Waging the battle against dirty air

Here’s the rundown on using HEPA filters to clear the air.

By Russ Kratowicz, Executive Editor

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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.

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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.

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.

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.

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