ETL 1110-3-498
24 Feb 99
The minimum TFA overpressure will be 75 Pa (0.3 inch wg). This corresponds to a wind speed
impact pressure normal to a wall of 40 km/hr (25 mph). After installation of the overpressure
system, it is possible that a TFA pressure higher than the 75 Pa (0.3 inch wg) will result. A higher
pressure provides a higher factor of safety for the CP system and should not be intentionally
lowered to maintain a 75 Pa (0.3 inch wg) overpressure.
C-8. Toxic-Free Area Envelope Air Leakage Rate and Sealing Measures.
a. Existing Facilities. To determine the envelope air leakage rate for existing facilities, an
air leakage measurement 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 pressurization 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 also be identified by physical inspection or with smoke testing.
Leakage areas will be sealed with a good quality sealant or, if necessary, reconstructed. Weather
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 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 air leakage rate will be calculated
using the effective leakage area procedures in the ASHRAE Handbook of Fundamentals. The
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. Appendix G will be used as a guide to confirm the TFA envelope
unit leakage rate as determined by the calculations. 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.
C-9. Collective Protection Overpressure System Design.
a. Airflow Filtration Capacity. The airflow capacity of the CP overpressure filtration system
is the sum of the following three components: TFA envelope air leakage rate at the design pressure
differential, the ventilation air intake rate that meets exhaust requirements, and the airlock airflow
necessary to achieve the required purge rate. The CP filtration system blower total static pressure
will be designed to include the filtration system with dirty filters, ductwork system pressure losses,
and the overpressure requirement of the TFA. The HVAC system must be designed, operated, and
maintained to provide uncontaminated air to the TFA. It will be located in a contamination-free
C-4
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