Cascade Systems - Coordination And Injection - Danfoss ADAP-KOOL AK-PC 781 Design Manual

Cascade systems – coordination and injection
On cascade systems coordination is necessary between the two
compressor groups for low temperature (LT) and middle tempera-
ture (MT) respectively –LT compressors must not start before the
MT compressors are running.
In addition it is necessary to give a signal to the injection control
of the cascade refrigerator so that injection is started and stopped
in synchronisation with start/stop of the compressors
(Coordination with parallel compression on CO2 system (MT and
IT) is described under parallel compression on page 122.)
Coordination
The coordination between LT and MT compressors can be carried
out in three ways:
1) MT/LT compressor release
Here the MT group is the controlling circuit.
The MT compressors must not start before the load on the
MT circuit requires it and the LT group must not be allowed
to start before at least one MT compressor has been started.
This function is achieved by connecting the output signal
from the MT controller "MT compressor release" to the in-
put signal from the LT controller "LT compressor release".
MT comp. release
output
LT comp. release
Input
When a compressor is running in the MT circuit, the controller will
also pull the relay with the release signal into the LT circuit.
The LT controller must receive the signal as an On/Off signal.
Either as a contact signal on analogue input or as voltage signal
on a DI input.
Thread the connections between the two controllers so that the
controllers are kept galvanically separate.
Sequence
MT compressor
MT/LT release
LT compressor
T1: First MT compressor starts and the release signal is activated
T2: When the need arises, the first low-pressure compressor starts
T3: Last LT compressor stops
T4: Last MT compressor stops
(If the last MT compressor stops "before T3", the release signal will
drop out and thereby stop the LT compressors.)
AK-PC 781 Capacity controller RS8GG602 © Danfoss 2015-03
Example
MT controller:
- LT/MT coordination = MT compressor release
- MT controller uses an output "MT compressor release", which is activated
when the first MT compressor starts.
LT controller:
- LT/MT coordination = LT compressor release
- The LT controller uses an input "LT compressor release", which is con-
nected to the output signal from the MT controller. When the input
receives the signal from the MT controller, the first LT compressor is
released for start.
2) LT/MT coordination
Here the MT compressors can start either as a result of:
- Load on the MT circuit
- Requirements from the LT circuit
The MT circuit will still ensure that the LT circuit is only permitted to start
when at least one MT compressor has started. It will also ensure that secu-
rity timers and compressor timers are complied with.
MT comp. release
output
LT comp. release
Input
Here both a relay output and an On/off input are used on both
controllers.
(Thread the connections between the two controllers so that the
controllers are kept galvanically separate.)
- The output signal from the MT controller "MT compressor
release" gives a signal for the input signal of the LT controller "LT
compressor release".
- The LT controller's output signal "LT compressor requirement"
gives a signal to the MT controller's input signal "MT compressor
requirement".
When the LT controller requires a compressor to start, it will acti-
vate the "LT compressor requirement signal".
When the MT controller receives the signal, it will start the com-
pressor and simultaneously send a release signal to the LT control-
ler via the relay output "MT compressor relay".
Sequence
MT comp. request
MT comp
MT comp release
LT comp.
T1: The load on the LT circuit requires that compressor capacity be
connected.
MT comp. request
input
LT comp. request
output
103