Defrost and reverse cycle
The Danfoss CH scroll compressor have the ability to withstand a certain amount of liquid refrigerant dynamic slug.
When compressors are installed in parallel, in order to limit liquid amount handled per compressor when beginning
and ending defrost, it is recommended to avoid running part load (keep all compressors running or keep them
stopped when moving 4-way valves).
For further details, please refer to Parallel application guidelines
EXV can also be opened when compressors are stopped and before 4 way valve is moving in order to decrease
pressure difference. Opening degree and time have to be set in order to keep a minimum pressure difference for 4
way valve moving. Each application design however should be thoroughly tested to ensure acceptable operating
characteristics.
Suction line accumulator
The use of a suction line accumulator is strongly recommended in reversible-cycle applications. This because of the
possibility of a substantial quantity of liquid refrigerant remaining in the evaporator, which acts as a condenser
during the heating cycle.
This liquid refrigerant can then return to the compressor, either flooding the sump with refrigerant or as a dynamic
liquid slug when the cycle switches back to a defrost cycle or to normal cooling operations.
Sustained and repeated liquid slugging and flood back can seriously impair the oil’s ability to lubricate the
compressor bearings. This situation can be observed in wet climates where it is necessary to frequently defrost the
outdoor coil in an air source heat pump. In such cases a suction accumulator becomes mandatory.
Water utilizing systems
Apart from residual moisture in the system after commissioning, water could also enter the refrigeration circuit
during operation. Water in the system shall always be avoided. Not only because it can shortly lead to electrical
failure, sludge in sump and corrosion but in particular because it can cause serious safety risks.
Common causes for water leaks are corrosion and freezing.
Corrosion
: Materials in the system shall be compliant with water and protected against corrosion.
Freezing
: When water freezes into ice its volume expands which can damage heat exchanger walls and cause leaks.
During off periods water inside heat exchangers could start freezing when ambient temperature is lower than 0°C.
During on periods ice banking could occur when the circuit is running continuously at too low load.
Both situations should be avoided by connecting a pressure and thermostat switch in the safety line.
Reduce moisture in the system
Excessive air and moisture
• can increase condensing pressure and cause high discharge temperatures.
• can create acid giving rise to copper platting.
• can destroy the lubricating properties of the oil.
All these phenomena can reduce service life and cause mechanical and electrical compressor failure.
Requirements
• The compressors are delivered with < 100ppm moisture level.
• At the time of commissioning, system moisture content may be up to 100ppm.
• During operation, the filter drier must reduce this to a level between 20 and 50ppm.
Solutions
To achieve this requirement, a properly sized and type of drier is required. Important selection criteria’s include:
© Danfoss | Climate Solutions | 2022.09
AB236986442743en-000501 | 39
Scroll compressors, CH290 and CH485 | Application