In air-cooled machines, cycling the fans with a head pressure controller will ensure that the fans remain off until the
condensing pressure has reached a satisfactory level. Variable speed fans can also be used to control the
condensing pressure. In water-cooled units, the same can be performed using a water regulator valve that is also
operated by head pressure, thereby ensuring that the water valve does not open until the condensing pressure
reaches a satisfactory level.
The minimum condensing pressure must be set at the minimum saturated condensing temperature shown in the
application envelopes.
Under very low ambient conditions, in which testing has revealed that the above procedures might not ensure
satisfactory condensing and suction pressures, the use of a head pressure control valve is recommended. Note: This
solution requires extra refrigerant charge, which can introduce other problems. A non-return valve in the discharge
line is recommended and special care should be taken when designing the discharge line.
For further information, please contact Danfoss.
Sump heaters
Sump heaters are strongly recommended on all systems where the compressor is exposed to low ambient
temperatures, especially split and remote condenser installations. The sump heater will minimize refrigerant
migration caused by the large temperature gradient between the compressor and the remainder of the system,
please refer to section “Accessories”.
Low load operation
The compressors should be run for a minimum period in order to ensure that the oil has sufficient time to properly
return to the compressor sumps and that the motor has sufficient time to cool under conditions of lowest
refrigerant mass flows.
Brazed plate heat exchangers
A brazed plate heat exchanger needs very little internal volume to satisfy the set of heat transfer requirements.
Consequently, the heat exchanger offers very little internal volume for the compressor to draw vapour from on the
suction side. The compressor can then quickly enter into a vacuum condition. It is therefore important that the
expansion device be sized correctly and that a sufficient pressure differential across the expansion device be
available to ensure adequate refrigerant feed into the evaporator. This aspect is of special concern when operating
the unit under low ambient and load conditions. For further information on these conditions, please refer to the
previous sections.
Due to the small volume of the brazed plate heat exchanger, no pump-down cycle is normally required. The suction
line running from the heat exchanger to the compressor must be trapped to avoid refrigerant migration to the
compressor.
When using a brazed plate condenser heat exchanger, a sufficient free volume for the discharge gas to accumulate
is required in order to avoid excess pressure build-up. At least 1 meter of discharge line is necessary to generate this
volume.
To help reduce the gas volume immediately after start-up even further, the supply of cooling water to the heat
exchanger may be opened before the compressor starts up so as to remove superheat and condense the incoming
discharge gas more quickly.
Electronic expansion valve
The use of an electronic expansion valve requires a specific compressor start / stop control.
Specific compressor start sequence control has to be set when an electronic expansion valve (EXV) is used. The
sequence must be adjusted according to the EXV step motor speed to allow time for the EXV to open before the
compressor starts to avoid running under vacuum conditions.
The EXV should be closed at compressor stop not to let refrigerant in liquid phase entering the compressor. Ensure
that the EXV closes when the supply voltage to the controller is interrupted (ie power cut off) by the use of a battery
back-up.
© Danfoss | Climate Solutions | 2022.09
AB236986442743en-000501 | 37
Scroll compressors, CH290 and CH485 | Application