KKT chillers vBoxX 10, Instruction Manual

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8.7.4. Compressor control system
The control system is designed for a refrigerating circuit with a speed-controlled compressor.
The compressor is requested depending on the controller output signal and is controlled according
to the load demand. The compressor switches off if refrigerating capacity is no longer requested.
The high pressure is monitored on the hardware side by means of the high-pressure limiter. When
it is triggered the compressor switches off immediately. In addition the high pressure is controlled
by means of a pressure transmitter. This also switches off the compressor if the setpoint is
exceeded, but it re-releases it when the pressure falls below the release value. Before switching
off, the high-pressure transmitter signals a warning message.
The low pressure is also monitored by pressure transmitters. If the value falls below the setpoint
the compressor switches off. If the switching off value is exceeded by the hysteresis the compressor
is released again. The triggering of the alarm is delayed by two timing elements. The first timing
element is started with the compressor request. If no suction pressure has built up after the time
has expired the low-pressure fault alarm is triggered. After the start time has expired the low
pressure may fall below the switching off value for a short time. If this happens more often than
three times within an hour, the compressor is locked. Before triggering a low-pressure fault a low-
pressure warning is signalled.
8.7.5. Fan speed control
The continuous adjustment of the fan speed keeps the condensation pressure constant, by means
of a PI controller, depending on the ambient temperature. As the condensing pressure rises very
quickly at the moment at which the compressor switches on, the PI controller is superimposed with
a P controller with a limiting setpoint. This only intervenes if the PI controller is too slow.
8.7.6. Electronic expansion valve control
An electronic expansion valve with PI controller is used to keep the overheating constant.
8.7.7. Temperature limit monitoring
The temperature of the cold water flow is monitored for a minimum and maximum limit value if the
chiller is switched on (pump is running). If the temperature falls below or exceeds the limit value,
after a time delay an alarm is triggered which switches off the compressor.
8.7.8. Group fault message
A group fault is triggered if an alarm occurs. All alarms are included in the group fault, however, not
the warnings. The group fault alarm relay has a floating changeover contact and is pulled up in
fault-free operation to ensure wire break monitoring.