Waging the battle against dirty air

The intent of government regulation is to improve the health and welfare of the citizenry subject to those regulations. Some regulations are aimed at making the plant a safer place to spend eight or more hours every day.

It is not surprising that air quality in the industrial environment is a topic of concern. The microscopic particulate matter, both live and dead, floating in the air cause upper respiratory distress in susceptible people. Air in a typical plant environment contains between one and two million particles per cubic foot, most of which are in the sub-micrometer range. Because these minute particles cannot settle of their own accord, every industrial HVAC system uses some sort of filter to capture dust and dirt. This keeps them from settling inside air handling units and ductwork and migrating through the plant and office.

In general, filter porosity is chosen on the basis of the size of the offending particles to be captured. Every filter can capture gross material, such as airborne sawdust, but the materials that cause respiratory distress are much smaller. For example, the major dimension of many bacteria and viruses is less than one micrometer. Removing such material from an air stream requires a more intense degree of separation.

People are not the only elements of the plant environment sensitive to microscopic material suspended in the air. Some particulate-sensitive industrial areas, such as semiconductor cleanrooms, require that the HVAC system be equipped with high-efficiency filters that can remove all the suspended solids in an air stream.

How clean is clean?

The degree of cleanliness in a cleanroom is specified by a figure of merit known as its class, a number that indicates the maximum number of airborne particles expected to be within one cubic foot of air. For example, the air in a Class 100 cleanroom will have, at most, 100 particles per cubic foot. Cleanrooms range from Class 1 to Class 100,000 or more.

The filters that achieve such intense degrees of particle removal are known as HEPA filters, an acronym that means high efficiency particulate air. To even be considered a HEPA filter, it must be capable of capturing at least 99.97% of the particles that have a size of 0.3 micrometer or larger. HEPA filters were first used as a means to control airborne radioactive contamination during the development of the atomic bomb. A similar filter type is the ULPA (ultra low penetration air, which removes 99.999% of airborne particles as small as 0.1 micron.

Because HEPA filters are capable of removing extremely small particulates, their use assumes that the air handling system has prefilters to remove larger particles, which would otherwise overload the HEPA filter. Because HEPA filters remove solids, not vapors or odors, some HVAC systems use activated carbon to remove volatile organic compounds and similar contaminants. More recent innovations include the use of baking soda for odor control.

HEPA construction

The filters feature a non-woven medium held in a rigid frame. The HEPA filter medium is made from glass fibers having a diameter of 0.1 micrometer. The fibers are formed into a wet-laid, non-woven mat held together with a synthetic binder. The manufacturing process is similar to that used for making paper. This stands in contrast to standard, low-efficiency fiberglass filters, which use a dry-laid manufacturing process, much thicker fibers and a much larger open area.

Because the spaces between the HEPA fibers are so small, the filter exhibits a relatively large pressure drop, even when new. Whereas a typical clean low-efficiency fiberglass filter might have a pressure drop of 0.2 in. water column (WC), HEPA filters of the same physical size would be expected to exhibit a pressure drop of at least 1 in. WC. This greater pressure drop is the primary reason why HEPA filters cannot simply serve as a retrofit to replace a standard filter. The system fan may not be capable of moving enough air through the HEPA unit.

Minimizing the inherently large pressure drop requires maximizing the ratio of filter area to volumetric airflow. But, the cross-sectional area of the ductwork through which the air passes is limited. That is why HEPA filters are pleated, a geometry that increases the total area available for filtering while keeping the overall size within a limited form factor.

The capture mechanism

Although air does pass through the fiberglass filter medium, particle capture does not occur on the surface. Catching dirt with a sieve-like action would overload a HEPA filter and make its use uneconomical. As a result, HEPA applications rely on inexpensive, expendable filters to capture the gross dirt load, which leaves the HEPA filters to do what they do best — capture the smallest particles.

The mechanisms by which a HEPA filter captures the microscopic material is both complex and elegant. The first mechanism is called impaction, in which larger particles crash into individual glass fibers head-on and simply remain there. Another capture mechanism is interception, wherein particles in the air stream can’t quite get out of the way and collide with a fiber, perhaps a bit off-center, where they remain.

The smallest particles are subject to Brownian motion. They are so small that, in the absence of a solid surface, random collisions with air molecules are sufficient to keep them in constant motion. In the last capture mechanism, diffusion, the particulates jostle around under the influence of Brownian motion, touch the fibers and stick there.

These finest particles are then held in place by Van der Waals forces, the intermolecular attraction that becomes significant only at the very smallest molecular scale. The positive nucleus of one molecule is attracted to the negative electron cloud of another. It is possible for these smallest particles to be captured on the downstream side of the filter fibers.

Life expectancy

When handling properly prefiltered air streams, a HEPA filter should exhibit an extraordinarily long service life. According to Dale Montgomery, engineering product manager at Flanders Precision Air, a manufacturer of the filters in Smithfield, N. C., HEPA filters used in semiconductor plants should last for the lifetime of the cleanroom.

Disposal of filters

Unless they have been used for either nuclear or medical applications, HEPA filters removed from service can be treated as ordinary waste material subject to local disposal regulations. If they are loaded with radionucleotides or pathological biota, they are considered hazardous waste and are subject to different regulations.

Commissioning and field validation

Obtaining the maximum performance from a HEPA filter requires that it not only be selected properly, but that it be installed properly as well. If the filter edges are not sealed to the ductwork adequately, dirty air “short circuits” and carries the particulate matter around the unit that should have stopped it. Qualifying the performance of the installed filter is an important part of the commissioning process.

The manufacturer’s quality control test involves determining what fraction of an engineered “smoke” (having only particles about 0.3 micrometers in diameter) the filter captures. When dioctyl phthalate (DOP) is heated, it produces such “smoke” particles. DOP ratings are commonly featured in filter specifications. DOP testing is, however, a destructive test that leaves a stain on the filter medium. Qualification testing in the field relies on “smoke” from polystyrene latex, which does not leave an esthetically objectionable stain.

Typical plant applications

HEPA filters are costly compared to standard fiberglass filters. As a result, they are rarely used for general air filtration applications. Rather, they find use for specialized purposes where other types would not function. One example is spot filters for controlling fumes and smoke, such as in welding booths. Another application is a process that must avoid particulate-induced failures.

R. Vijayakumar, marketing manager at Hollingsworth & Vose Co., a manufacturer of HEPA filters based in East Walpole, Mass., offers some advice to plant professionals considering HEPA filters. “HEPA is the most abused term in the industry. You get what you pay for. Remember, you are not buying HEPA filters — you are buying clean air. When you talk to vendors, make sure that you know your allowable pressure drop, the flow rate you need and the degree of removal desired.”