How HEPA Filters Work
HEPA stands for "High Efficiency Particulate Air." These kind of air filters were originally developed as a part of top secret Manhattan project to be used in case of any radioactive outbreak. This was in direct relation to the development of the atom bomb and the United states DOD was looking for a way to develop a device capable of filtering air contaminated by radioactive materials and possibly pathogens. The first HEPA filter trials were using human worn portable filters but were later dropped due to the bulkiness and the complexity of the device. During that time many more attempts were made to create better and more efficient air filtration devices like electrostatic precipitators and HEGA (High Efficiency Gas Absorption ) filters, but it was HEPA air filters that won the race. With the declassification of the Manhattan project the HEPA technology was later commercialized and many companies had started manufacturing HEPA air filters. Till date the basic technology remains the same with a few additions like the use of ultra violet light for killing air borne pathogens.
With many manufacturers now selling HEPA air filters the quality of these commercial air filters must meet the strict Military Standard 282 HEPA filtration efficiency test. Basically for a HEPA filter to pass the test , the device should be able to achieve 97 percent efficiency. In layman’s terms that means for a contaminant size of 3 microns and at the rate of 10,000 per sec, the filter should only let out 3 contaminant particles and filter out the remaining 9,997 ones. This sums up to the 99.97 per cent efficiency. Lately the New European Standard for Coarse and Fine Filters has issued several benchmarking parameters for testing and measurement of air filtration systems. Largely called as the EN779 system or BS EN779 in the United Kingdom. The table values for the EN 779 is as under –
|EU10||95 < 99.9 %
|EU11||99.9 < 99.97|
|EU12||99.97 < 99.99|
|EU13||99.99 < 99.999|
One more important in determining the efficiency of heap filter is the air flow rate. This is the rate at which air filtered out. Normally a flow rate of > 4 Nm3/min ( netwon meter cube ) is accepted as the industry standard.
The primary reason for the popularity of HEPA air filters is due to its high efficiency. An efficiency of 99.995% can be easily be achieved by industrial grade HEPA filters. This had led to the use of HEPA filters in hospitals, laboratories and highly sanitized environment like micro chip manufacturing facilities. In hospitals HEPA air filters are widely used in critical care units and neo natal units as they are one of the most efficient air filtration systems that effectively filter out air borne pathogens and viruses. An average size of a pathogen is somewhere between 1.02 to .004 microns. The largest being the anthrax having a size of 1.2 microns. While a HEPA filter dopes a good job of removing certain pathogens but it has to rely on additional techniques for sterilization. Some viruses and bacterias have a much smaller footprint and cannot be captured by a HEPA air filter. To combat this special techniques such as Ultraviolet lighting is used. A single 254 nanometer wavelength UV light operating at 6500 mW /cm2 is enough to kill (Vibrio cholerae) or the cholera virus. For molds an operating intensity of 60,000 mW /cm2 is sufficient. The total power of the UV lamp is a product to the power of the light source multiplied by the exposure time.
Apart from the ability to clear out pathogens HEPA air filters are used in creating dust free & allergen free environments. People who are allergic to pollens, cat dander, microscopic dust particles are advised to install HEPA systems at home or their workplace. HEPA filters are considered to be of excellent use in removing pollens and mold spores from free air. Thereby creating a dust and allergen free environment.
How HEPA works
The heart of a HEPA filter is densely compacted fiberglass padding on the interior. The fiberglass strands are randomly arranged and form a matting structure. Which is placed around the air suction duct. A basic HEPA filter works on a simple rule Interception, Impaction and Diffusion. Interception means that the particles are impacted on the fiber mesh and stick on to it. Impaction means when larger particles collide with the fibermesh and are retained by the padding. Diffusion means when the smaller particles collide with surrounding gas molucles and their velocity is reduced. This allows them to be trapped by the mesh structure. Below is a diagram that illustrates the working on a HEPA filter.
HEPA Filter Maintenance
Over time the fiber mesh may get dirty due to the accumulation of dust and other particles. The best way to clean a HEPA filter is using a soft brush. Using a hard brush will rip the fiber glass arrangement and create tear marks to the mesh. If there are stubborn impurities sticking on to the padding then a gentle knock on the mesh can dislodge the stuck particles. If this doesn’t work then highly compressed air blown through the mesh can help in removing the stuck particulates.
Once done it is advisable to soak the filter in a mild germicidal solution before replacing it again. The reason being that since the filter would have accumulated a lot of pathogens on its surface so it is better to disinfect it before putting it back again.