120 Capacity controller RS8HE202 © Danfoss 2017-07 AK-PC 781A
Parallel compression
Principle
On transcritical systems installed in slightly warmer surround-
ings than normal, the COP will be significantly improved by using
parallel compression.
One or more compressors are used to help maintain the receiver
pressure during warm periods when the outdoor temperature
becomes high — primarily during the summer months.
The ordinary capacity regulation is carried out by two AK-PC 781A
units. One is for low temperature regulation (LT), and the other is
for medium temperature regulation (MT). MT regulation also con-
trols the entire condenser circuit, with heat recovery, gas cooling
and receiver pressure.
The parallel compression (intermediate temperature, ‘IT’) is con-
trolled by the third AK-PC 781A. It receives signals from two pres-
sure transmitters and from the controller on the MT circuit, and it
will start the compressor as needed, so that the receiver pressure
is kept at the desired level. The compressor capacity will be vari-
able, and the controller will emit a 0-10 V signal that indicates the
desired capacity.
The function is activated by the MT controller, which regularly
records the opening degree of the Vrec valve. When the opening
degree is greater than the set value, the MT controller will pull a
relay. This signal is recorded by the IT controller, which starts the
IT compressor. The controller will now speed-regulate the IT com-
pressor so that the pressure in the receiver is kept at the desired
level.
When the IT compressor is running, the IT controller will pull a
relay. This is recorded by the MT controller, which then closes the
Vrec valve.
The reference for the receiver pressure is raised to the max. when
the IT compressor is running.
When the IT capacity’s need drops to min. capacity, the compres-
sor will stop, the signal (IT relay) to the MT controller will disap-
pear, and the MT controller will assume pressure control of the
receiver by regulation of the Vrec valve.
IT comp. running
output
(IT comp. running)
IT comp. running
input
(IT comp. running)
IT comp. release
output
(IT comp. release)
IT comp. release
input
(IT comp. release)
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