Fridgewatch 2100 Controller Mk4
Section 3
Publication 3-58
Issue 1.1 : 01/08
Page 31 of 152
5.9. Limiting
Inputs
It may sometimes be necessary to use limiting to restrict the refrigeration
capacity of a compressor or number of compressors. The facility
prevents system operation outside design parameters or chosen limits,
and allows the plant to keep running under difficult conditions rather than
tripping on a fault.
Examples of limiting conditions are listed below and illustrated as inputs
to Fridgewatch 2100 in Fig 9:
Current limiting for individual compressors
- limits the current
taken by the compressor motor. This function is normally provided
by Fridgewatch 2100 measuring the motor current direct from a
current transformer. Current limits are entered by the
commissioning engineer during configuration. Inputs from an
external current limiting device can also be used as illustrated in
Fig 9.
P4 input limiting for individual compressors
- unloads or holds
the compressor load to prevent the measured value read by the P4
analogue input from falling below the limiting values entered by the
commissioning engineer; refer to 5.10. Limiting on P4 Input.
Total current limiting value for all compressors
- limits the total
current taken by all compressors and hence places an upper limit
on the plant’s total power consumption.
Suction pressure limiting
- limits the evaporation temperature
and can be used to prevent overcooling the cooled medium.
Condenser pressure limiting
- can be used to avoid HP cut-out
trips by restricting compressor capacity when direct head pressure
control methods, such as condenser water pump control, fan
cycling, dampers etc., cannot cope.
It is worth remembering that these are not the only possibilities. The
Fridgewatch 2100 Controller can be configured to perform limiting based
on the following analogue inputs: P1, P2, P4 or P5.
In addition, two digital inputs (3MIO2 terminal 133 and RAMP4 terminal
0) are provided to enable limiting from any definable plant condition which
can be transmitted to Fridgewatch 2100 as a signal: contacts open or
closed.
Two levels of limiting can be applied by the Fridgewatch 2100 Controller:
Start/load inhibit signal
- inhibits any further loading, and if
applied to the compressor when it is not running, prevents the
compressor from starting.
The second limiting condition will be invoked if the condition worsens.
Forced unload signal
- causes the running compressor to unload
all the time the signal is applied (contacts closed).
It is important to note that, while load inhibit or force unload conditions
exist, Fridgewatch 2100 prevents further loading or unloads the
compressor
even though
the Fridgewatch PID algorithm may be calling
for duty. Limiting appears on the front panel display when limiting is
taking place.
Limiting signals such as total current, common condensing pressure and
common suction pressure, should set start/load inhibit for all compressors
as, obviously, a capacity increase would only exacerbate the limiting
condition(s).
