Fridgewatch 2100 Controller Mk4
Publication 3-58
Section 3
Page 26 of 152
Issue 1.1 : 01/08
If the duty requirement only slightly exceeds that available from the
compressor(s) running at maximum load, when the S8 Next Compressor
Start Delay timer expires, the next compressor in the running order would
automatically start even though it may be quite acceptable to run with a
small deviation from the set point. Under these circumstances S8 could
be set to a fairly long delay to hold off starting the next compressor. This
leads to the second limitation of the conventional solution, in that a large
deviation from the set point could occur during a lengthy next start delay
if there is a genuine need for more duty.
The same principle applies to a fall in duty. The S9 Next Compressor
Stop Delay timer, started when the last compressor to start unloads to
minimum, expires and stops this compressor while capacity modulation
passes to the previously started compressor. This prevents stopping a
compressor prematurely, but can cause large deviations from the set
point if the trend for less duty is falling rapidly.
To overcome these disadvantages, Fridgewatch 2100 supplements the
delay before starting and stopping timers (S8 and S9) with two additional
timers: S12 Set Point Deviation to Start Next Compressor and S13 Set
Point Deviation to Stop Next Compressor. S12 and S13 are configurable
by the operator as the deviation from the set point that must be exceeded
before any start/stop action occurs. When the Fridgewatch 2100
Controller detects there is an increase in duty which cannot be met by the
compressor(s) already running, the S8 Next Compressor Start Delay
timer starts and expires as before. However, another compressor still
cannot start until the measured value exceeds the set point start value by
more than the S12 Set Point Deviation to Start Next Compressor value.
This means that the time delay before starting the slave can be reduced
without causing unnecessary cycling of this compressor when it is not
required. However, if the additional capacity provided by another
compressor is genuinely required, the measured value will rapidly rise
and should start this compressor before the deviation becomes
unacceptably large. An identical but opposite control action occurs when
stopping a compressor.
5.5. Master/Slave
Operation
As already mentioned, the Fridgewatch 2100 Controller can be linked
with another Fridgewatch 2100 Controller, or with a Fridgewatch 2000
Controller, and then act as either the master or slave controller on a
common control system.
Before discussing master/slave operation, it is necessary to define the
function of the master and slave Fridgewatch 2100 Controllers:
•
Master Controller
- responsible for sensing the measured
variable (pressure or temperature), starting and stopping the
master and slave compressors and modulating their
capacity.
•
Slave Controller
- starts and stops the slave compressor(s),
adjusts slave compressor capacity, as commanded by the
master Fridgewatch Controller.
The master and slave Fridgewatch 2100 Controllers must be wired via a
changeover switch to prevent both controllers being select as master at
the same time; refer to 6.7. Summary of Master/Slave Connections.
5.5.1.
Starting the Master Compressor
For the moment we shall assume the master and slave Fridgewatch
Controllers each control only one compressor; multi-compressor
applications are described later.
