Compressor capacity control and inverter drive
Capacity control adapts compressor output to the required load. It can change running time, active displacement or speed; these methods have different operating limits.
Guide
ContentsOn/off and staged control
A fixed-speed compressor can cycle on and off; a bank of compressors can enable machines in steps. Controls must respect allowed starts, minimum run and stop periods and oil management. Parallel machines share duty but do not automatically provide independent backup or successive compression stages.
Variable speed and inverter
An electronic drive controls motor speed within its approved range. The compressor, motor, inverter and controller must be compatible. Inverter is not synonymous with scroll or rotary, and installing a frequency converter does not automatically make a fixed-speed model suitable for variable-speed duty.
Mechanical unloading and flow control
Depending on construction, capacity may be adjusted by cylinder unloading, a screw slide valve, scroll unloading or centrifugal guide vanes. These affect gas delivery differently and require the model’s intended control system. Hot-gas bypass can support system control but does not imply a proportional reduction in compressor power.
Part-load comparison
Check minimum stable output, the whole operating envelope, motor cooling, oil return and protective logic. Compare useful output and total input at the same load and temperatures. A seasonal saving depends on the system and its operating schedule; a control label alone cannot establish it.
Prepare a compressor request
Pressure conversion and pressure-ratio calculation · model selection unavailable
The pressure tool converts the supplied values and calculates the absolute discharge-to-suction pressure ratio. The request form records other requirements. Neither function selects a compressor, calculates capacity or confirms a refrigerant cycle, diagnosis or interchangeability.
For a technical review, identify the exact model and refrigerant, duty and rating conditions, electrical supply and control, oil, dimensions/connections, and the reason for replacement or service. Missing information remains unknown.
Selection / replacement · Service
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Absolute pressures and pressure ratio
Use suction and discharge pressures for the same stated operating condition and compatible measurement definitions. This is a pressure calculation; it does not check whether a compressor can operate at those pressures.
Method: 1 bar = 100 kPa; 1 MPa = 1000 kPa. Convert each input to kPa. For gauge pressure, p(abs) = p(gauge) + p(atm). For absolute pressure, use the entered value directly. r = p(discharge, abs) / p(suction, abs); r has no unit.
Both absolute pressures must be positive. Enter a positive absolute atmospheric pressure whenever either input is gauge. A negative gauge reading is allowed if its absolute pressure remains positive. Atmosphere is never assumed.
Example: suction 2 bar absolute and discharge 8 bar absolute give 200 kPa, 800 kPa and r = 4. The same result follows from 1 bar gauge and 7 bar gauge with an explicitly supplied atmosphere of 1 bar absolute.
The calculation does not provide cooling capacity, power, discharge temperature, an operating envelope, refrigerant/oil compatibility, diagnosis or a replacement model. Pressure sources, measurement uncertainty, operating stability and model limits remain unverified. Displayed results are rounded; extreme values outside finite numeric precision are rejected.