AK-PC 781A
Capacity
controller RS8HE202 © Danfoss 2017-07
103
Injection signal to heat exchanger control
An injection into the cascade heat exchanger must usually be
coordinated with the start-up of the first compressor.
AK-PC 781A allows the injection to begin when the controller is
ready to run with the first compressor.
Depending on system type/design, it will be advantageous to
synchronise the injection with the signal "MT comp.release output
before start"..
The relay output can e.g. be used to control a magnet valve or to
signal to a controller. E.g. an EKC 313.
Here an injection signal from the LT controller can be used to
request start of MT compressors.
- The LT controller’s injection signal is connected to the MT con-
troller’s input signal “MT compressor requirement”.
When the LT controller starts the first compressor, the injection
signal will be activated and thereby request MT compressor start.
When any delay in the MT control has expired, the first MT com-
pressor will start.
Sequnens
T1: The load on the LT circuit requires compressor capacity.
LT starts compressor and activates injection signal and thereby
the input “MT request” on the MT controller.
T2: First MT compressor starts after expiry of delays.
T3: Last LT compressor stops which removes the compressor
requirement signal and the last MT compressor stops.
MT compressor
LT compressor
LT inj.heat.excha.
MT comp. request
input
Injec. heat
exchanger
output
Injec. heat
exchanger
output
Injection ON
The electronic expansion valves in the refrigeration appliances
must be closed when all the compressors are prevented from
starting
. In this way the evaporators will not be filled with liquid
which is subsequently passed on to a compressor when regulation
is restarted.
One of the compressor control relays may be used for this func-
tion, or the function can be obtained via data communication
The function is described based on the sequence of events below:
T1) The last compressor is cut-out
T2) The suction pressure has increased to a value corresponding to
Po Ref + ½ NZ + 2 K” but no compressor can start due to re-start
timers or safety cut-out
T3) The time delay “Injection OFF delay” elapses and the injection
valves are forced to close via relay signal or via network signal.
T4) The first compressor is now ready to start. The forced closure
signal via the network is now cancelled.
T5) The time delay “Comp. Start delay” expires and the forced
closure signal via the relay switch is cancelled simultaneously
with the first compressor being allowed to start.
The reason why the forced closure signal via the network is
cancelled before the first compressor starts, is that it will take
some time to distribute the signal to all appliance controllers via
the network.
with relay
with data communication
Summary of Contents for AK-PC 781A
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