Engineering manual - UniSAB III 1.10.8
001930 en 2021.06
95/346
Compressor control and surveillance
Suction ramp
With the Suction ramp timer it can be indicated at which speed the compressor is allowed to lower its
suction pressure 1°C. The function, which is used for as long as it takes the compressor to reach its nor-
mal operating point, is a combination of low suction pressure limiting function and a ramp function.
If the suction pressure ramp limiting function prevents the compressor from increasing its capacity above
5%, the timer Delay before stop will not be activated and the compressor will continue operating.
The suction pressure limitation without the ramp function can stop the compressor if capacity falls below
5% for a long period.
High discharge pipe temperature (one-stage reciprocating compressors)
This capacity limiting function works for all one-stage reciprocating compressors using refrigerant R717.
This function is activated at certain operating conditions, especially at excessive differential pressure,
corresponding to the operating limits diagram zone 2 (zone 4 for CMO, SMC 100 S/L and SMC 180).
The diagrams are included in the Engineering manual for the specific compressor.
At such operating conditions, CMO/HPO and SMC/HPC compressors must minimum load 50% of their
capacity when starting up and during operation. If the compressor is running at lower capacity, the dis-
charge temperature will increase which may cause a shutdown. In most cases, a minimum capacity of
50% can prevent a compressor shutdown on high discharge temperature.
At present the limit curve (which is a straight line) is fixed on the basis of a max. permissible discharge
pipe/oil temperature (T
2max
) of 150°C as well as suction superheating of 10°C.
When the limiting function is active, the following will occur at start-up:
– Capacity stages corresponding to a minimum of 50% will be loaded.
When the limiting function is active during operation, with 50% (step) capacity or more, the following will
occur:
– It is impossible, both in Manual, Auto or Remote to unload capacity stages so that capacity
drops below 50%, i.e. 50/66% are the lowest capacity stages - also when total unloading has
been selected.
– In Auto and Remote the timer Delay before stop will be started at 50/66% if the regulator sends
out a down regulating signal. If one of the limiting functions is going to unload capacity, the com-
pressor will stop at Delay down timer at 50/66% capacity.
If the compressor is operating at capacity below 50% when the limiting function is active, the limiting
function will not actively begin to load stages. However, the following will occur:
– In Manual the compressor will be stopped on the total unloading timer if capacity stays below
50% for more than approx. 4 min.
– In Auto and Remote the compressor will be stopped on the total unloading timer if the regulator
is neutral and capacity remains below 50% for more than approx. 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 compressor will stop at De-
lay down timer.
– If the regulator sends out a regulating up signal and the compressor is loading stages so that ca-
pacity 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 ca-
pacity 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 val-
ue). Since the limit curve is directly depending on the suction superheating of the plant, the actual
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