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 MIL-HDBK-1012/1
cable grid network, and if the outlets are located above the network, the air
may tend to short circuit. When the cable is under the floor, poor air
distribution may also occur due to cables or cable troughs blocking the
limited under floor space. Extending the supply or return ducts to floor
level at columns and walls should be considered in high-load or congested
areas.
4.2.3.7
Air Filtration. All outside and recirculated air shall be
filtered. Replaceable media or filters are preferred in lieu of cleanable
filters because of their lower manpower requirements. Where filtration is a
problem (as in dusty locations), prefilters of lower efficiency can be used in
conjunction with higher efficiency filters to extend their life. All filter
banks should have permanently installed indicators to give a warning when
pressure drop exceeds the maximum recommended. Roll filters should be
controlled by an adjustable timer (first choice) or a static pressure
controller (second choice); however, note that static pressure controllers are
sometimes upset by the action of dampers and other occurrences resulting in a
waste of filter media. Table 2 shows the recommended minimum filter
performance as specified in American Society of Heating, Refrigerating and Air
Conditioning Engineers (ASHRAE) Test Standard 52-76, Method of Testing Air-
Cleaning Devices used in General Ventilation for Removing Particulate Matter.
Table 2
Minimum Filter Performance
(% Efficency)
PERCENT
ELECTRONIC EQUIPMENT AREAS
60
Equipment rooms requiring near clean room conditions
45
Telephone switchgear, computer rooms and relay rooms
40
UPS rooms and transmitter spaces
30
All other, in general (see BESEP)
AREAS NOT CONTAINING ELECTRONIC EQUIPMENT
20
Air conditioned spaces
15
Spaces without air conditioning
20
Pre Filters (when used)
15
Roughing Filters (when used)
4.2.3.8
Energy Conservation. Operating economy and energy conservation are
highly desirable but these goals are secondary to the reliability of the
system for electronic facilities. Where energy monitoring and control systems
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