24 Feb 99
D-5. Toxic-Free Area Overpressure.
For existing facilities being modified or new facilities being designed with a Class II CP system, the
air intakes will be protected with a CBR filtration system. The TFA will be designed for a
minimum overpressure goal of 5 Pa (0.02 inches wg). This overpressure corresponds to a wind
speed impact pressure normal to a wall of 12 km/hr (7 mph). This wind speed condition is most
favorable for directing a plume of agent with minimum dispersion toward an outside air intake.
After installation of the overpressure system, it is possible that a TFA pressure may be higher than
the 5 Pa (0.02 inch wg). A higher pressure provides a higher factor of safety for the CP system
and should not be intentionally lowered to maintain a 5 Pa (0.02 inch wg) overpressure.
a. Existing Facilities. For existing facilities, the ventilation design will be analyzed to
determine if an overpressure can be achieved by supplying additional air through the existing
ventilation system, restricting exhaust airflow rates such as from an economizer air exhaust or from
other building exhaust systems. The IAQ will still be met since the air is simply exhausted through
the TFA envelope. The IAQ and exhaust airflow rates required by building ventilation codes will
be maintained. If an overpressure can be achieved with the existing ventilation system, an air
leakage measurement test using a blower door assembly will be performed in accordance with
ASTM E779. If an overpressure cannot be achieved in existing facilities, good protection is still
provided by protecting the outside air intakes.
b. New Facilities. For new facilities, overpressure can be achieved by supplying a higher
rate of conditioned fresh air to the TFA than is exhausted. To obtain a TFA minimum over-
pressure of 5 Pa (0.02 inch wg), the additional air required can be approximated by the unit
leakage values presented in Appendix G. In addition, the air leakage calculation procedures in
ASHRAE Handbook of Fundamentals can be used to determine the additional air intake
D-6. Toxic-Free Area Envelope Air Leakage Rate.
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 tabulation, 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.