TM-5-855-4
(3) The chemical-sorbent or dessicant-type dehumidifier is not ordinarily affected by temperature
levels and will operate effectively over a wide range of temperatures. Solid dessicant units require only
power hookups and exhaust outlets for discharging moisture vapors from the dessicant beds during the
heating or regeneration cycle. Liquid dessicant-type equipment usually requires auxiliary steam for the
treatment of the moisture-laden dessicant. Solid dessicant equipment generally returns more heat
through the dried air stream to the occupied space than the liquid-dessicant-type. These and other
equipment selection factors are covered in ASHRAE Handbook, Equipment.
4-9. Computer area cooling.
a. Criteria. Command centers, radar installations, missile launch facilities and similar areas
will generally contain computers and ancillary equipment which are sensitive to extremes of
temperature, humidity, and the presence of dust. The environmental criteria requirements for
computers and for areas housing the machines will vary widely with the computer manufacturer and the
computer cooling configuration.
b. Air-cooled computers.
(1) Air-cooled units will have either cooling air drawn from the room and circulated through the
unit by an internal fan; air forced through the unit by fans from a remote central supply system; or self-
contained computer room air-conditioner units within the computer room. The latter two systems utilize
under-floor ducts or plenums to supply the computer units with cooling air. Vertical space is at a
premium in underground facilities. This is reflected in the height of raised-floor plenums. Therefore,
particular attention must be given to the following:
- Locating the computer room units preferably in center of room.
- Keeping cables and wire bundles from blocking the airflow.
- Large room underflow ducting for uniform air distribution.
- Controlling supply air dewpoint to prevent mildew and fungus.
(2) Remote central air supply systems will multiple fans for reliability are more adaptable for
hardened facilities than unitary units. The requirements for complete redundant unitary units for
reliability and the necessity of routing condenser water lines in the computer room for unitary units will
influence the computer air-cooling system selection to favor a remote central air supply system. The
necessity of repairing the unitary equipment in the computer area is an additional detriment.
(3) The ASHRAE Handbook, Applications lists the typical design conditions for computer room
cooling systems; however, the equipment manufacturers' requirements will govern.
c. Water-cooled computer equipment.
(1) Some computer equipment on the market requires cooling water to remove a portion or all of the
heat generated within the cabinets. The cooling water system configuration within the cabinets will
vary, but in almost all cases, the cooling media circulating within the cabinets will be distilled or
demineralized water. Computers may be furnished with integral closed-loop. cooling systems made up of
water to water heat exchanger and pump, or the demineralized water may be pumped to the computer
cabinets from central demineralized water/ chilled water heat exchangers.
(2) Some computer systems are cooled by circulating demineralized water from a central system
through electronic racks at pressure below atmospheric pressure. The system is designed to ingest air
into the cooling water circuits, should a leak occur, in lieu of leaking water onto the rack electronic
components, as would be the case with a system operating at a positive pressure.
(3) Leak detection devices should be installed in the plenum beneath the computer floor to warn of
leaks in the cooling water lines. All critical functions such as water flow, pressure, and temperature
should be monitored locally and remotely for each computer cabinet.
4-10. Boilers and heat recovery.
a. The requirement for supplemental heating will be greatest when the facility is on standby. The
major requirements for heating during normal operation will be heating fresh air to interior design
conditions and for reheating.
(1) The greatest liabilities of combustion type boilers are space, combustion air, and flue gas
requirements, which eliminate them from consideration for underground service. For aboveground
facilities, fuel and ash handling requirements and the necessity to shut the boiler down and button-up on
short notice render coal-fired boilers unsuitable but oil fired package boilers are acceptable for
aboveground service.
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