P Band Factor - Johnson Controls SABROE Unisab III Control Engineering Manual

Set-up

6.5.7 P band factor

For screw compressors, the following picture will appear:
Start delay
Start delay can be changed in the range from 1 to 100. Stop delay can be changed in the
range from 0 to 100. The function of these zones must be seen in connection with the applied
regulator (suction pressure, brine, etc).
If selecting the value 1, the timer will not start counting until the measured value of the regu-
lator is 1% inside the proportional band above the neutral zone. If the value 100 is selected,
the timer will not start counting until the measured value of the regulator is 100% inside the
proportional band above the neutral zone.
Whether selecting 1% or 100%, the timer - when it is started - will count in seconds without
extension (this is not the case with reciprocating compressors, see next subsection).
Stop delay
If selecting the value 0, the timer will start counting when the measured value of the regula-
tor is in the neutral zone.
If selecting the value 1, the timer will not start counting until the measured value of the regu-
lator is 1% inside the proportional band below the neutral zone.
If selecting the value 100, the timer will not start counting until the measured value of the
regulator is 100% inside the proportional band below the neutral zone.
Sp = Set point
Nz = Neutral zone
Pb = Proportional band
Example 1: Unisab III has been set for suction pressure regulation
Sp = -10°C,
Nz = 2 K,
Pb = 10 K,
Start delay = 90 sec and factor = 50.
Stop delay = 60 sec and factor = 30.
In the neutral zone and up to -4°C the compressor will not be in operation. If the measured
suction pressure is a little higher than -4°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.
In the neutral zone and up to -14°C the compressor will be in operation. If the measured suc-
tion pressure is a little lower than -14°C and the capacity slide is below 5%, the Stop delay
timer will start counting and the compressor will stop after 60 seconds.
Engineering manual - Unisab III 1.10
001930 en 2014.09
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