Mechanical Filters

Mechanical filters may be used in central filtration systems as well as in portable units using a fan to force air through the filter. Mechanical filters capture particles by straining larger and then smaller particles out of the airstream thorough increasingly smaller openings in the filter pack. Very small submicron-sized particles are captured by being drawn toward the surfaces of the filtration medium, where they are held by static electric charges. This is the factor responsible for the effectiveness of the highest efficiency mechanical filters' removal of submicron-sized particles. There are three major types of mechanical filters: panel or flat filters, pleated filters, and high-efficiency particulate air (HEPA) filters.

Flat or panel filters (Fig. 20-4) usually contain a low packing density fibrous medium that can be either dry or coated with a sticky substance, such as oil, so that particles adhere to it. Less-expensive lower efficiency filters that employ woven fiberglass strands to catch particles restrict airflow less, so smaller fans and less energy are needed. The typical, low-efficiency furnace filter in many residential HVAC systems is a flat filter, 13 to 25 mm (|-1 in.) thick, that is efficient in collecting large particles, but removes only between 10 and 60 percent of total particles, and lets most smaller, respirable-size particles through.

Older buildings were designed with only crude panel filters in HVAC equipment. Engineers now also use a combination of high-efficiency particle filters and adsorption filters to achieve high IAQ. Panel filters are

Figure 20-4 Dry mat panel air filter.

placed ahead of the HVAC unit's fan (upstream), and the high-efficiency systems are located downstream from the HVAC's cooling units and drain pans. This way, microbiological contaminants in wet components of the system are removed before they are distributed with the air through the entire building.

Not all pollutants can be removed by filters. Large sized particles are the easiest to remove, but smaller particles may be the most dangerous. Panel filters come with HVAC equipment, and are designed primarily to protect fans from large particles of lint and dust, not for proper air cleaning. Standard commercial grade filters remove 75 to 85 percent of particles from the air.

Media filters use much finer fibers. However, any increase in filter density significantly increases resistance to airflow, slowing down the air flowing through the filter. Media filters are around 90 percent efficient. They are usually a minimum of 15 cm (6 in.) deep, and have a minimum life cycle of six months. Filters, and especially media filters, require regular maintenance. If blocked, they can damage HVAC equipment, so they must be replaced frequently. Filters for large units can cover an entire wall in a room-size air-handler plenum.

The most effective approach to increasing effectiveness in a filter is to extend the surface area by pleating the filter medium. This slows down the airflow velocity through the filter and decreases overall resistance to airflow to reduce the drop in pressure. Pleated filters use highly efficient filter paper in pleats within a frame. Pleating of filter media increases the total filtering area and extends the useful life of the filter. The efficiency of pleated media filters is much higher than for other dry-type filters.

High-efficiency particulate air filters provide the best protection. Such HEPA filters were originally developed during World War II to prevent discharge of radioactive particles from nuclear reactor facility exhausts. They are now found in special air cleaners for very polluted environments, and for spaces that demand the highest quality IAQ. High-efficiency filters are used in hospitals and laboratories, as well as in portable residential air cleaners. They are generally made from a single sheet of water repellent fiber that's pleated to provide more surface area with which to catch particles. The filter is made of tiny glass fibers in a thickness and texture very similar to blotter paper. To qualify as a HEPA filter, the filter must allow no more than three particles out of 10,000 (including smaller respirable particles) to penetrate the filtration media, a minimum particle removal efficiency of 99.97 percent. Because they are more densely woven than other filters, HEPA filters require larger and more energy-intensive fans, making them more expensive and noisier. Consequently, HEPA filters are generally reserved for hospital operating rooms, manufacturing clean rooms (for example, where computer chips are made), and other especially sensitive places. HEPA filters are generally not applied to central residential HVAC systems due to their size and horsepower requirements. They need a powerful fan, leading to increased energy costs. Replacement filters range from $50 to $100, but last up to five years when used with a prefilter.

Similar HEPA-type filters with less efficient filter paper may have 55 percent efficiencies. These filters, which are still very good when compared to conventional panel type and even pleated filters, have higher airflow, lower efficiency, and lower cost than their original version.

In summary, there is little reason to use inexpensive tabletop, appliance-type air cleaners, regardless of the technology they employ. In general, high-efficiency particle collection requires larger filters or electronic air cleaners.

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