108 Capacity controller RS8HF102 © Danfoss 2017-07 AK-PC 772A
Parallel compression
Principle
On transcritical systems installed in slightly warmer surroundings
the COP will be significantly improved by using parallel compres-
sion.
One compressor is used to help maintain the receiver pressure
during warm periods when the outdoor temperature becomes
high — primarily during the summer months.
The parallel compression (intermediate temperature, ‘IT’) is
controlled by the IT suction group. It receives signals from two
pressure transmitters and it will start the compressor as needed,
so that the receiver pressure is kept at the desired level.
The compressor capacity will be variable, and the controller will
emit a 0-10 V signal that indicates the desired capacity.
The function is activated by the IT control, which regularly records
the opening degree of the Vrec valve. When the opening degree is
greater than the set value, the IT compressor will start up.
But it only starts up if the temperature at Sgc is higher than the set
value in IT setting "IT Comp. Sgc min."
The controller will now speed-regulate the IT compressor so that
the pressure in the receiver is kept at the desired level.
The parameter “IT comp delay” defines the time that the valve will
continue to regulate before the compressor takes over.
The parameter “IT end delay” defines the time that the IT compres-
sor must be stopped before regulation is transferred to the valve.
A high value will increase the operating hours of the IT compres-
sor.
When the IT compressor starts, the Vrec valve will be closed by
increasing the reference for the receiver pressure to max. minus
the P-band.
When the capacity requirement of IT decreases and the pressure
in the receiver reaches Prec.Min, the compressor will stop and the
pressure control will be assumed by the Vrec valve.
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