US Army Corps of Engineers
Technical Letter
US Army Corps of Engineers Design of Collective Protection Shelters to Resist Chemical, Biological, and Radiological (CBR) Agents
Technical Letter Number ETL 1110-3-498, selected appendix entries
Complete text of this document (PDF)
APPENDIX C
CLASS I, FILTRATRION WITH PRESSURIZATION
C-7. Toxic Free Overpressure.
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 TFA overpressure 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.
APPENDIX D | CLASS II FILTRATRION WITH LITTLE OR NO PRESSURIZATION | D-5 | Toxic Free 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 plum of agent with minimum dispersion toward an outside air intake. After installation of the of the overpressure system, it is possible that a TFA pressure may be higher than the 5 Pa (0.02 inch wg) overpressure. 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.
APPENDIX E | FILTRATION EQUIPMENT | E-1 | Collective Protection Equipment
If the CP filtration 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 contaminated air through a ductwork 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 requirement of the TFA.
E-3 | Filtration Systems
(2) Roughing Filter. Continuously operated filter systems will have a roughing filter with an average efficiency of 25 to 30 percent when tested in accordance with ASHRAE 52.1. The roughing filter extends the life of the intermediate filter or prefilter and reduces its change frequency.
(3) Prefilter. The prefilter or intermediate filter will have an average efficiency of 80 to 85 percent when tested in accordance with ASHRAE 52.1. The prefilter extends the life of the HEPA filter and reduces its change frequency.
(4) HEPA Filter. The HEPA filter frame and filter media will meet the construction, material, testing, qualification, and documentation requirements of ASME N509, ASME N510, and UL 586 and will have a filter efficiency of 99.97 percent at a 0.3 Fm diameter partial size when tested in accordance with the MIL-STD-282 dioctyl phthalate (DOP) test method. The filter frames meet the requirement of ASME AG-1a, section FC. The HEPA filter medium will meet the requirements of MS MIL-F-51079D.
(5) Adsorption Filter. The adsorber charcoal media will be designed to adsorb aerosol with a minimum residence time of 0.25 seconds and will meet the requirements of MIL-PRF-32016(EA). Typically, for commercial filters, two stages are required to achieve the 0.25 second residence time at airflow rates of 330 L/s (700 cfm) to 590 L/s (1,250 cfm). One stage of filter adsorption can be used for airflow rates from 165 L/s (350 cfm) to 295 L/s (625 cfm). For unknown threats and adsorption of volatile agents, ASZM-TEDA carbon conforming to EA-C-1704 will be used. If the threat is known, the use of ASZM-TEDA may not be required. This will depend upon the chemical volatility of the threat. If the agents of higher volatility, such as hydrogen cyanide and cyanide chloride are not a threat, activated carbon (not impregnated carbon) will suffice. The static pressure drop through an ASZM-TEDA adsorber that meets the requirements of MIL-PRF-32016(EA) is approximately 625 Pa (2.5 inches wg) for a 325 L/s (500cfm) adsorber. A sample of the ASZM-TEDA carbon must be provided by the filter manufacturer for testing at the U.S. Army Edgewood Research, Development and Engineering Center (ERDEC) as stated in MIL-PRF-32016(EA). Funding for ERDEC testing is provided by the user. Filter trays not contaminated by chemical surety materials or by super toxic materials can be refilled by the manufacturer, but any contaminated carbon must be disposed of by the owning activity in accordance with local, state, and federal regulations. A license must be obtained from the U.S. Department of Commerce before an adsorption filter containing ASZM-TEDA carbon can be shipped outside the United States.
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