MIL-HDBK-1003/3
values of supply fan flow. If an
economizer outside air cycle is
provided, the outside air, return
air, and exhaust air dampers
are automatically controlled. An
outdoor air dry bulb sensor
disables the economizer cycle and
maintains minimum ventilation
air when the outside air dry bulb
temperature exceeds the
changeover point. Fans of series
type VAV terminal units shall
run continuously.
d) Unoccupied Mode: Cycle the VAV terminal units at
maximum heating capacity to maintain minimum heating temperature
in the space while providing no ventilation. Outside air and
exhaust air dampers are closed and return air dampers are opened.
e) Maintenance and Safety Mode: A differential
pressure sensor measuring static pressure drop across the filter
shall initiate an alarm when the pressure drop exceeds the
recommended value.
A freezestat with extended surface sensing element
located ahead of the chilled water coil shall stop the supply
fan, return system components to their fail-safe position, and
initiate a low-temperature alarm if the temperature at any point
of the sensing element falls below the setpoint. Normal
operation will occur only after the freezing condition is
corrected and the freezestat is manually reset.
f) Supply Fan Discharge Air Mode: A dry bulb
temperature sensor located in the supply air fan discharge air
shall, on a rise in temperature, first gradually close the
preheat coil valve, next gradually open the outside air damper
from its minimum ventilation air position while simultaneously
gradually close the return air damper (ensure thorough mixing)
and gradually open the exhaust air damper, and then gradually
open the chilled water coil valve. A dry bulb temperature low
limit sensor located in the mixed air (ensure thorough mixing)
shall, on a fall in temperature, gradually open the preheat coil
valve. An outdoor air dry bulb sensor disables the economizer
cycle and maintains minimum ventilation air when the outside air
dry bulb temperature exceeds the changeover point.
g) Supply Duct Static Pressure Mode: A supply duct
static pressure sensor located two-thirds the distance from the
fan of the length of the longest supply duct run shall, on a rise
in static pressure, gradually reduce the speed of the electric
motor variable speed drive and close the fan vortex damper. A
high-limit duct static pressure sensor located at the fan
discharge shall override the supply duct static pressure sensor
to limit pressures to design values.
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