HEPA filter, High-Efficiency Particulate Air/ High-Efficiency Particulate Arrestance Filter / High-Efficiency Particulate Absorbing Filter is known as HEPA filter denote higher degree of efficiency. The efficiency level denote the grade of the HEPA filters. There are several grades of HEPA filters available as per their area of application.
A standard HEPA filter must remove the particles from the air at least 99.5%[EU standard] or 99.97%[ASME,US.DOE] having particle diameter equal to 0.3 μm and subsequently increase efficiency with particle diameter greater than or equal to0.3 μm.
All type of standards HEPA filter generally retain particle and microorganism like dust, pollen, dirt, moisture, virus (0.02-0.3 μm), bacteria (0.2-2.0 μm), Bacillus subtilis, submicron liquid aerosol (0.02-0.5 μm), Penicillium citrinum, Aspergillus niger, Clostridia, Staphylococcus epidermidis, Bacilli, Bacteroidia, photocatalytic oxidation (PCO) etc.
HEPA filters use in wide area since its commercialization at 1950 where need to control contamination-
- Aerospace industry
- Cosmetic industry
- Materials dispensing area
- Electronics
- Food Industry
- Hard disk drives mfg. industry
- Hospitals
- Medical devices
- Microbiological Lab
- Nuclear Plant
- Pharmaceuticals
- Sterile products mfg. facility
- Semiconductor industry
- Vaccine and blood products mfg. area
- Vehicle industry
Type of HEPA filters
Generally HEPA filters can be classified as [A], [B], [C], [D], [E] and [F]. Types testes are demonstrates on the following table.
How it works?
All type of HEPA filters are generally composed of mat where fibers are randomly arrange, these fibers are actually set of poly propylene or fiberglass varies diameter between 0.5 and 2.0 micrometers. Frequently use tangled bundles of fine fibers to prepare these filters. Air passes through the convoluted pathway crated by the tangled bundles of fine fibers.
Actually this fibers act like sieve which retain the big particles which subject to try pass through it. Most time the smaller particles can’t hold its motion when they are passed with air, in result the small particle crush with the fibers. As per Brownian motion, the small particle possess the little materiality to move randomly with air as these particles are suffered to bombard action.
As the small particles crush with the fibers, here the key factors are mainly filter thickness, fiber diameter and velocity of the air. The air space between the HEPA filter subject to greater than or equal to 0.3 μm provide high level of efficiency. In sieves or membrane filters, particles which are less than the pore or openings can pass through but in HEPA filters the particles are attached to the fibers. There are several mechanisms has been identified which play the major role trap the particle to the pore.
Diffusion
All the particles under 0.3 μm retained by the HEPA filter in diffusion method. This the main theme of the Brownian motion where smaller particle below or equal to 0.1 μm in diameter are hindered and trapped in the filter. This mechanism is not prominent is low velocity of air.
Interception
Particles which are subject to flow in a line come with same radius/diameter of a fiber and adhere to it, this is very prominent to mid-size particle and which are retain by this process.
Impaction
Particles with higher diameter subject to unable to pass the fibers and directly retain on them, the effect increase subsequently with the air velocity.
Particle having size at or below 0.1 μm diameter, diffusion method is predominant and particle above0.4 μm predominant to impaction and interception. The MPPS [Most Penetrating Particle Size] is 0.21 μm, here the impaction and interception are remarkably inadequate. And this this point consider the weakest point of HEPA filter. The classification of the HEPA filters remarkably depend on the particle retention capacity at or below or equal to 0.3 μm particle size.
What is Gas filtration?
All types of HEPA filters are designed to retain particles but HEPA filters are unable to retain odor and gases molecules. HEPA filters can’t filter the odor or gases molecules. Alternative have been developed by British Armed Forces[BAF]. To filter the chemical vapors, organic compounds, volatile compounds, pet, cigarette, or flatulence odors, activated carbon filter or other filters are used.
HEGA [High Efficiency Gas Adsorption] filters plays the major role here, composed of Carbon cloth filters which is more efficient than granular activated carbon form. This filters [HEGA] designed by BAF to face chemical warfare.
