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 MIL-HDBK-1012/1
c) Ductwork and controls. Exhaust ducts should be routed to a
location that will minimize mixing of exhaust and makeup air. Modulating
dampers should be of the low-leakage type, and controls should be arranged to,
maintain the specified temperature by modulating exhaust, return, and intake
air dampers in response to the room thermostat. Dampers should fail to the
maximum outside air (safe) position. Ducting should be arranged so that
return (tempering) air is thoroughly mixed with the supply air to give a
uniform room temperature devoid of raw air drafts. This is even more
important in colder climates. A firestat shall stop the fans on a high-
temperature alarm signal; note that the firestat must sense room temperature
or, if of the duct-mounted type, must be set at an appropriate value over the
provided in accordance with NFPA 90A, Standard for the Installation of Air
Conditioning and Ventilation Systems.
d) Other arrangements. When variations in the illustrated schemes
are necessary, the design should provide the same degree of reliability and
performance. Automatic duty switching of normal and standby fans in the
parallel arrangement is not recommended on the basis of limited benefit versus
additional complexity that reduces reliability. One such variation is where
the UPS cannot be ventilated due to the inaccessability to outside air,
outside air quality or outside air temperature. For these applications, a
chilled water air handler tied either to a dedicated small chiller or to the
central system can be used. However, since the chiller will stop when the
outage occurs, a thermal inertia ride-through tank must also be supplied along
with a small circulating pump to supply the air handler during the outage.
The UPS must supply power to both the air handler and the small pump motor for
the inertia tank in addition to its normal load.
Figures 3 and 4 show examples of thermal inertia designs. The
degree of reliability required and the space available will dictate whether
dualization of the air handlers and chillers in these applications is
necessary. Caution is advised on the use of other air conditioning
arrangements that do not have a thermal inertia capability to ride through a
15 minute outage since the UPS must continue to operate for 15 minutes when an
outage occurs. During this period waste heat must continue to be removed from
the equipment room or the temperature in the average sized room will rise to
the over-ambient cut out point as sensed by the UPS safety thermostat at which
point it will preserve itself by stopping and the load will be dropped. UPS
environmental control systems or components subject to failure must be
dualized in order to meet the uninterruptible requirements of the system.
Lack of ride through capability will preclude use of D-X air conditioning
systems even if dualized. Chilled water systems augmented with chilled water
storage tanks for ride through can be utilized when necessary.
e) Auxiliary heating. UPS equipment rooms require backup heat
whenever the equipment is down for maintenance or repair. Electric resistance
heaters are normally preferred, since usage is minimal. The heaters should be
sized to maintain a minimum of 60 degrees F (16 degrees C) in the space with
the UPS equipment off (winter), and 90 degrees F (32 degrees C) (summer) to
remain above the highest likely dewpoint, with the ventilation equipment de-
energized.
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