary purpose is to extend the life of the final filters. Pre-filters are highly recommended in systems requiring high efficiency filtration, especially where a high concentration of lint is present. Two stages of pre-filters are recommended in clean room applications.

Pressure Drop

The difference in static air pressure between the upstream and downstream sides of an air filter due the resistance of the filter. Usually measured in inches of water, gauge (w.g.).

Resistance

Air passing through an air handling system roust travel through a number of ducts, the filters, coils, and other obstacles which tend to restrict its flow because of friction. This restriction is referred to as resistance. There is a total resistance of the system as well as the resistance caused by the filters. The general reference to resistance is that of the filters, measure in inches of water.

Safing

Sheet metal used to seal off the edges of the filter bank when the bank does not run from floor to ceiling or wall to wall in the plenum.

Surface Loading

The condition occurring when collected particles build up on the surface of the media plugging the spaces between the fibres. Also known as blocking. As a rule, the finer the media, the more susceptible it is to surface loading by "coarse" particles.

Straining

A method of filtration that removes larger particles.Straining occurs when a particle is larger than the space between fibres and cannot pass through them.

Unloading

The process by which dirt, originally stopped by the filter is released back into the air stream. Also called "blow- off".

Velometer

An instrument used to measure the velocity of air.

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