Sterlco GP Series 20-60 HP Packaged Chillers - Manual (Page 13)

GP Series Portable Chillers Chapter 2: Functional Description 13 of 93
differential” value the controller will enable the compressor; if equipped with two
compressors the controller will enable the one with the least amount of hours. The second
compressor will be enabled if the leaving fluid temperature remains above the setpoint plus
the “compressor on differential” value for more than 60 seconds.
The hot gas valve is designed to trim the load of one compressor and will modulate in order
to meet the desired leaving fluid setpoint via a PID algorithm. After the compressor starts,
the hot gas valve is allowed to modulate. The compressor will shut off if the leaving fluid
temperature drops below the “compressor off differential” and the hot gas valve is at 100%
for the “compressor off delay” time. The hot gas will reset to 0% when the compressor is off.
There is a “compressor anticyle” timer that will delay the time between compressor starts to
prevent short cycling of the compressor. This is a start to start timer set at 5 minutes. For
example, if the compressor has been running for 5 minutes and shuts off, then the compressor
can start immediately if the demand is there. If it has only been running for 2 minutes, then it
would not be able to start again for 3 minutes.
For a unit with two compressors, the second compressor or lag compressor starts when the
lead compressor is on, the hot gas is at 0%, and the leaving fluid temperature is above the
setpoint plus the “compressor on differential”. There is also an adjustable “lag compressor on
delay” timer. Before starting either compressor, the compressor with the least amount of
runtime hours is considered the lead compressor and the first to start. When both
compressors are running, the compressor with the most hours is considered the lead
compressor and is the first to stop. The lead compressor will shut off if the leaving fluid
temperature drops below the “compressor off differential” and the hot gas valve is at 100%
for the “lead compressor off delay” time. The compressor anti-cycle timers are active with
the two compressor units.
There will be two PID controls running simultaneously for determining the position of the hot
gas valve. One PID output will be based on leaving fluid temperature and the other based on
a minimum saturated suction temperature for freeze protection. The hot gas valve position
will be determined by which output is greater. If it is controlling to the saturated suction
temperature to prevent freezing, a warning will display on the screen that the hot gas is in this
mode. Once the control goes back to setpoint control, the warning will disappear. The
saturated suction temperature is calculated from the suction pressure and the type of
refrigerant.
The discharge pressure is controlled using an analog output signal to drive a fan for air-
cooled units or a water regulating valve for water-cooled units. The output signal is
determined from a PID algorithm using a discharge pressure transducer as the process
variable. The VFD or water regulating valve is controlled from the analog output to an
adjustable discharge pressure setpoint. The analog output starts at the initial “discharge
pressure start” % for a given time delay (discharge pressure hold). After the time delay, it
will control to the discharge pressure setpoint via the PID control. There is also a setting to
control the discharge pressure to the most efficient value.
2-3 Standard Features
Mechanical Features
Compressor – Hermetic scroll compressors.
Evaporator – Stainless steel copper brazed plate evaporators.