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 MIL-HDBK-1012/1
density, yielding ratios of 50 to 75 square feet per ton of cooling. Computer
areas are therefore susceptible to drafts if care is not taken in the layout
of supply registers and diffusers. Hot spots can also occur because of the
concentrated loads.
8.11.2.3 Air Conditioning Systems. Systems serving data processing centers
should be independent of other systems in the building if the building central
system is secured during the winter season. Chilled water systems may be
cross-connected to provide backup. The air conditioning systems available for
use are listed in Section 4. They may be classified as self-contained units
(using air or water for condensing), decentralized air-handling equipment, and
central systems.
a) Self-contained Units. Units are completely packaged, with
reciprocating compressors and refrigerant coil, filters, humidifiers,
provisions for a reheat coil, operating controls, and instrumentation such as
signal lights, dirty filter alarms, and other status/alarm lights.
Construction of major items should be corrosion-resistant. Units can be
arranged to supply air down into a floor plenum or up to overhead ducts and/or
a plenum with diffusers. The units are located in the spaces served, and are
provided with water-cooled condensers connected to a remote cooling tower,
water-to-air heat exchanger, or other heat rejection device. Multiple units
are usually provided, and the outdoor ventilation air is normally introduced
through one of them.
b) Decentralized Air-handling Units. These are similar to self-
contained units, but are served by remotely located refrigeration
equipment, usually through chilled water coil connections. This arrangement
is appropriate in facilities where both air-cooled computing equipment and
water-cooled equipment (such as computers associated with simulators) is
installed. Chilled water normally should be in the range of 47 to 50 degrees
F (8.3 to 9.4 degrees C) to provide the high sensible cooling ration required
in computer areas. This type of system requires central chiller equipment
capable of 24- hour-per-day, year-round operation.
c) Central Systems. These have an advantage in that no computer
room floor space is taken up by the air-handling units, and all servicing and
maintenance takes place outside the computer rooms. Central systems must be
designed, however, to accommodate future expansion of the computer space or
changes in loads, and are not as widely used as the other systems.
8.11.2.4 Supply Air Distribution.
As indicated in Section 4, air
distribution is important in all electronic equipment areas. Data processing
centers impose even more stringent requirements to accommodate load changes
and future expansion, with minimal changes to the mechanical equipment.
Under-floor or ceiling plenum supp ly is commonly used to provide the required
flexibility.
a) Under-floor Systems. Where raised floors are provided, it is
often advantageous to use them as a supply plenum for conditioned air. If air
must be introduced directly to the machine, more stringent restrictions are
imposed on allowable temperature and relative humidity than if it were
introduced to the room; this approach should therefore be used only if
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