Recommendations for existing systems

Generally avoid unfavourable influences and operating conditions on the guide compressor, e.g:

Daytime operation with frequent start-up and shut-off of the downstream compressors and unstable (fluctuating) operating conditions caused by poor control (Parameters and recommendations for optimal system efficiency and operational safety),
continuous operation at low compressor frequency, with simultaneous a high pressure ratio and high suction gas temperatures,
Night operation with low compressor frequency and regular pump-down cycles, with a high pressure ratio and high suction gas temperatures.

Recommendations:

Increase of the control accuracy (control factor CF) in the medium temperature stage

Use of a guide compressor with speed control. Here, first check whether the thermal load for the guide compressor is still permitted with the suction gas superheat and operating conditions that occur.
If the frequency-controlled motor is star-connected, there must also be a sufficient motor current reserve.
Retrofitting of a downstream compressor with mechanical capacity control (CR 100% or CR 50%) (CRII Mechanical capacity control for transcritical compressors). For this purpose, the compound controller must be able to control another speed controlled compressor!

Install an external desuperheater in the low temperature compressor stage. This reduces suction gas superheating in the medium temperature compressor stage in part load operation.
The use of a liquid injection can also be effective. Before doing so, check whether the capacity control of the system can cover the minimum load conditions without frequent pump-down and on-off cycles. If this is not the case, ensure that the liquid injection does not take place immediately before the pump-down set point is reached.

Control of the system components in a not too dynamic manner (see Stable control of the refrigerant mass flow).
Reduction of the maximum permitted high pressure in heat recovery mode, standard value 80 bar.
Keep the system clean and dry.
Eliminate causes of frequent oil alarms.

Further information:

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