HEPA is abbreviation of “High Efficiency Particulate Air” filter (As per definition by US Department of Energy). The HEPA filter is a pleated mechanical filter made of glass fiber or filter paper. It has large filter surface, established high quality and performance even in difficult environments. HEPA filter is capable of removing hypothetically 99.97% of dust, pollen, bacteria dander, allergens from the fur of pets and any-other airborne particle of size 0.3 microns (µm). This particle size has been used as a standard because it refers to most penetrating particle size, hence the worst case situation. Therefore, efficiency of HEPA filter would be higher for all other particle sizes.
History and Application of HEPA Filters
HEPA filters were developed in 1940s and were initially put in use to prevent spread of radioactive contaminants. In the following decade HEPA filters witnessed their use on commercial scale to eliminate pollen, bacteria, viruses and dander. Hospitals, pharmaceutical manufacturing, electronics, aerospace and nuclear power were few sectors that needed highly efficient filters.
Other application areas of HEPA include HVAC industry, Industrial and domestic vacuum cleaners and airlines.
Types of HEPA Filter
To be very precise, HEPA filter do not have types. Filter can be either HEPA or not HEPA. They may be classified on the basis of their efficiency and maybe categorized and compared on the basis of efficiency. When looking for HEPA filter, you will come across the terms like HEPA type, HEPA like, HEPA style, 99% of HEPA. These terms are futile and create confusion among consumers. And, if the user is not well aware of filters; he might end up trapped and buying lower quality filters.
True HEPA filters are standard filters meeting desired specifications. Term “true” refers to standard HEPA filters and to differentiate from other HEPA like filters (not HEPA). True HEPA doesn’t have any added or special qualities as compared with standard HEPA filters. Standard/True HEPA filters are widely used in air purifier, vacuum cleaners, furnace filters etc.
Classification of HEPA Filters
HEPA filters are classified on the basis of efficiency levels. The classification is used to categorize HEPA filters among each other. Higher grade depicts the superiority of filter over other.
- HEPA H10: pass more than 15% of 0.1 micron particles per liter of air.
- HEPA H11: pass a maximum of 5% of 0.1 micron particles per liter of air.
- HEPA H12: pass more than 0.5% of 0.1 micron particles per liter of air.
- HEPA H13: pass up 0.05% of 0.1 micron particles per liter of air.
- HEPA H14: pass up 0.005% of 0.1 micron particles per liter of air.
According to above classification shows HEPA H14 and H13 are high end filters and are referred as Medical Grade HEPA filters. These medical grade HEPA filters are generally used in pharmaceutical manufacturing and electronic control rooms as they have a greater particulate retention rate. As evident for cleanest air, HEPA H13 or H14 is recommended.
What is Working Principle of HEPA Filters?
Working principle of HEPA filters may be divided into three stages to be exact inertial impaction, direct interception, diffusion and sieving. Let’s look at working of each stage.
Inertial impaction is first stage of filtration, where inertia acts on large particles suspended in flow stream. These particles are heavier than the fluid surrounding them. When fluid changes direction to enter the space, particles continue their motion in straight line, due to inertia. These straight moving heavy particles collide with media fibers, whereby they’re trapped and detained. By this mechanism contaminants like pollen are filtered out.
Interception acts on mid size particles that are not big enough to be captured by inertial impact and not small enough to diffuse with flow stream. These semi range size particles chase the fluid stream and are captured when they touch a fiber.
Diffusion acts on small sized particles. Being small, these particles are not held in place by the viscous fluid and diffuse within the flow stream. This type of movement is referred to as Brownian movement. When smaller particles transverses the flow stream, they collide with the fiber and hence detained.
It is most common mechanism in filtration. Sieving occurs when the particle is too bulky to fit between the fiber spaces. In many cases, the particles that accumulate on the surface of the filter contain a large percentage of clothing lint and fibers. Since they do not penetrate into the body of the filter media, they do not significantly contribute to pressure drop in the filter. Instead, large percentage of fibers trapped by sieving acts as additional filter media increasing the efficiency of the filter.
Below is the graph showing efficiency of HEPA filters versus particle size. It also shows working principle for each range of particle size.
In the following graph efficiency of HEPA filters has been depicted, alongside showing working principle of each stage.
It is evident from graph that larger the particle, better a HEPA filters out the particle. Further, note the dip in graph and observe corresponding particle size. It shows that HEPA has lowest efficiency of 99.975% for 0.3 micron particle size. It is the most penetrating size of particle. HEPA performs better for smaller or larger particle size than this 0.3 micron.
Importance of HEPA filters in Vacuum Cleaners
Nowadays domestic vacuum cleaners with HEPA filters are available in market. Quality of air in home is greatly affected by type of vacuum cleaner you own. Vacuum cleaners at home are used to collect dust, allergens, pet dander and hair. If any person at your home suffers from allergy, asthma or other respiratory issues raised by particle allergens, then you must look for vacuum cleaner equipped with HEPA filter. Studies have revealed that traditional vacuum cleaners can aggravate allergies by redistributing allergens on floor back in air that you are breathing. Hence, vacuum cleaners with HEPA filter can alleviate allergens from spreading in air, ultimately mitigating issues like asthma etc.