HEPA Filters - h3010v10226

DOE-HDBK-3010-94

5.0 Surface Contamination; HEPA Filters

5.4

H E P A F I LT E R S

5.4.1

T h erm al S tres s

The ARF from the heat-induced damage to a HEPA filter is estimated to be very small.

HEPA filters resisted temperature as high as 825 oC for period of tens of minutes before loss

of efficiency and 500 oC for in access of 45 min (Hackney, 1983). The filter medium is very

fine diameter glass fiber that softens and melts when heated and thus, tends to retain

materials adhering to the fibers. The release rate for several types of HEPA filter in flowing

air at elevated temperatures less than required to induce failure (up to 400 oC) are very low

(Ammerich et al., 1989).

HEPA filters, both unused and removed from service due to high differential pressures

(clogged), were tested using solid particles at a range of temperatures less than required for

failure. The efficiencies of the filters prior to testing for 1.8 m particles ranged from

99.97% to 99.9999989%. Two high flow (2000 cfm) and one 1000 cfm HEPA filters with

glass fiber media and various sealant and gasket materials were tested. No releases were

found at temperatures below 150 oC (175 oC for one of the high flow filters). For the

1000 cfm type filter, the release rates for temperatures from 175 oC and 190 oC started at

1E-6/min and reduced to 5E-8/min within 1 hour (the lower limit of detection was

2E-8/min). The high flow HEPAs were tested to temperatures of 200 oC and 250 oC with

release rates starting at 2E-4/min and 2E-5/min and reducing to 3E-7/min in 30 min and

2E-8/min in 60 min. The decay in release was exponential during the initial 30-minutes

approaching the 60-min rate asymptotically. There was no release of contamination from a

oven-fired, mineral sealant, high flow type filter at temperatures up to 350 oC and the release

in other types of HEPA filters is associated with the emission of smoke (binder, degradation

of inert dust on filter, pyrolysis of gaskets). Thus, it appears that the heat-induced release

from 1000 cfm HEPA filter prior to failure may be as high as 1E-5. It is assumed that

HEPA filters destroyed by flame intrusion or by the impact of air at a temperature

sufficiently high to melt the glass fiber are subjected to high temperature air to result in the

release given above for heat-induced release. The RF is assumed to be 1.0 without an

experimental basis. ARFs for high-flowrate HEPA filter may be an order of magnitude

higher (1E-4). On these bases, bounding ARF and RF values for the impact of heat upon

loaded HEPA filters are assessed to be 1E-4 and 1.0.

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