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Filtration

General

Air filtration involves the cleaning of both ventilation air and re-circulated air for the interior conditioning of building. Complete air cleaning may require the removal of airborne particles, microorganisms, and gaseous pollutants. This segment discusses only the removal of airborne particles. Atmospheric dust is a complex mixture of smokes, mists, fumes, dry granular particles, and natural and synthetic fibers. A sample of atmospheric dust usually contains soot and smoke, silica, clay, decayed animal and vegetable matter, organic materials in the form of lint and plant fibers, and metallic fragments. It may also contain living organisms, such as mold spores, bacteria, and plant pollens, which may cause diseases or allergic responses.

Ventilation Air Cleaning

Different applications require different degrees of air cleaning effectiveness. In industrial ventilation, removing only the larger dust particles from the airstream may be necessary for cleanliness of the structure, protection of mechanical equipment, and employee health. In other applications, surface discoloration must be prevented. Unfortunately, the smaller components of atmospheric dust are the worst offenders in smudging and discoloring building interiors. Electronic air cleaners or medium- to high-efficiency dry filters are required to remove smaller particles, which often must be controlled for health reasons. In clean room applications or when radioactive or other dangerous particles are present, high- or ultrahigh efficiency filters should be selected. Further information can be found in the latest ASHRAE HVAC Systems and Equipment Handbook.

Particle size may be defined in numerous ways. Particles smaller than 2.5 mm in diameter are generally referred to as the fine mode and those larger than 2.5 mm as the coarse mode. The cutoff for particles impacting on the respiratory function is considered to be 2.0 or 2.5 mm.

The fine and coarse mode particles typically originate by separate mechanisms, are transformed separately, have different chemical compositions, and require different control strategies. Fine mode particles generally originate from condensation processes or are directly emitted as combustion products. These particles are less likely to be removed by gravitational settling and are just as likely to deposit on vertical surfaces as on horizontal surfaces. Coarse mode particles are typically produced by mechanical actions such as erosion and frictional wear. Coarse particles are more easily removed by gravitational settling, and thus have a shorter lifetime in the airborne state.

Rating Air Cleaners

In addition to criteria affecting the degree of air cleanliness, factors such as cost (initial investment and maintenance), space requirements, and airflow resistance have encouraged the development of a wide variety of air cleaners. Accurate comparisons of different air cleaners can be made only from data obtained by standardized test methods. The three operating characteristics that distinguish the various types of air cleaners are efficiency, airflow resistance, and dust-holding capacity.

Efficiency measures the ability of the air cleaner to remove particulate matter from an airstream. Average efficiency during the life of the filter is the most meaningful for most filters and applications. However, because the efficiency of many dry-type filters increases with dust load, the initial (clean filter) efficiency should be considered for design in applications with low dust concentrations.

Airflow resistance (or simply resistance) is the static pressure drop across the filter at a given airflow rate. The term pressure drop is used interchangeably with resistance. Dust-holding capacity defines the amount of a particular type of dust that an air cleaner can hold when it is operated at a specified airflow rate to some maximum resistance value or before its efficiency drops seriously as a result of the collected dust.

Complete evaluation of air cleaners therefore requires data on efficiency, resistance, dust holding capacity, and the effect of dust loading on efficiency and resistance. When applied to automatic renewable media devices (roll filters), the rating system must evaluate the rate at which the media is supplied to maintain constant resistance when standard dust is fed at a specified rate. When applied to electronic air cleaners, the effect of dust buildup on efficiency must be evaluated.

Mechanisms Of Particle Collection

In the collection of particulate matter, air cleaners rely on the following five main principles or mechanisms:

  • Straining
  • The coarsest kind of filtration strains particles through a membrane opening smaller than the particulate being removed. It is most often observed as the collection of large particles and lint on the filter surface.

  • Direct Interception
  • The particles follow a fluid streamline close enough to a fiber that the particle contacts the fiber and remains there. The process is nearly independent of velocity.

  • Inertial Deposition
  • Particles in the airstream which are large enough or of sufficient density contact the fiber, and remain there. At high velocities (where these inertia effects are most pronounced), the particle may not adhere to the fiber because of high drag and bounce forces. In this case, a viscous coating is applied to the fiber to improve adhesion of the particles and is critical to the performance of an adhesive-coated, wire screen impingement filter.

  • Diffusion
  • Very small particles have random motion about their basic streamlines (Brownian motion), which contributes to deposition on the fiber. This deposition creates a concentration gradient in the region of the fiber, further enhancing filtration by diffusion. The effects increase with decreasing particle size and velocity.

  • Electrostatic Effects
  • Particle or media charging can produce changes in the collection of dust.

Types Of Air Cleaners

Common air cleaner groups are:

Fibrous media unit filters, in which the accumulating dust load causes pressure drop to increase up to some maximum recommended value. During this period, efficiency normally increases. Beyond the maximum value (high dust loads), dust may adhere poorly to filter fibers and efficiency drops due to offloading. Filters in such condition should be replaced or reconditioned, as should filters that have reached their final (maximum recommended) pressure drop. This category includes viscous impingement and dry-type air filters, available in low-efficiency to ultrahigh-efficiency construction.

