Fridgewatch 2100 Controller Mk4
Publication 3-58
Section 3
Page 116 of 152
Issue 1.1 : 01/08
•
A slave Fridgewatch 2100 Controller, on detecting P2 limiting,
will unload its own compressors;
•
A master Fridgewatch 2100 Controller, on detecting P2
limiting, will divert unload pulses to the currently modulating
compressor which may be the slave.
Low P2 trips stop all running compressors. Any non-running
compressors may start and run successfully provided that the trip
condition has cleared before the compressor starts.
The numerical value,
x
, (after removing the negative sign if present) is
used to specify the number of unload pulses to be given to the unloading
gear in any one sample time when the force unload limit threshold value
has been crossed. This is to prevent too severe unloading occurring if
there is a delay between unload pulses being sent and the actual
measured value responding. For example heat exchanger outlet
temperature responds more slowly to unloading than does discharge or
suction pressures. (Contrast this with current limiting where this facility is
not offered as motor current responds immediately when the slide valves
move).
A non-zero value ‘turns on’ both Limiting AND Trip functions, if only one
of these functions is required, the unwanted function must effectively be
disabled by entering a value in settings S95 and S96 or S97 that will
never be attained.
Note that if limiting is enabled and an E42 Input/Output Error is present
because of an over or under current being applied to this channel which
would otherwise provide limiting, unless the package unit is in ‘manual’
start/stop - ‘manual’ capacity control mode Fridgewatch 2100 will trip the
compressor. In ‘manual’ mode limiting is normally applied, but when an
E42 error is present, it is the responsibility of the user to provide another
means of measuring the condition and ensuring that the loading state
does not cause damage to the equipment or process.
Note also for a Fridgewatch 2100 operating in ‘auto capacity control’
mode you should not normally apply limiting to the same analogue that
the Fridgewatch is using for auto capacity control unless you have
thought through carefully the conflicts that may occur.
S95 P2 (pressure) limiting: hold low limit (units: bar g)
With S94 enabled, S95 defines the pressure limit below which
compressor loading is inhibited; this overrides the output from the PID
algorithm or external load/unload signals.
NOTE: the value assigned to S95 Hold Limit must
normally be higher than the S96 Unload Limit.
If a Trip Limit on P2 is required but limiting is not, set S95 to a value that
will never be attained in practice.
S96 P2 (pressure) limiting: unload low limit (units: bar g)
With S94 enabled, S96 defines the pressure limit below which the
Fridgewatch 2100 Controller sends unload pulses (as defined by S94)
until the value displayed by P2 does not exceed this limit; this overrides
the output from the PID algorithm or external load/unload signals.
NOTE: the value assigned to S96 Unload Low Limit must
normally be lower than the S95 Hold Low Limit.
If a Trip Limit on P2 is required but limiting is not, set S96 to a value that
will never be attained in practice.
