6
to meet capacity requirements at a pressure differential between discharge pressure and design suction pressure. If
discharge pressure is allowed to drop below a point which will not maintain this design differential, the suction will
also drop due to "starving" of the evaporator. When this happens, nuisance tripping of low-pressure control, or low
evaporator temperature, will occur.
Normal system design will allow satisfactory operation with discharge pressure down to approximately 180.3
PSI (95
°
F).
On air cooled equipment, a number of methods are used to maintain this minimum discharge pressure:
FAN CYCLING
- For applications where ambient temperatures are not extreme, the condenser fan(s) is cycled
on/off in response to condenser pressure.
FAN SPEED CONTROL
- A fan speed control device activated by a temperature or pressure sensor controls
the condenser fan motor speed in response to the saturated condensing temperature or discharge line pressure. For
outdoor temperatures down to - 20
°
F, it maintains condensing temperature at 100 ±10
°
F.
On water-cooled equipment, discharge pressures are much easier to control. The most common method would
be by the use of condenser water regulating valve directly sensing discharge pressure.
