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Different operating states reduce the number of cooling levels required by one cooling level:
High pressure warning
Low pressure warning
Difference between high and low pressure is too large.
8.7.5.
Compressor control
The control is defined for a refrigeration circuit with a maximum of four compressors. The number of
compressors is selected by means of coding resistance. The number of compressors is determined once
when the control is restarted (switching on of the operating voltage) and is compared with the number of the
last restart. If these are not the same the "Coding resistance changed" alarm is generated. When the alarm
is acknowledged the new number of compressors is stored in the E-EPROM and the factory setting is accepted
as the new number of compressors.
The compressors are requested depending on the controller output signal. The compressor with the least
number of operating hours switches on first. If no more refrigerating capacity is reqstuired the compressor
that switched on first also switches off first. The compressors are switched on with a time delay (comment:
only applies if several compressors exist)
In the event of a fault in a compressor (motor protection switch) a fault changeover is activated. The high
pressure is monitored on the hardware side by means of the high-pressure limiter. If it is triggered all
compressors switch off immediately. In addition, the high pressure is controlled by means of a pressure
transmitter. This also switches off the compressors if the setpoint is exceeded, but releases them again if the
value falls below the release value. Before switching off the, high-pressure transmitter signals a warning.
Low pressure is also monitored by pressure transmitters. If the value falls below the setpoint the compressors
switch off. If the switch-off value is exceeded by the hysteresis, the compressors are released again. The
triggering of the alarm is delayed by two timing elements. The first timing element is started with the request
for the compressor. If no intake pressure has built up by the time the time expires the low-pressure fault
alarm is triggered. After the start time has expired the low pressure may exceed the switch-off value for a
short time. If this occurs more than three times within an hour the compressors are blocked. Before a low-
pressure fault is triggered a low-pressure warning is signalled.
8.7.6.
Fan speed control
Due to the continuous adjustment in the number of fans, the condensing pressure is kept constant depending
on the ambient temperature by means of a PI controller. As the condensing pressure at the moment in which
the compressor is switched on rises very fast, the PI controller is superimposed by a P controller with limiting
setpoint. This only intervenes if the PI controller is too slow.
8.7.7.
Electronic expansion valve control
An electronic expansion valve with PI controller is used to keep the overheating constant. To prevent the
maximum operating pressure (MOP) from being exceeded, a P controller is used, which counteracts when
the MOP is approached and limits opening of the electronic expansion valve.
8.7.8.
Temperature limit monitoring
The temperature of the cold water feed is monitored for a minimum and maximum limit value if the chiller is
switched on (pump is running). If the limit value is exceeded or the minimum is not reached, an alarm is
triggered with time delay, which switches off the chiller.
