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 MIL-HDBK-1012/1
4.2.4.2 UPS Equipment Rooms. Figure 2 shows several approaches for the
environmental control of these areas. These methods are also applicable for
transmitter buildings, except that transmitter buildings do not require backup
heating and ventilation.
a) Fans. Provide two fans capable of handling the amount of air
exhausted through the equipment cabinet plus about ten percent for good sweep
at the hood. Fans shall operate with one on the utility circuit and the other
on the UPS circuit. The system shall be designed to ensure continuous
operation of either fan. Note that the UPS must be sized to carry this
additional fan load in addition to its technical load.
For the parallel arrangement, the primary fan shall be connected to
the UPS circuit and the standby fan to the utility circuit. Normal operation
shall be with the primary fan only operating via manual start, and automatic
start of the backup fan in the event of failure of the primary fan. Both fans
shall be equipped with sail switches to operate the failure alarm.
For the series arrangement, both fans shall operate during normal
operation via manual start, and failure of either must be sensed by a
differential static pressure switch across each fan to operate the failure
alarm.
Both fans in the series configuration shall be of the non-
overloading type, such as those having backward inclined air-foil blades and a
fairly shallow static pressure versus CFM performance curve, where CFM is
plotted as the ordinate, so that a loss of one fan will not drastically reduce
the cooling air flow. The parallel arrangement requires more space than the
series arrangement. Ducting for the parallel arrangement must be split so it
is more complicated and low-leakage backdraft dampers must be provided to
prevent short-circuiting. Therefore, the series arrangement is the preferred
arrangement. The supply fan plus return fan arrangement is a series
arrangement in the sense that failure of either fan would still provide
ventilation.
b) Hoods. Hoods shall be provided over the UPS exhaust ports to
prevent waste heat from spilling into the equipment room. This will limit
room temperature to the supply air (outside air) temperature except for
insignificant gains in the summer. In the winter air temperature will be
limited to a value determined by the room thermostat setting. Hoods should be
approximately 6 inches (152.4 mm) larger all around than the ventilating
discharge and should extend to within 18 +/- 6 inches (457.2 +/- 152.4 mm) of the
discharge port. Refer to the ASHRAE Handbook Series for a detailed discussion
of exhaust hood design. If space for hoods is not available, an alternate
means should be designed to minimize spillover of exhaust air into room air.
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