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 MIL-HDBK-1012/1
BESEP. If the manufacturer's recommended conditions differ from those
specified by this manual or the BESEP, the matter should be referred to SPAWAR
and NAVFAC for resolution.
a) Specialized equipment. HVAC units specifically designed for
computer rooms and similar cooling applications are sometimes referred to as
process coolers. They generally differ from units designed for comfort
cooling, in that the cooling coils are designed for a much higher sensible-to-
total cooling ratio; discharge air relative humidity may be limited to about
80 percent, as opposed to nearly 100 percent in comfort cooling applications;
and the cubic feet per minute (CFM)-per-ton value may be as high as 600 to
800, as opposed to 400 for comfort cooling. These units are designed for
year-round use rather than the 1000 to 3000 hours of operation of comfort air
conditioners. Both in-room equipment and central systems must be tailored to
the needs of the spaces served. This includes providing the proper degree of
air filtration, adequate humidification capability, the ability to provide
cooling in cold weather, and the proper type of cooling coils.
b) System types. HVAC equipment for electronic equipment/computer
room areas may be of the following types:
(1) Direct expansion, glycol- orwater-cooled;
(2) Direct expansion, air-cooled;
(3) Central chilled water system with in-room air units;
(4) Central chilled water system with central air handlers;
(5) Process chillers (for direct water-cooled computer
equipment).
Advantages and disadvantages are discussed in the ASHRAE Handbook Series and
in Section 8 of this manual. In general, systems with glycol are quite
popular due to the simplicity of installation and ease in maintaining winter
operation. However, it should be noted that these type systems are the least
efficient of all systems when evaluated on a BTU/watt power use basis.
c) Provisions for standby. The preferred approach is to divide
the total required capacity into increments of reasonable size and then
provide one or more extra increments/units for backup. With central systems,
consider configuring piping and ductwork to allow systems serving noncritical
areas to serve critical areas in an emergency. On the water side, multiple
chillers, pumps and cooling tower cells should be provided where applicable.
Consult the BESEP for guidance in regard to areas requiring backup HVAC
equipment.
d) Auxiliary equipment. Areas requiring close temperature and
humidity control year-round shall be provided with 7-day temperature and
relative humidity recorders. Alarms should indicate out-of-tolerance room
temperature and relative humidity and appropriate HVAC equipment off-line or
off-normal conditions.
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