Johnson Controls SABROE Unisab III Control, Engineering Manual, Page: 148 / 348

Engineering manual - UniSAB III 1.10.8
148/346
001930 en 2021.06
Compressor control and surveillance
Superheat regulator
The PID regulator will control an analogue expansion valve in order to regulate the liquid level according
to the superheat set point. If the “Suction superheat” is lower than the set point, the opening position will
be decreased. If the “Suction superheat” is higher than the set point, the opening position will be in-
creased. The opening value depends on the PID calculation.
The suction superheat is calculated by measuring the suction temperature using the internal suction
temperature PT100 sensor or an external 4-20 mA PT100 transmitter. The external 4-20 mA PT100
transmitter is enabled in
Set up/Configuration/Plant/Ext PT100 .
The PID regulator settings are made in the picture Control values/Process Temp/Superheat/Control . If
the regulator is too fast or too slow you may adjust the Proportional band and/or the Integral time values
up or down accordingly.
The expansion valve is a motorised valve. In series with this valve there is a solenoid stop valve which is
open when the compressor is running. The output range is 0 to 100%.
Start position for expansion valve
When the compressor starts and the expansion valve starts to regulate, the expansion valve will be
forced to a start opening position. This valve start position set point is default 50%. This feature ensures
a very steady superheat during start up. The opening position can be set at any value. When the com-
pressor is stopped, the expansion valve is closed.
Feed forward
Many processing plants have a brine system with a limited buffer volume. This might result in many and
large changes in the compressor capacity. To avoid that these changes affect the superheat, a
Feed for-
ward factor is integrated in the superheat controller to obtain a steadier superheat. When the capacity of
the compressor is changed, a feed forward signal is sent to the expansion valve. The opening position
of the expansion valve will be changed. The feed forward function will change the position of the expan-
sion valve if there is a sudden change in compressor capacity, and this is performed before the primary
superheat controller can measure a change in the superheat.
The feed forward contribution is calculated as:
Low superheat limit
To protect the compressor against liquid hammering, it is possible to set a limit for minimum superheat.
If the superheat falls below the Reduction Set point limit the expansion valve will close (or reduce the
opening degree radically) until the superheat again becomes higher than this limit. The superheat con-
troller continues to regulate although the expansion valve is closed or reduced in its opening. The open-
ing value of the expansion valve is reduced by the Opening reduction factor when the suction superheat
is below the low superheat limit.
The valve reduction is calculated as:
Valve position (with reduction) [%] = Valve position (without reduction) [%] x (1 - Valve opening reduction
factor /100)
By normal operation, this feature will not be active, but for plants with large and sudden changes in ca-
pacity, this feature will provide extra protection of the compressor.
Change in valve position [%] = (Compressor capacity at this cycle - compressor capacity at last cycle) x FF factor
Compressor capacity at last cycle