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Condenser / Gas cooler
Principle
The condenser in a transcritical CO2 system is also called a gas-
cooler. Unlike in an HFC system, subcooling is not controlled by a
condenser, but by the high-pressure valve Vhp.
The gas-cooler control must regulate the temperature in the gas-
cooler's discharge, so that it has the lowest possible value and the
energy consumption of the fans is minimal. However, this should
not be so low as to prevent the receiver pressure from being
maintained.
Capacity control of the condenser (gas cooler) can be accom-
plished via step regulation or speed control of the fans.
• EC motors
An analogue output signal is used here, which controls the fans
from 0 to maximum capacity.
• Step regulation
The controller can control up to 4 condenser steps that are cut in
and out sequentially.
• Speed control
The analog output voltage is connected to a speed control. All
fans will now be controlled from 0 to max. capacity. If an ON/
OFF signal is required it can be obtained from a relay output.
Regulation can be carried out based on one of the following
principles:
- all fans operate at the same speed
- Only the necessary number of fans is cut in.
• Combination with one fan speed regulated and the rest step
regulated.
Capacity control of condenser
The cut-in condenser capacity is controlled by the condenser pres-
sure's actual value and depends on whether the pressure is rising
or falling. Regulation is performed by a PI controller.
The controller cuts in capacity in such a way that the deviation
between the actual condensing pressure and the reference value
becomes as small as possible.
Regulating sensor selection
The capacity distributor regulates from the temperature sensor
Sgc, located at the outlet for the gas cooler.
Cap. Ctrl sensor = Sgc
Pc is used as the safety function for high condenser pressure and
will therefore ensure cut-out of the compressor capacity when
condenser pressure is too high.
Handling sensor errors:
In the event of Sgc failure the controller switches over to an
"emergency cooler sequence" that attempts to maintain regula-
tion.
100
Capacity controller RS8HF102 © Danfoss 2017-07
Reference for condensing pressure
The reference for the regulation can be defined in two ways. Either
as a fixed reference or as a reference that varies according to the
outdoor temperature.
Fixed reference
The reference for the condensing pressure is set in °C.
Floating reference (recommended)
This function allows the condensing pressure's reference value to
vary according to the outdoor temperature within a defined area.
The reference is based on:
- the outdoor temperature measured with Sc3 sensor
- The minimum temperature difference between the air
temperature and the condensing temperature at 0% compressor
capacity.
- the condenser's dimensioned temperature difference between
the air temperature and the condensing temperature at 100%
compressor capacity (Dim tmK)
- how large a part of the compressor capacity has been cut in.
The minimum temperature difference (min tm) at low load should
be set at approximately 2 K as this will eliminate the risk that all
fans will be running when no compressors are running.
Set the dimensioned difference (dim tm) at max. load (e.g. 3 K).
The controller will now contribute with a value to the reference
which depends on how large a part of the compressor capacity
has been cut in.
Heat recovery
The condensing pressure may be increased during heat recovery.
This is mentioned in the "Heat recovery" section.
Limitation of the reference
To safeguard yourself against a too high or too low regulation
reference, a limitation of the reference must be set.
PcRef
Max
Min
(The maximum value can be overridden by the heat recovery
function).
AK-PC 772A
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