Example 2: Unisab III has been set for suction pressure regulation
Sp = -20°C,
Nz = 4K,
Pb = 10 K,
Start delay = 90 sec and factor = 10.
Stop delay = 60 sec and factor = 0.
In the neutral zone and up to -17°C the compressor will not be in operation. If the measured
suction pressure is a little higher than -17°C, the Start delay timer will start counting and the
compressor will start after 90 seconds. If necessary, a compressor start can thus be delayed
or blocked.
As long as the measured suction pressure is in the upper proportional band, the compressor
will be in operation. If the measured suction pressure is a little lower than -18°C and the ca-
pacity slide is below 5%, the Stop delay timer will start counting and the compressor will stop
after 60 seconds.
For reciprocating compressors the following picture will appear:
Each factor can be selected in the range 1-10. In Auto and Remote mode, the various P Band
Factors are influencing the decision of when to start or stop the compressor or when to en-
gage or disengage another capacity stage.
Basically, starting and stopping as well as engaging and disengaging are delayed by the
timers Start delay, Stop delay, Delay up and Delay down. See subsection 6.5.3. Timer set-up.
The relevant timers start as soon as the regulated input (e.g. suction pressure) is just outside
the neutral zone. There is one P band factor for each of these timers. If the P band factor is 1,
the time delay will be the same no matter if the regulated input is just outside the neutral
zone or far outside the neutral zone. By selecting a P band factor higher than 1 (up to 10), it
is possible to have the timer run slower when the regulated input is close to the neutral zone
as illustrated in Fig. 23.
0
1
Timer step [sec]
PBF
PBF = P. band factor
Diff. = Measuring value - setpoint
Diff.
PBNZ-NZ-PB
Fig. 23: Principle for P. band factors on reciprocating compressors
When the regulated input is outside the P band (PB), the timer will run at normal speed as it
is counting in seconds. When the regulated input is just outside the neutral zone (NZ), each
countdown will last one PBF second. This way a compressor start can be delayed or blocked if
Set-up
Engineering manual - Unisab III 1.10
001930 en 2014.09
211/319
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