Renewable media filters in which fresh media is introduced into the airstream as needed to maintain essentially constant resistance and, consequently, constant efficiency. Roll filters are one example.

Combination air cleaners, which combine the above types. For example, an electronic air cleaner may be used as an agglomerator with a fibrous media downstream to catch the agglomerated particles blown off the plates. Electrode assemblies have been installed in air handling systems, making the filtration system more effective. Also, a renewable media filter may be used upstream of a high-efficiency unit filter to extend its life.

Charged media filters are also available that increase particle deposition on media fibers by an induced electrostatic field. In this case, pressure loss increases as it does on a fibrous media filter. The benefits of combining different air cleaning processes vary.

Very high-efficiency dry filters, HEPA (high-efficiency particulate air) filters, and ULPA (ultralow penetration air) filters, are made in an extended-surface configuration of deep space folds of submicrometre glass fiber paper. Such filters operate at duct velocities near 250 fpm, with resistance rising from 0.5 to more than 2.0 in. of water over their service life. These filters are the standard for clean room, nuclear, and toxic-particulate applications.

Membrane filters are used predominately for air sampling and specialized small-scale applications where their particular characteristics compensate for their fragility, high resistance, and high cost. They are available in many pore diameters and resistances and in flat sheet and pleated forms.

Moving-Curtain Viscous Impingement Filters. Automatic moving-curtain viscous filters are available in two main types. In one type, random-fiber (non-woven) media is furnished in roll form. Fresh media is fed manually or automatically across the face of the filter, while the dirty media is rewound onto a take-up roll. When the roll is exhausted, the tail of the media is wound onto the take-up roll, and the entire roll is thrown away. A new roll is then installed, and the cycle is repeated. Moving-curtain filters may have the media automatically advanced by motor drives on command from a pressure switch, timer, or media light-transmission control. A pressure switch control measures the pressure drop across the media and switches on and off at chosen upper and lower set points. This control saves media, but only if the static pressure probes are located properly and unaffected by modulating outside air and return air dampers. Most pressure drop control systems do not work well in practice. Timers and media light-transmission controls help to avoid these problems; their duty cycles can usually be adjusted to provide satisfactory operation with acceptable media consumption.

Filters of this replaceable roll design generally have a signal indicating when the roll of media is nearly exhausted. At the same time, the drive motor is de-energized so that the filter cannot run out of media. The normal service requirements involve insertion of a clean roll of media at the top of the filter and disposal of the loaded dirty roll. Automatic filters of this design are not, however, limited in application to the vertical position. Horizontal arrangements are available for use with makeup air units and air-conditioning units. Adhesives must have qualities similar to those for panel-type viscous impingement filters, and they must withstand media compression and endure long storage.

The second type of automatic viscous impingement filter consists of linked metal mesh media panels installed on a traveling curtain that intermittently passes through an adhesive reservoir. In the reservoir, the panels give up their dust load and, at the same time, take on a new coating of adhesive. The panels thus form a continuous curtain that moves up one face and down the other face. The media curtain, continually cleaned and renewed with fresh adhesive, lasts the life of the filter mechanism. The precipitated dirt must be removed periodically from the adhesive reservoir.

The resistance of both types of viscous impingement automatically renewable filters remains approximately constant as long as proper operation is maintained. A resistance of 0.40 to 0.50 in. of water at a face velocity of 500 fpm is typical of this class.

Moving-Curtain Dry-Media Filters

Random-fiber (non-woven) dry media of relatively high porosity are also used in moving-curtain (roll) filters for general ventilation service. Operating duct velocities near 200 fpm are generally lower than those of viscous impingement filters.

Special Automatic Dry Filters

are also available, which are designed for the removal of lint in textile mills and dry-cleaning establishments and the collection of lint and ink mist in pressrooms. The medium used is extremely thin and serves only as a base for the buildup of lint, which then acts as a filter medium. The dirt-laden media is discarded when the supply roll is used up. Another form of filter designed specifically for dry lint removal consists of a moving curtain of wire screen, which is vacuum cleaned automatically at a position out of the air stream. Recovery of the collected lint is sometimes possible with such a device.

Electronic Air Cleaners

can be highly efficient filters using electrostatic precipitation to remove and collect particulate contaminants such as dust, smoke, and pollen. The designation electronic air cleaner denotes a precipitator for HVAC air filtration. The filter consists of a pre-filter, an ionization section and a collecting plate section. Customized electronic air cleaning systems designed for 2000 to 30,000 CFM requirements are applicable for kitchen hood exhaust or metalworking fluids like oil mist, smoke, fumes and airborne dusts. Up to four stages of air cleaning with integral wash system can be supplied. Large field assembled customized units can be designed for large volume general ventilation with integrated wash system, making it suitable for industrial manufacturing plants or large commercial and institutional buildings. The can be mounted in exterior air house buildings.