Effective Usage of HEPA filter with Pre-filter
In a common air handing unit, a pre-filter[Carbon activated] use with the HEPA bag filter to extend the usage life/shelf life of more expensive HEPA filter. At the first stage of the filtering the pre-filter, removes the almost all type of dust, hair, particle from the air. Particles which can’t remove by pre-filters or escape from it then the fine particles remove by the HEPA filters in AHUs[air handling units].
Specifications of HEPA Filter
HEPA filters are defined by the US, DOE[United States Department of Energy] in almost all American industry which removes at least 99.97% of particulate at 0.3 μm in diameter. At the standard volumetric air flow, the minimal resistance of the air flow or pressure drop is around 0.044 psi[300 pascals].
In EU, filters are classified based on MPPS [Most Penetrating Particle Size] as HEPA, EPA, and ULPA where the average efficiency defined as “Overall” and the specific point efficiency defined as “Local”.
Marketing View Regarding HEPA Filter
Now a days some of the company highlighted the term “True HEPA” supposed to give assurance to the end user/customer to confirm that their filter is the original HEPA filter. This term “True HEPA” has no significant scientific value or no legal meaning. Some of the company marketed as designed as”99% HEPA”,”HEPA-style””HEPA-like,” “HEPA-type,” etc. which didn’t comply the HEPA standard[99.97% efficiency] and this type of material may not have the original test certificate of may not undergo any type test like filter integrity test.
Safety consideration of HEPA Filter
Mid-size particles ranges between 0.15 µm and 0.2 µm is considered hardest to filter through HEPA. The mechanism of working of HEPA filter is totally different from Ionic filters and Ozone filters which work through negative ions and ozone gas respectively. So HEPA plays a great role in pulmonary side-effects like asthma and allergies which are much lower with HEPA filters.
Where the HEPA filters subject to use in commercial grade, it is better practice to change the HEPA filter in every six month to hold its efficiency at desired level of work but if it use in residential purpose then it may be change in every two to three years. In the period of time, the efficiency of HEPA filters decrease day by day and failing to change the same in due to create hazard to the machine or system and particulate contamination spread day by day.
Applications of HEPA filters
Biomedical
HEPA filters generally remove the air borne particle, bacteria, virus and various type of organism resulting infections. High energy ultraviolet light units or panels with anti-microbial coating integrated HEPA filters are specially use to serve medical purpose to kill the live bacteria and virus which trapped by the filter media. Airborne disease transmission successfully protect by the special design HEPA filter which efficiency declared as 99.995%.
Heating, ventilation, and air conditioning[HVAC]
HVAC [Heating, ventilation, and air] technology uses HEPA filters to remove airborne particulate, dust, microorganism etc. in indoors and vehicle, greatly use in sensitive pharmaceutical product manufacturing where subject to clean room class. To ensure better environment and better health, HVAC play a major role various organization who are concern this activities.
COVID-19
HEPA filters plays a great role to get entrapped SARS‑CoV‑2 Airborne droplets which size approximately 0.125 µm even if they are present on the floor. Various type of HEPA incorporated mask are available at the current market which is the best solution to prevent the pandemic situation for personal use.
Motor vehicles
HEPA filters is using in the latest car, “Tesla Mode X” using HEPA filters since 2016 and now their update “Model S” also using the additional HEPA filter.
Vacuum cleaners
HEPA filters are also use in many well designed vacuum cleaners as a part of their filtration systems. This option of the vacuum cleaners plays beneficial role for the patients having asthma and allergy problem due to HEPA filters traps fine particles reduce to size 0.1 µm which is mainly responsible to induce asthma and allergy. Before using the Vacuum cleaner integrated with HEPA filters, check the filter efficiency [at least 99.97%] unless authorized. HEPA like, True HEPA, HEPA are not the actual term to declared standard HEPA efficiency.
