Johnson Controls SABROE Unisab III Control, Engineering Manual

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Engineering manual - UniSAB III 1.10.8
001930 en 2021.06
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Set-up
6.5.7 P band factor
For screw compressors, the following picture will appear:
Each P band factor can be selected in the range 1-10. In Auto and Remote mode, the various P Band
Factors influence the decision of when to start or stop the compressor. Basically, starting and stopping
are delayed by the timers Start delay and Stop delay . 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 regardless of whether the regulated input is just outside the neutral zone or far away from it.
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. 26.
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, the compressor start can be delayed extra if there is a vague cooling require-
ment. However, if there is a large deviation, the timer will count down fast, and start the compressor.
Example 1:
Suction pressure regulation has been selected. The measured suction pressure equals the set point.
The compressor is stopped and ready to start.
Set point, Sp = -10°C/R
Nz = 2°K
Pb = 5°K
Start delay = 60 sec.
P band factor start delay = 5.
When the measured suction pressure exceeds -9°C/R, the Start delay timer will start counting with an in-
terval of 5 seconds (P band factor) each time the value in the display counts one down. If the pressure
steadies, the timer will run for 5 x 60 = 300 seconds.
Should the measuring value reach -6.5°C/R, the counting interval will be 3 seconds. If the pressure is
still moving upwards and exceeds -4°C/R (the Pb limit), the timer will count in seconds. The resulting to-
tal delay will range between 60 and 300 seconds, and, after this, the compressor will start.
Example 2:
Discharge pressure regulation has been selected on a screw compressor. The measured discharge
pressure equals the set point. The compressor is running at 0% capacity.
Set point, Sp = 30°C/R
Nz = 2°K
Pb = 5°K
Stop delay = 180 sec.
P band factor stop delay = 3.
When the measured discharge pressure exceeds 31°C/R, the Stop delay timer will start counting with
an interval of 3 seconds (P band factor) each time the value in the display counts one down. If the pres-
sure steadies, the timer will run for 3 x 180 = 540 seconds.