In the ionization section, small-diameter wires with a positive direct current potential between 6 and 25 kV are suspended equidistant between grounded plates. The high voltage on the wires creates an ionizing field for charging particles. The positive ions created in the field flow across the air stream and strike and adhere to the particles, thus imparting a charge to them. The charged particles then pass into the collecting plate section. The collecting plate section consists of a series of parallel plates equally spaced with a positive direct current voltage of 4 to 10 kV applied to alternate plates. Plates that are not charged are at ground potential.As the particles pass into this section, they are forced to the plates by the electric field on the charges they carry; thus, they are removed from the airstream and collected by the plates. Particulate retention is a combination of electrical and intermolecular adhesion forces and may be augmented by special oils or adhesives on the plates. In lieu of positive direct current, a negative potential also functions on the same principle, but more ozone is generated.

Electronic air cleaners typically operate from a 120- or 240-V ac single-phase electrical service. The high voltage supplied to the air cleaner cells is normally created with solid-state power supplies. Safety measures are required and a typical arrangement makes the air cleaner inoperative when the doors are removed for cleaning the cells or servicing the power pack.

The electrical power consumption ranges from 20 to 40 watts per 1000 cfm of air cleaner capacity. This type of air filter can remove and collect airborne contaminants with average efficiencies up to 98% at low airflow velocities (150 to 350 fpm) when tested according to ASHRAE Standard 52.1.

Efficiency decreases (1) as the collecting plates become loaded with particulates, (2) with higher velocities, or (3) with non-uniform velocity. As with most air filtration devices, the duct approaches to and from the air cleaner housing should be arranged so that the airflow is distributed uniformly over the face area. Panel pre-filters should also be used to help distribute the airflow and to trap large particles that might short out or cause excessive arcing within the high-voltage section of the air cleaner cell. Electronic air cleaner cells must be cleaned periodically with detergent and hot water. Some designs incorporate automatic wash systems that clean the cells in place; in other designs, the cells are removed for cleaning. The frequency of cleaning (washing) the cell depends on the contaminant and the concentration. Industrial applications may require cleaning every 8 hours. The timing of the cleaning schedule is important to keep the unit performing at peak efficiency.

Optional features are often available for electronic air cleaners. After-filters such as roll filters collect particulates that agglomerate and blow off the cell plates. These are used mainly where heavy contaminant loading occurs and extension of the cleaning cycle is desired. Cell collector plates may be coated with special oils, adhesives, or detergents to improve both particle retention and particle removal during cleaning. High-efficiency dry-type extended media area filters are also used as after-filters in special designs. The electronic air cleaner used in this system improves the service life of the dry filter and collects small particles such as smoke.

Negative Ionizer

uses the principle of particle charging but does not use a collecting section. Particulates enter the ionizer of the unit, receive an electrical charge, and then migrate to a grounded surface closest to the travel path. Particulates that pass through an ionizer and are charged, but not removed, carry the electrical charge into the space. If continued on a large scale, a space charge builds up, which tends to drive these charged particles to walls and interior surfaces. Thus, a low-efficiency electronic air cleaner used in areas of high ambient dirt concentrations, or a malfunctioning unit, can blacken walls faster than if no cleaning device were used.

Ozone

All high-voltage devices are capable of producing ozone, which is toxic and damaging to paper, rubber, and other materials. When properly designed and maintained, electronic air cleaners produce ozone concentrations that only reach a fraction of the EPA levels acceptable for continuous human exposure and are less than those prevalent in many American cities. Continuous arcing and brush discharge in an electronic air cleaner may yield ozone levels that are annoying or mildly toxic; this will be indicated by a strong ozone odor. Although the nose is sensitive to ozone, only actual measurement of the concentration can determine that a hazardous condition exists.

ASHRAE Standard 62, Ventilation for Acceptable Indoor Air Quality, defines acceptable concentrations of oxidants, of which ozone is a major contributor. The United States Environmental Protection Agency (EPA) specifies a 1-hour average maximum allowable exposure to ozone of 0.12 ppm for outside ambient air. The United States Department of Health and Human Services specifies a maximum allowable continuous exposure to ozone of 0.05 ppm for contaminants of indoor origin.

Selection And Maintenance

To evaluate filters and air cleaners properly for a particular application, the following factors should be considered:

  • Degree of air cleanliness required
  • Disposal of dust after it is removed from the air
  • Amount and type of dust in the air to be filtered
  • Operating resistance to airflow (pressure drop)
  • Space available for filtration equipment
  • Cost of maintaining or replacing filters
  • Initial cost of the system

Savings from reduction in housekeeping expenses, protection of valuable property and equipment, dust-free environments for manufacturing processes, improved working conditions, and even health benefits-should be credited against the cost of installing and operating an adequate system.

The capacity and physical size of the required unit may emphasize the need for low maintenance cost. Operating costs, predicted life, and efficiency are as important as initial cost because air cleaning is a continuing process. Panel filters do not have efficiencies as high as can be expected from extended-surface filters, but their initial cost and upkeep are generally low. Compared to moving-curtain filters, panel filters require more attention to maintain the resistance within reasonable limits.

The figure below illustrates some of the many filter arrangements.

Source: 1996 ASHRAE Handbook, HVAC Systems and Equipment, Chapter 24

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