Johnson Controls SABROE UniSAB III Engineering Manual page 234

Should the measuring value reach 33.5°C/R, the counting interval will be 2 seconds. If the pressure is
still moving upwards and exceeds 36°C/R (the Pb limit), the timer will count in seconds. The resulting to-
tal delay will range between 180 and 540 seconds, and, after this, the compressor will stop.
For reciprocating 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 are influencing the decision of when to start or stop the compressor or when to engage or disen-
gage another capacity stage.
Basically, starting and stopping as well as engaging and disengaging are delayed by the timers Start de-
lay, 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. 26.
Timer step [sec]
-PB -NZ
Fig. 26: 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 there is no particular cooling re-
quirement. However, if deviation is large 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°C/R
Pb = 5°C/R
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.
PBF = P. band factor
Diff. = Measuring value - setpoint
PBF
1
0 NZ
Diff.
PB
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Engineering manual - UniSAB III 1.10.8
001930 en 2021.06
Set-up