Danfoss ADAP-KOOL AK-PC 783 Design Manual page 105

Time delays with safety cut-out:
In connection with safety monitoring of a compressor it is possible
to define two delay times:
Cut-out delay time: Delay time from alarm signal from the
safety circuit until the compressor outlet cuts out (note that the
delay time is common to all security inlets for the compressor
concerned)
Safety re-start time: The minimum time a compressor must be OK
after a safety cut-out until it may start again.
Monitoring of superheat
This function is an alarm function which continuously receives
measured data from suction pressure P0 and suction gas Ss.
If superheat is registered which is lower or higher than the set lim-
it values, an alarm will be given when the time delay has passed.
Monitoring of max. discharge gas temperature (Sd)
Common Sd monitoring
The function gradually cuts out compressor steps if the discharge
temperature becomes higher than permitted. The cutout limit can
be defined in the range from 0 to +195°C.
The function is started at a value that is 10 K below the set value.
At this point the entire condenser capacity is cut in at the same
time as 33% of the compressor capacity is cut out (but minimum
one step). This is repeated every 30 seconds. The alarm function is
activated.
If the temperature rises to the set limit value all compressor steps
are immediately cut out.
The alarm is cancelled and renewed cutin of compressor steps is
permitted when the following conditions are met:
- the temperature has dropped to 10 K below the limit value
- the time delay prior to restart has been passed. (see later)
Normal condenser control is permitted again when the tempera-
ture has dropped to 10 K below the limit value.
Individual Sd monitoring
The affected compressor will be disconnected here when the
temperature exceeds the threshold value.
- The piston compressor will be reconnected when the tempera-
ture has dropped 10 K.
- The screw compressor will be reconnected when the tempera-
ture has dropped 20 K.
- Scroll compressors will be reconnected when the temperature
has dropped 10 K.
If signals are also obtained from the embedded NTC sensor,
the disconnect value for this temperature will always remain at
130°C and the reconnect value at 120°C.
AK-PC 783 Capacity controller RS8GN202 © Danfoss 11-2013
Monitoring of min. suction pressure (P0)
The function promptly cuts out all compressor steps if the suction
pressure becomes lower than the permitted value.
The cutout limit can be defined in the range from -120 to +30°C.
The suction is measured with pressure transmitter P0.
At cutout the alarm function is activated:
The alarm is cancelled and renewed cutin of compressor steps is
permitted when the following conditions are met:
- the pressure (temperature) is above the cutout limit
- the time delay has elapsed (see later).
Monitoring of max. condensing pressure (Pc)
The function cuts in all condenser steps and cuts out compressor
steps one by one if the condensing pressure becomes higher than
permitted. The cutout limit is set in bar. The condensing pressure
is measured with pressure transmitter Pc_.
The function takes effect at a value which is 3 K below the set
value. At this time the entire condenser capacity is cut in at the
same time as 33% of the compressor capacity is cut out (but min.
one step). This is repeated every 30 seconds. The alarm function is
activated.
If the temperature (pressure) rises to the set limit value, the follow-
ing will happen:
- all compressor steps will immediately be cut out
- the condenser capacity will remain cut in
The alarm will be cancelled and renewed cutin of compressor
steps is permitted when the following conditions are met:
- the temperature (pressure) falls to 3 K below the limit value
- the time delay for restart has been passed.
Delay of Pc max alarms
It is possible to delay the "Pc max alarm" message.
The controller will still disconnect the compressors, but the send-
ing of the alarm itself is delayed.
The delay is useful on cascade systems where the max. Pc limit is
used to disconnect compressors in the low-pressure circuit if the
high-pressure compressors have not started.
Time delay
There is a joint time delay for "Monitoring of max. discharge gas
temperature" and "Min. suction pressure".
After a cutout, regulation cannot be recommenced until the time
delay has been passed.
The time delay starts when the Sd temperature has again dropped
to 10 K below the limit value or P0 has risen above the P0 min.
value.
Alarm for too high suction pressure
An alarm limit can be set which will become effective when the
suction pressure becomes too high. An alarm will be transmitted
when the set time delay has been passed. The regulation
continues unchanged.
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