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 compressor 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 forward 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 expansion valve if there is a sudden change in
compressor capacity, and this is performed before the primary superheat controller can meas-
ure a change in the superheat.
The feed forward contribution is calculated as:
Change in valve position [%] = (Compressor capacity at this cycle - compressor capacity at last cycle) x FF factor
Compressor capacity at last cycle
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 controller continues to regulate although the expansion valve is
closed or reduced in its opening. The opening 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 capacity, this feature will provide extra protection of the compressor.
Pump down
To the compressor stop sequence is added a “Pump down function”. The expansion valve
closes and the compressor keeps running until the suction pressure is below the Pump down
Set point [°C] limit or until the Pump down stop timer has run out, whichever comes first.
Compressor control and surveillance
132/319
Engineering manual - Unisab III 1.10
001930 en 2014.09
To Next Page
Loading...
Need help?
General
