1
CP
EXV
2
3
4
C
C
-0
0
0
0
4
9
1
2
3
4
CP
EXV
Opened
Closed
On
Off
Compressor
Electronic expansion valve
Figure 26: specific compressor start / stop control
Reversible heat pump systems
Transients are likely to occur in reversible heat pump systems, i.e. a changeover cycle from cooling to heating,
defrost or low-load short cycles. These transient modes of operation may lead to liquid refrigerant carry-over (or
flood back) or excessively wet refrigerant return conditions. As such, reversible cycle applications require specific
precautions for ensuring a long compressor life and satisfactory operating characteristics. Regardless of the
refrigerant charge in the system, specific tests for repetitive flood back are required to confirm whether or not a
suction accumulator needs to be installed.
The following considerations cover the most important issues when dealing with common applications. Each
application design however should be thoroughly tested to ensure acceptable operating characteristics.
Sump heaters
Sump heaters are mandatory on reversible cycle applications given the high probability of liquid migration back to
the compressor sump during off -cycles due to the outdoor location of most units and operations during low
ambient conditions.
Discharge temperature thermostat
Heat pumps frequently utilize high condensing temperatures in order to achieve a sufficient temperature rise in the
medium being heated. At the same time, they often require low evaporating pressures to obtain sufficient
temperature differentials between the evaporator and the outside temperature. This situation may result in high
discharge temperature; as such, it is mandatory that a discharge gas thermostat be installed on the discharge line to
protect the compressor from excessive temperatures. Operating the compressor at too high discharge temperatures
can result in mechanical damage to the compressor as well as thermal degradation of the compressor lubricating oil
and a lack of sufficient lubrication. The discharge gas thermostat should be set to shut down the compressor in the
event discharge gas rises above 135°C.
Discharge line, reversing valve, solenoid valves
The Danfoss CH scroll compressor is a high volumetric machine and, as such, can rapidly build up pressure in the
discharge line if gas in the line becomes obstructed even for a very short period of time which situation may occur
with slow-acting reversing valves in heat pumps. Discharge pressures exceeding the operating envelope may result
in nuisance high-pressure switch cutouts and place excess strain on both the bearings and motor.
To prevent such occurrences, it is important that a 1-meter minimum discharge line length be allowed between the
compressor discharge port and the reversing valve or any other restriction. This gives sufficient free volume for the
discharge gas to collect and to reduce the pressure peak during the time it takes for the valve to change position. At
the same time, it is important that the selection and sizing of the reversing or 4-way valve ensure that the valve
switches quickly enough to prevent against too high discharge pressure and nuisance high-pressure cutouts. Check
with the valve manufacturer for optimal sizing and recommended mounting positions.
In applications with heat recovery or condenser partialisation, servo piloted solenoid valve have to be properly sized
or associated with a second small valve in parallel, in order to avoid quick discharge pressure drops when opening.
This phenomenon could lead to hammering effects and create constraints on the non-return valve integrated in
discharge fitting.
Scroll compressors, CH290 and CH485 | Application
© Danfoss | Climate Solutions | 2022.09
AB236986442743en-000501 | 38