- In Auto and Remote the compressor will be stopped on the total unloading timer pro-
vided that the regulator is neutral and capacity remains below 50% for more than ap-
prox. 4 minutes.
- If the regulator sends out a regulating down signal, the timer Delay before stop will
be started.
- If one of the other limiting functions is about to unload capacity, the timer Delay down
will stop at 50/66%.
- If the regulator sends out a regulating up signal and the compressor is loading stages
so that capacity rises to 50% or more, the limiter will be deactivated until the capacity
is decreased to 50% again.
In the Alarm picture the text Limiting discharge temperature will appear if the compressor is
about to be stopped on the total unloading timer due to insufficient capacity or if the limiting
function prevents capacity from being reduced in Manual, Auto or Remote.
The limit curve (limiting zones 2/4) corresponds as standard to suction superheating of 10°C
(factory value). Since the limit curve is directly depending on the suction superheating of the
plant, the actual superheating of the plant must be entered as Set point 2 in the picture Con-
trol values/Suction/Superheat/Control if differing from the above factory value.
Adjusting range: -10 - +30°C
By adjusting Set point 2 for Suction superheat upwards or downwards, the limit curve will be
dislocated accordingly.
If Set point 2 is set at -10°C, the limit curve will be dislocated so much upwards that the limit-
ing function will be disabled.
Separator velocity (screw compressors)
For all screw compressors a capacity limiting function for high separator gas velocity can be
activated. It will limit the compressor capacity to a calculated limit based on the current run-
ning conditions. To activate the limiter, two values calculated from the Refrigeration Plant
Computation program are required:
• Actual separator velocity limit reference
• The compression ratio at which the above value was calculated.
The high capacity limit for the separator is calculated by means of the measured compression
ratio and the two mentioned reference values. When the limiter inhibit function is active (i.e.
capacity cannot be increased) the text Running overload is shown in the status field on the
display.
The Alarm picture reads: Limiting high separator flow. When the limiter force-unload function
is active (i.e. capacity will actively be decreased) the status text Running overload is shown in
the status field on the display.
The text in the Alarm picture reads: Limiting high separator flow.
The set up values for the limiter are entered in the Setup/Configuration/Oil system picture.
Min. Max.
Sep.vel.ref 0.49 0.00 10.00
Sep.vel.cr 4.72 0.00 100.00
The calculated (intermediate) values for the separator limiting function are shown in the Diag-
nosis picture Service/Diagnosis/Software/Separator Velocity.
Compressor control and surveillance
90/319
Engineering manual - Unisab III 1.10
001930 en 2014.09
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