HEPA Filter Overview
HEPA is an acronym for ‘High Efficiency Particulate Absorbing’ or ‘High Efficiency Particulate Arrestance’ or, as officially defined by the Department of Energy (DOE), ‘High Efficiency Particulate Air.’ This type of air filter can theoretically remove at least 99.97% of dust, pollen, mould, bacteria and any airborne particles with a size of 0.3 micrometres (μm) at 85 litres per minute (Lpm). In some cases, they can even remove or reduce viral contamination. The diameter specification of 0.3 responds to the most penetrating particle size (MPPS). Particles that are smaller or larger are trapped with even higher efficiency. Using the worst case particle size results in the worst case efficiency rating (i.e., 99.97% or better for all particle sizes).
HEPA filters are also employed to filter out highly hazardous aerosols, such as those that are radioactive, bio-hazardous, and highly toxic (e.g. carcinogens). In the event of a nuclear, biological, or chemical outbreak, HEPA filters are the last line of defence between contamination and the those who could be exposed to it.
History of HEPA Filters
The first HEPA filters were developed in the 1940’s by the United States Atomic Energy Commission to fulfil a top-secret need for an efficient, effective way to filter radioactive particulate contaminants. They were needed as part of the Manhattan Project, which was the development of the atomic bomb. The first HEPA air filters were very bulky compared to the HEPA air filters that are produced today. HEPA filter technology was declassified after World War 2 and then allowed for commercial and residential use. The following is a chronological list of events in the history of HEPA filters:
- Developed during WWII atomic bomb research for containment of radioactive aerosols called ‘superimpingement’ or ‘superinterception’ filters and later referred to as ‘absolute’ filters
- First prototype filters used esparto grass as the filter medium
- In the 1950s, glass fibres were introduced into the paper
- In 1960, the first laminar flow bench was invented at Sandia National Laboratory
- In the 1960s, specifications were standardized and called HEPA filters
- In the 1970s, asbestos was removed
Industries and Applications
Construction and Function
HEPA and ULPA filters are best applied in situations where a high collection efficiency of submicron Particulate Matter (PM) is required, where toxic and / or hazardous PM cannot be cleaned from the filter, or where the PM is difficult to clean from the filter. HEPA and ULPA filters are typically utilized for applications involving chemical, biological, and radioactive PM. HEPA and ULPA filters are installed as the final component in a PM collection system, downstream from other PM collection devices, such as electrostatic precipitately or baghouses (Heumann 1997).
HEPA and ULPA filters are specifically designed for the collection of submicron PM at high collection efficiencies. They are best utilized in applications with a low flow rate and low pollutant concentration. Filter outlet air is very clean and may be re-circulated within the plant in many cases (AWMAI 1992).
They are not sensitive to minor fluctuations in gas stream conditions (Heumann. 1997).
Corrosion and rusting of components are usually not problems. Operation is relatively simple. Unlike electrostatic precipitators, HEPA and ULPA filter systems do not require the use of high voltages; therefore, flammable dust may be collected with proper care (AWMAI 1992).
Filters are available for a range of dimensions and operating conditions. Commercial filter systems and housings are available in several types of configurations to suit a variety of installation and operation requirements. These systems have many built in features such as testing and monitoring equipment. HEPA and ULPA filters are useful for collecting particles with resistivities either too low or too high for collection with electrostatic precipitated (AWMA. 1992).
Unlike bag-houses, which require workers to enter the collector to replace bagel, HEPA and ULPA filters systems are designed to replace filters outside the collector housing. This makes them ideal for applications involving hazardous air pollutants (HAPs) or toxic PM. The collected PM is tightly adhered to the filter media for subsequent disposal. Bag in / bag out procedures that may be required by OHSA are easily performed with the filters (Heumann. 1997).
HEPA Filtration Technical Referances
When designing HEPA filters, there are several items that must be considered: application, environment, efficiency required, physical geometry constraints, structural requirements, system volumetric flow requirements, system operational pressures, existing air handling equipment and its capabilities, as well as maintenance, ergonomics, cost, and manufacturability.
Most HEPA filters are constructed from a mat of randomly arranged special glass fibre sheet pleated in a “V” pattern like a folded paper fan with corrugated aluminium separators between the folds. This is attached to a sturdy base, forming the core of the filter.
HEPA and ULPA filters generally contain a paper media. Newer filter designs may contain nonwoven media, which utilizes recently developed fine fibre technology (INDA, 2000). Generally, the filter media is fabricated of matted glass fibre such as borosilicate microtines (EPA, 1991). The small fibre diameter and high packing density of both the paper and nonwoven media allow for the efficient collection of submicron PM (Gaddish, 1989). The waste gas stream is passed through the fibrous filter media causing PM in the gas stream to be collected on the media by sieving and other mechanisms, as mentioned below. The dust cake that forms on the filter media from the collected PM can increase collection efficiency (EPA, 1998a).
The filter media is pleated to provide a larger surface area to volume flow rate. For this reason, HEPA and ULPA filters are often referred to as extended media filters. Close pleating, however, can cause the PM to bridge the pleat bottom, reducing the surface area (EPA, 1998a). Corrugated aluminium separators are often employed to prevent the media from collapsing (Heumann, 1997). The pleat depth can vary from 2.5 centimetres (cc) (1 in.) up to 40 cm (16 in.). Pleat spacing is generally between 12 to 16 pleats per in., with certain conditions requiring fewer pleats, 4 to 8 pleats per in. (EPA, 1998a).
The most common designs are a box filter cell and a cylindrical filter cell. In a box cell the pleated media is placed in a rigid, square frame constructed of wood or metal. The air flows from the front to the back of the filter. Box packs are approximately 60 cm (24 in.) in height and width and 6 to 30 cm (3 to 12 in.) in length (EPA, 1991). The media in a cylindrical filter cell is supported by inner and outer wire frameworks. A metal cap seals the media at one end. Air flows from the outside to the inside of the filter. This allows a higher air flow rate than a box cell since more surface area is exposed (Vokes, 1999). Typical cylindrical packs are 50 centimetres (cm) (20 in.) in diameter and 35 to 60 cm (14 to 24 in,) in length (Vokes, 1999).
Both the box and cylindrical cells seal the media to the frame or cap using polyurethane, epoxy, or other commercially available adhesive. A metal grill protects the media face from damage. The filter cell is mounted to a holding frame using a gasket or fluid seal. The filter is generally mounted on the clean air plenum (EPA, 1991). The filter can be mounted directly in the duct or in a separate housing. HEPA and ULPA filter systems require pre-filtering for large diameter PM. HEPA and ULPA filter systems are generally the final component in a PM removal system (Heumann, 1997).
The HEPA and ULPA filter cells are generally utilized as a disposable-type filter. As discussed previously, when the filter cake build-up results in unacceptable air flow rates, the filters are replaced. In most designs, replacement of the filter cell takes place at the clean air plenum and outside of the housing unit. This reduces the risk of exposure to PM by the maintenance workers. This feature is especially important. HEPA filters differ in terms of filtration efficiency, configuration (size and shape), materials of construction, and fire resistance.