Hepa this Hepa that, is there such thing as a true Hepa vacuum?


Courtesy of Hypervac Technologies

We all see it, the big Hepa sticker on the side so we automatically believe the vacuum must be a Hepa rated vacuum, right? - Sorry the chances are the Vacuum would never pass a Certified D.O.P test.

Just because a vacuum has a Hepa filter doesn't make it a Hepa vacuum, many factors have to be taken into account such as:
The Hepa Filter needs to be sized according to the airflow that the impeller will generate. Quite often in the quest to produce a vacuum with more suction manufacturers will install larger impellers to create the extra draw with out taking into account the Hepa filter may only be able to handle at most 2000 -2500 Cfms , the filter may allow some of the extra air to go through, for a while until a bit of dust gets into the pleats and once any type of dust load gets into the filter it causes the Hepa Paper to breakdown and tear from the increased static pressure being created. Many times the Hepa filter can be destroyed from the very first time the vacuum is turned on and you could be contaminating the airspace and probably would never know as the tears can be microscopic in size.

The filter needs to be designed for its intended purpose. The large square Hepa filters you see being used in many duct vacuums were never actually intended to be used for that purpose. Hepa Paper is incredibly fragile, (think Rice paper) it was never intended to be put under any type of pressure, If you have ever been in a Hepa Bank in a large HVAC unit you would see a massive wall of filters in a large room far bigger then you would expect, this is all so the Hepa paper never undergoes any type of pressure or stress from direct air pressure, especially from directly underneath. Using an of the shelf HVAC hepa filter and placing it into a Duct Vacuum does not make it a HEPA vacuum. These types of HEPA filters should really only be used in Negative air machines where the static pressures are quite low.

Gaskets and air seal with HVAC Hepa filters.
Hepa filters when being used in the proper way are fitted with special gaskets to ensure they are sealed properly to the mounting frame, once they are mounted and securely in place you would never think of moving the Hepa filter again until the next replacement , now think about putting one in a cabinet and bounce the unit up and down a bunch of stairs all day.
The square Hepa filters you see in many of todays duct vacuums are just not tough enough for repeated use, if the vacuum pressure doesn’t destroy the filter on start up then just moving the unit around to much certainly will finish it off. The vacuum may pass a DOP test from brand new , but will it pass it over and over again., not likely.

DOP Testing:
HEPA filters are normally used only where an extremely high level of cleanliness or purity is required. The requirement may be due to problems caused by the presence of particulates or physiological problems caused by viable airborne organisms. In any event, the efficiency of every filter is of paramount importance and must be measured in an appropriate way.

Mil-Std-282 is recognized as the standard for 'hot' DOP efficiency testing and is used for compliance with many HEPA filter specifications. It is also recognized as being 'monodisperse 0.3 micron particles' as referenced in EPA and OSHA definitions for HEPA filters.

DOP (dioctylpthalate) is an oil commonly used with vinyl resins to make soft vinyl plastics. It is also used by air filter manufacturers and various testing agencies to make an aerosol to test the effectiveness of air filters. Other oil-like materials, like DOS, can be substituted with similar results.

The DOP aerosol used to challenge HEPA filters to test for efficiency by this standard is known as 'hot' or 'thermally generated' DOP because it is derived from heated dioctylpthalate oil. Sophisticated equipment is used for carefully controlling oil and air temperatures, air flow rates and mixing conditions. This 'hot' DOP aerosol has a very narrow particle size distribution (monodisperse). Because the only way to determine the efficiency of a filter on a specific particle size (fractional efficiency) is to test with particles of that size, DOP is used to produce a high concentration of 0.3 micron particles - that which theory indicates and has historically been considered to be the most penetrating of filter media.

For each test, the average aerosol concentration is measured both upstream and downstream of the filter with a photometer. The inefficiency or penetration in percent can therefore be determined and recorded on the filter label. For example, a filter with a penetration of .008% would mean it was 99.992% efficient, well above the minimum of 99.97% for HEPA efficiency.

DOP aerosol can be generated in the field but the equipment used, while relatively simple and portable, cannot produce truly 'hot' DOP that is monodisperse. The DOP generated by such equipment is 'cold' DOP which having a broad particle size distribution is polydisperse. Such an aerosol is useful for field testing for leaks and ensuring the integrity of an installation, however, without the ability to particle count the 0.3 micron size particles, 'cold' DOP does not provide the ultimate test of filter efficiency.

The penetration or efficiency of a filter is strongly affected by the particle size of the challenge aerosol. A small change in particle size can have a significant effect on penetration. The smaller the particle, the lower the efficiency until the maximum penetrating particle size is reached.

As indicated earlier, 'cold' DOP has a broad particle size with larger average size than 'hot' DOP. Efficiencies are, therefore, higher with 'cold' DOP than with 'hot' DOP. The control of temperatures and flow rates with the equipment is critical to maintaining a consistently tight particle distribution which allows for consistent and reproducible efficiency measurements.

Where 'cold' DOP can be useful in determining HEPA filter efficiency is when testing in accordance with IEST-RP-CC007.1. For each test, particle counters are calibrated to simulaneously count the number of 0.3 micron particles both upstream and downstream of the filter. Providing the 'cold' DOP challange aerosol contains a statistically significant number of 0.3 particles, the inefficiency or penetration in percent can determined. In this test, the polydisperse nature of 'cold' DOP is irrelavent because the other particle sizes are not measured.

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