13 May 98
illuminate when the airflow rate drops below design values. The static air pressure drop from the
TFA to the airlock and from the airlock to the exterior will not be more than 25 pascals (0.1 inch
wg) at each location, giving a total airlock static pressure drop of 50 pascals (0.2 inch wg).
B-7. TFA Overpressure.
The minimum TFA overpressure will be 75 pascals (0.3 inch wg). This corresponds to a wind
speed impact pressure normal to a wall of 40 km/hr (25 mph). At wind speeds of 24.1 km/hr (15
mph) and higher, airborne contaminants are diluted to a level of greatly reduced hazard. After
installation of the overpressure system, it is possible that a TFA pressure higher than the 75 pascals
(0.3 inch wg) will result. The resulting higher pressure will provide a higher factor of safety for
the CP system and should not be intentionally lowered to maintain the 75 pascals (0.3 inch wg)
B-8. TFA Envelope Air Leakage Rate and Sealing Measures.
a. Existing Facilities. For existing facilities, a pressurization test using a blower door assembly
will be performed in accordance with ASTM E779. Test data will be plotted on a log-log graph
for ease of data extrapolation and review. Air leakage locations can be identified during pressur-
ization testing when the blower door assembly is operated in the negative pressure mode and
draws outside air into the proposed TFA. These leakage locations can be identified by physical
inspection or with smoke testing. Leakage areas will be sealed with a good quality sealant or, if
necessary, reconstructed. Weatherization-type sealing measures can be expected to achieve
leakage reductions in the range from 5 to 50 percent depending on the type and quality of facility
construction. Sealing of the TFA envelope will reduce the air leakage rate and thus reduce the
required amount of filtered air. Sealing measures must be economical when compared to the cost
of the filtration and HVAC equipment and, for continuously operated CP facilities, energy usage
needs must also be considered. After sealing, a second blower test will be conducted to determine
the final TFA envelope air leakage rate.
b. New Facilities. For new facilities, the TFA envelope leakage rate will be calculated using
the effective leakage area procedures in the ASHRAE Handbook of Fundamentals. Leakage
calculations will be performed for the TFA envelope including the walls, roofs, floors, doors,
windows, sole plates, mechanical and electrical penetrations, ceiling-wall joints, isolation dampers,
etc. The overpressure of the TFA will be used as the differential pressure in determining the TFA
envelope leakage rate. Care should be taken during design and construction to ensure that proper
sealing of penetrations is performed and that continuous air leakage control barriers are used in the
TFA envelope. A blower door test of the TFA envelope should be performed to verify the leakage
rate and ensure that the CP overpressure filtration system has sufficient capacity.