The extremely high solubility in all common oils results in a reduction of the oil viscosity in the compressor, in particular at low oil temperatures and high suction pressure.
Consequences:
Strong gas discharge effect in the oil sump and lubrication gaps upon reduction in pressure.
Foaming, resulting in increased oil carry over.
Reduced performance and heavy wear on drive gear parts (Oils).
Measures:
Use oil with high basic viscosity, high viscosity index and good lubricating properties.
Select the minimum possible refrigerant charge.
Use a generously sized oil heater.
Install a solenoid valve in liquid line and, if necessary, a check valve in discharge gas line (as additional safety against refrigerant migration during standstill).
Install compressor in temperature-controlled environment and insulate it if necessary.
When there is a risk of high shut-off pressures on the suction side and long cut-out periods:
Provide an automatic or at least a one-off pump down system! Set cut-out pressure only slightly lower than the lowest evaporation pressure during operation. In doing so, avoid vacuum operation under all circumstances!
Ensure sufficiently high suction gas superheat. The discharge gas temperature for reciprocating compressors must be at least 20 K and that for compact screw compressors at least 30 K above the condensing temperature in summer and winter operation. For medium-temperature and low-temperature applications and heat pump systems, use the heat exchanger between the suction gas and liquid lines.
Secure system against ''wet operation'' during start-up and operation – use expansion devices with stable control behaviour. With electronic expansion valves, set a specific degree of opening after defrosting and the like, if necessary, use a suction accumulator.
Recommendation for systems with foreseeable long shut-off periods (e.g. only seasonally operated systems or precharged systems that are stored or transported for several weeks before commissioning): To prevent refrigerant migration during the shut-off periods, the shut-off valves on the compressor should be closed after a single pump-down.