48 TGZ 040A through TGZ 190A OMM TGZ-1
If Unit Mode = Cool AND
Number of compressors running > Demand limit AND
Stage down timer expired THEN
Stage Down Now = True
If Unit Mode = Heat AND
LWT error < -0.5 * Control band AND
More than one compressor running AND
Stage down timer expired THEN
Stage Down Now = True
If Unit Mode = Heat AND
LWT error < (-0.5 * Control band - stop delta) AND
One compressor running AND
Stage down timer expired THEN
Stage Down Now = True
Compressor Sequencing
Compressor staging is based primarily on compressor run hours and starts. Compressors
that have less starts will normally start before those with more starts. Compressors that
have more run hours will normally shut off before those with less run hours. In the event of
a tie on number of starts, the lower numbered compressor starts first. In the event of a tie
on run hours, the lower numbered compressor shuts off first. Run hours are compared in
terms of tens of hours.
If possible, only one compressor per circuit will start before starting the second compressor
on any circuit. If a circuit is unavailable for any reason, the other circuit shall be allowed to
stage the second compressor on. When staging down, one compressor on each circuit shall
be left on until each circuit has only one compressor running.
Circuit Capacity Overrides
The following conditions shall override the automatic capacity control when the chiller is in
cool mode only. These overrides keep a circuit from entering a condition in which it is not
designed to run.
Low Evaporator Pressure
If a compressor in a circuit is running and the evaporator pressure drops below the Low
Evaporator Pressure Hold set point, no more compressors will be allowed to start on that
circuit. The limit shall be active until the evaporator pressure reaches the hold Low
Evaporator Hold set point plus 5.0 psi. A Low Evaporator Pressure Hold event will be
recorded in the Event Log.
If two or more compressors are running in a circuit and the evaporator pressure drops below
the Low Evaporator Pressure Unload set point, the circuit will begin reducing capacity. If
two compressors are running, one of the compressors will be stopped. If three compressors
are running, then one compressor will stop immediately. Ten seconds later, if the pressure
has not risen above the unload set point; an additional compressor will be stopped. The last
compressor on a circuit will not stop due to the unload condition. The low evaporator
pressure unload event will clear when the evaporator pressure rises 5.0 psi above the Low
Evaporator Pressure Hold set point. A Low Evaporator Pressure Unload event will be
recorded in the Event Log.
If the evaporator pressure drops below the Low Evaporator Pressure Unload setpoint and
one compressor on the circuit is running then the following table applies.
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