ETL 1110-3-490
13 May 98
B-9. CP Overpressure System Design.
The airflow capacity of the CP overpressure filtration system is the sum of the TFA envelope air
leakage rate at the design pressure differential and the airlock airflow necessary to achieve the
required purge rate. If the CP filtration system is located in a contaminated environment; i.e.,
outside the TFA envelope, the CP filtration system will be designed as a blow-through system with
the blower located before the CP filtration system. If the CP filtration system is located in a clean
environment; i.e., inside the TFA envelope, and draws in the contaminated air through a ductwork
system, it will be designed as a draw-through system with the blower located after the CP filtration
system. 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 require-
ment of the TFA.
a. Filtration Systems. The filtration system is the most critical part of the CP overpressure
system. A number of filtration systems are available from both the military and commercial
suppliers. If commercial filter systems are used, the mechanical system designer should have the
technical expertise to prepare specifications that meet military filter system requirements. For
continuously operated filter systems, accessory equipment such as moisture eliminators and sand
filters will be considered for protection of the filter system as required by site conditions and design
requirements.
(1) Military Filtration Systems. Military filtration units are typically provided as Government
furnished equipment (GFE). Military equipment provided as GFE has the advantage of being pre-
approved for use on Government installations while commercially available equipment requires
additional Government quality testing.
(a) Fan Filter Assembly (FFA)-580. The FFA-580 filtration unit provides chemical filtration
with a designed capacity of 1,020 m3/hr (600 cfm) at 3,750 pascals (15.0 inches wg) using a 2.24
kW (3 HP) motor. The FFA-580 contains a high-efficiency particulate air (HEPA) filter and a
chemical filter designed for an adsorption residence time of 0.25 seconds. The FFA-580 is
designed for standby operation and is not intended for continuous duty.
1. Air Tempering. The FFA-580 unit does not provide air tempering of the filtered air. For
Class I and II facilities, the filtered air can be either ducted directly into the facility air handling unit
(AHU) return ductwork or discharged directly into the TFA. The existing mechanical system
provides tempering of the air and must be located within the TFA envelope. If the FFA-580 unit is
used to pressurize Class III facilities, the filtered outside air will be discharged directly into the
TFA and the TFA temperature will approach outside ambient conditions.
2. Filtered Airflow Rate. The FFA-580 is provided with an adjustable iris valve (variable
diameter orifice) at the blower inlet. This iris valve is used to maintain the airflow rate for differing
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