Functional Control ESIE10-01
2–44 Part 2 – Functional Description
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2.3.3 Compressor Control
The compressor should run only when the circuit is in a run or pumpdown state. This means the
compressor should not be running any time the circuit is off or preopening the EXV.
Cycle timers A minimum time between starts of the compressor and a minimum time between shutdown and start
of the compressor shall be enforced. The time values are set by global circuit set points.
These cycle timers shall be enforced even through cycling of power to the chiller.
Compressor run
timer
When a compressor starts, a timer will start and run as long as the compressor runs. This timer is used
in the alarm log.
Compressor
capacity control
After starting, the compressor should be unloaded to the minimum physical capacity and no attempt
to increase compressor capacity should be made until the differential between evaporator pressure
and oil pressure meets a minimum value.
After the minimum differential pressure is met, compressor capacity shall be controlled to 25%.
■ Auto Capacity Control
Changes to capacity shall be performed as needed to meet unit capacity requirements (see unit
capacity control section).
Capacity shall not be increased above 25% until discharge superheat has been at least 22°F for a
time of at least 30 seconds.
■ Manual capacity control
The capacity of the compressor may be controlled manually. Manual capacity control is to be ena-
bled via a set point with choices of auto or manual. Another set point should allow setting the
compressor capacity from 25% to 100%.
Capacity control shall revert back to automatic control if either:
■ The circuit shuts down for any reason.
■ Capacity control has been set to manual for four hours.
■ Slide Control Solenoids
The required capacity should be achieved by controlling one modulating slide and one non-modu-
lating slide. The modulating slide can control 10% to 50% of the total compressor capacity, infi-
nitely variable. The non-modulating slide can control either 0% or 50% of the total compressor
capacity.
Either the load or the unload solenoid for the non-modulating slide should be on any time the
compressor is running. For compressor capacity from 10% up to 50%, the non-modulating slide
unload solenoid should be on to keep that slide in the unloaded position. For capacity from 60% to
100%, the non-modulating slide load solenoid should be on to keep that slide in the loaded posi-
tion.
The modulating slide should be moved via pulsing of the load and unload solenoids to achieve the
required capacity.
An additional solenoid should be controlled to assist in moving the modulating slide in certain con-
ditions. This solenoid should be activated when the pressure ratio (condenser pressure divided by
evaporator pressure) is less than or equal to 1.2 for at least 5 seconds. It should be deactivated
when pressure ratio is more than 1.2.
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