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The LT circuit requests compressor start for the high-pressure
circuit.
T2: First MT compressor starts after expiry of recycle hours
T3: Last LT compressor stops
T4: Last MT compressor stops
Example
MT controller:
- LT/MT coordination = MT coordination
- The MT controller uses:
• An output "MT compressor release", which is activated when
the first MT compressor starts.
• An input "MT compressor requirement", which receives a signal
from the low-pressure controller.
LT controller:
- LT/MT coordination = LT coordination
- The LT controller uses:
• An input "LT compressor release" which is connected to the
output "MT compressor release" on the MT controller.
• An input "LT compressor requirement" which is connected to
the output "MT compressor requirement" on the MT control-
ler.
3) LT/MT coordination - LT-release before MT start
The LT compressors can start when the MT has signalled that it is
ready but has yet to start the compressors. The MT sends a signal
to the LT that MT is ready and that LT must start as needed. MT
then awaits LT starting up. When this takes place, MT will register
the pressure increase and immediately start the MT compressors
acc. to requested pressure.
MT comp. release
output before start
LT comp. release
Input
Here both a relay output and an on/off input on both controllers
are used.
(Make the connections between the two controllers, so that the
controllers are kept galvanically separate).
The LT controller is adjusted for LT coordination
The MT controller is set for MT coordination release before start.
104
MT comp. request
input
LT comp. request
output
Capacity controller RS8GG602 © Danfoss 2015-03
Time delays on signals
To achieve optimum coordination between the MT and LT circuits
it is possible to define time delays on all input and output signals.
Generally, though, the MT circuit's suction pressure will also
influence whether the release times for the LT circuit are met.
- If the MT pressure is over the neutral zone, LT must wait until
the pressure drops into the neutral zone.
- If the MT pressure is below the neutral zone, the LT must start
before MT starts.
MT release delay
Here the output signal from the MT controller is delayed.
This means that the MT compressors will be permitted to run for
the set delay before the LT compressors are released for start.
MT compressor requirement delay
Here the input signal "MT compressor requirement" is delayed on
the MT controller and thereby the start-up of the first MT com-
pressor.
This delay can be used if the LT circuit requires start-up of MT
compressors too often.
LT compressor release delay
Here the input signal "LT compressor release" is delayed on the LT
controller.
This means that the MT compressors will be permitted to run for
the set delay before the LT compressors are released for start.
LT compressor requirement delay
Here the output signal "LT compressor requirement" is delayed
from the LT controller. This delay can be used if the LT circuit
requires start-up of MT compressors too often.
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 781 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.
Injec. heat
exchanger
output
AK-PC 781
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