A powerful motor is essential to provide adequate cleaning power due to the high density of the HEPA filters. A vacuum cleaners containing washable HEPA filters subject to more expensive. A standard high quality HEPA filter generally trapped the particles having 0.3 µm in diameter where a natural human hair is 50 to 150 microns in diameter. So a HEPA filter traps the particle which are reduce to several hundred times smaller than the standard width of Human hair.
Sometimes, is remarkably noticeable that some of the manufactured declared HEPA 4/HEPA 3/HEPA 2 etc. without proper explanation behind them. This is actually declares their MERV [Minimum Efficiency Reporting Value] rating. This the ability of the air cleaner filter to remove the particle, dust, and microorganism etc. which passes with air through the filter. The MERV scale distributed between 1 and 16 which declare the efficiency of the filters remove particle between 10 to 0.3 µm in size.
Vehicles
Airlines
All types of modern airlines uses HEPA filters to reduce contamination in the air. As most of the air in air plane cabin is recirculated, so this is crucial to filter the air using high grade of HEPA filters to avoid cross contamination.
Maximum quantities of standard pressurized aircraft brought air from the outside and circulate it the cabin exhausted it through the outflow valves in the rear side of the respective aircraft. Almost 60% of air comes from outside of the plane and 40% cabin air passes through the HEPA filters in well-designed aircraft.
What are drawback of HEPA filters?
Every filtration technology has the shortcomings that no filter can pass the 100% contamination free air. The efficiency is almost 99.9997% not 100% same for HEPA filters. Besides this the HEPA filter has the following two types of drawback which can dissatisfy to you to buy a HEPA filter/HEPA filter integrated equipment’s-
Insignificant Pollutants Can Escape Filter:
HEPA filters are effectively act on the particle size at equal to or above 0.3 µm, there various type of microorganism which particles are reduce <0.1µm can easily pass the HEPA filters. So using HEPA filters you can trap/remove all type of organism, this myth is actually invalid. The smaller particulate cause the serious health hazard for the patient/personnel who are previously suffering from asthma or allergic problem. Alternative technology should be used to face this tiny size particles.
Various type of Mold & Bacteria Growth on Filter:
When bacteria and mold gather on the air system on the HEPA filter, this accumulate organism have the potential chance to grow in geometric order. As the number of the microorganism tend to grow outside the filter then the health hazard outside the controlled environment growth day by day. To solve this issue, a UV light integrated HEPA filter to be use to kill the mold and bacteria at your indoor environment.
So what is the best alternative of HEPA filters?
The best alternative of HEPA filters are ULPA [Ultra-low Penetration Air] filters. HEPA filters remove particles size equal to or above 0.3 µm having efficiency of 99.97% where ULPA filters remove particles size equal to or above 0.12 µm having efficiency of 99.999%.
Can HEPA filters be Wash or Reuse?
All type of HEPA filters can’t wash. Some of the HEPA filter can be wash and some are not. If your HEPA filter labelled with “Washable” then you can wash the HEPA filter but a non-washable HEPA filter may be wash by appropriate way.
To rinse a HEPA filter decrease its efficiency and proper study didn’t found that how much efficiency loss upon rinsing the HEPA filters. Moreover, this has been proved that upon rinsing the HEPA filter, its efficiency loss due to losing/damaging of fiber occur in this time.
During cleaning of HEPA filter, if individual face lack of awareness, fall in serious health hazard upon accidentally taking some of pollutants from HEPA filter. After cleaning, drying activity must complete for a long time. Check the same before use as the wet filter is the great source of mold.
HEPA filters can be reuse if you can clean it properly. Use a vacuum cleaner to clean the filter rather than rinse it. Even non washable filter can be clean this way, proper measures to be taken before cleaning the HEPA filter. Avoid direct touching it. If you rinse your filter, the dry the filter properly by placing it outdoor. Before placing, shake the it to remove the water properly. Set it at outdoor in such way that it get air from all direction and faster the drying process.
After washing the filter may withheld its efficiency maximum 99.7% and you can was your filter every six months or once in a year but it is the best practice to replace the same. Some of the filter labelled with “Permanent” then you can’t wash it, just replace it.
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