Johnson Controls SABROE Unisab III Control, Engineering Manual

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Engineering manual - UniSAB III 1.10.8
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001930 en 2021.06
Compressor control and surveillance
3.1.2 Regulator set-up
UniSAB III includes a number of compressor capacity regulators. In the modes Auto and Remote , one
(and only one) of these regulators is responsible for adjusting capacity according to the cooling (or heat-
ing) requirements. Which regulator to use is specified in line 3:
Control on in the picture Setup/Compr.
Control .
Control on can be set to one of the following:
• Suction pressure
• Process out
• Discharge pressure
• Hot water
• Ext. cool
• Ext. heat
Each regulator has its own set of parameters. This means that when changing the Control on setting
from one regulator to another and back again, the original parameters are kept. Each regulator parame-
ter set includes:
• Set point (SP)
• Neutral zone (Nz)
• Proportional band (Pb or P Band)
Depending on whether PID = Yes or No is selected in the picture Setup/Configuration/PID , PID parame-
ters will also be used for screw compressors and reciprocating compressors with VSD. See subsection
3.1.4 Capacity PID regulation (outer loop) below.
Set point is the desired pressure or temperature in the plant.
Neutral zone indicates how much the pressure or temperature is allowed to fluctuate in relation to the
Sp without the compressor changing its capacity. The set value of Nz is positioned symmetrically around
the set point (Sp +/- 1/2 Nz).
Proportional band indicates how powerful the regulating signal to the compressor capacity regulation
is going to be, depending on the difference between the desired value (Sp) and the actual value.
If the measured value is just outside the Nz, the regulating impulses will be very short whereas the regu-
lating impulse will be very long if the measured value is outside the P Band. The P Band is positioned
symmetrically around Sp outside the Nz. See Fig. 13.
Fig. 13: Regulation principle
3.1.3 Regulation of screw compressors
Screw compressors are capacity regulated (hydraulically or electrically) by moving the capacity slide.
This takes place via two digital outputs, which are controlled by UniSAB III, so that the slide moves to-
wards max. or min. capacity according to demand. The setting is stepless from 0 to 100%. UniSAB III
Rapidly up
Very slowly up
P. Band
SP
NZ
Very slowly down
Rapidly down
Error signal
P. band