The neutral zone NZ
suct,int
is found in the ../Suction/Internal Pressure/Control picture.
The neutral zone NZ
oil,rel
is found in the ../Oil/ Pressure/Control picture.
The ‘High Alarm limit’ value for P
suct,int
is found in the ../Suction/Internal Pressure/Limits pic-
ture.
The ‘Low Alarm curve’ Y value for P
oil,rel
is found in the ../Oil/ Pressure/Control picture.
The start-up limiter can maximum be active for the time set in timer “Max. start-up time”
(default 600 sec.) When the “Max. start-up time” timer is running the “Start-up limiter” alarm
is shown. If the “Max. start-up time” timer runs out the compressor is stopped by a shut-
down.
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 compres-
sor to reach its normal operating point, is a combination of low suction pressure limiting func-
tion and a ramp function.
If the suction pressure ramp limiting function prevents the compressor from increasing its ca-
pacity 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 refrig-
erant 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 discharge temperature will increase which may cause a shutdown. In most ca-
ses, a minimum capacity of 50% can prevent a compressor shutdown on high discharge tem-
perature.
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% capacity or more, the follow-
ing will occur:
- It is impossible, both in Manual, Auto or Remote to unload capacity stages so that ca-
pacity 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 regu-
lator sends out a down regulating signal. If one of the limiting functions is going to
unload capacity, the timer Delay down will be started at 50/66%.
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 provided that
capacity stays below 50% for more than approx. 4 min.
Compressor control and surveillance
Engineering manual - Unisab III 1.10
001930 en 2014.09
89/319
To Next Page
Loading...
