Max. sep. cap load is the calculated value for the maximum true capacity load in percent,
allowed at current running conditions. When this value is between 100 and 102% the capacity
inhibit function is activated. When the value is below 100% the capacity force unload is acti-
vated.
Max. cap. slide pos. is the calculated value for the maximum mechanical capacity load in
percent, allowed at current running conditions. This value, which will be between 0% and
100%, is the limiting value used when the capacity force unload function is active.
Pressure Ratio is the calculated pressure ratio = P
disch
/P
suct
K. Factor is the refrigerant constant, K (Kappa).
System Compr. Ratio is the calculated system compression ratio.
Syst. Sep.Vel. Ratio is the calculated system separator velocity ratio.
Utilisation Number is the calculated utilisation ratio for the separator at current running
conditions. Eg a value of 1.00 means that the separator is fully loaded and that the capacity
inhibit function is activated.
Insufficient main oil pressure (screw compressors)
Note in particular that this limiter only applies to SGC/GSV/RWF compressors (SAB 120, 151,
193, 233, 283 and 355).
If the absolute oil pressure is too low compared to the absolute suction pressure, capacity will
be reduced to 50%. If capacity is between 40-50%, it cannot be increased.
P
oil abs
< P
suct abs
x 1.5 + 1.0 bar.
See also Fig. 8: Insufficient main oil pressure during low differential pressure in section
2.2. Other alarms and shutdowns.
Forced speed active
Note that for SAB 120, 151, 157 HR, 193, 233, 283 and 355 with frequency converter you
may experience a problem with low oil pressure at compressor start-up at low rpm.
To avoid this problem Unisab III will increase the speed in the frequency converter (“forced
speed”) to make it regulate up to, and stay at, a predefined speed set between Motor rpm
min. and Motor rpm max.for as long as the main oil pressure is low. An alarm Forced speed
active will be issued.
When the main oil pressure (P
oil
abs - P
suct
abs x 1.5) is above a defined minimum limit of 1
bar the forced speed regulation is deactivated. The speed is then decreased to minimum
speed in Manual or the speed is set free to be controlled by the capacity regulator in Auto.
Compressor control and surveillance
Engineering manual - Unisab III 1.10
001930 en 2014.09
91/319
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