Introduction
A frequency inverter allows the stepless control of the refrigerating capacity to the cooling demand of the system by speed control. The following guidelines explain the design, operation, application range and special characteristics of
- Bitzer reciprocating compressors
- in combination with external frequency inverters for speed control, e.g. the Bitzer Varipack.
All Bitzer reciprocating compressors are suitable for operation above and below the supply frequency and can thus run over an exceptionally broad capacity range.
Features of the operation with a frequency inverter (FI):
- higher system efficiency, especially in part load
- more exact temperature control possible
- precise coolant temperature for sensitive process cooling resp. heat-transfer fluid temperature for heat pumps
- higher effective evaporation temperatures, therefore less dehumidification of unpacked foods and raw material in cold stores, as well as less icing at the evaporator
- fewer compressor starts
- less strain on the motor and the power network due to integrated soft start: starting current is lower than with direct start, softstarter, star-delta or part-winding start
- higher refrigerating capacity often possible by operation above supply frequency (allows use of a compressor with lower displacement at supply frequency 50 or 60 Hz, i.e. possibly lower costs per kW refrigerating capacity)
The figure below shows the smaller temperature fluctuations with frequency inverter control:
- On/Off regulation, left third: large temperature fluctuations, relatively low average effective evaporation temperature (fine dotted line)
- Stepped mechanical regulation, middle third: reduced temperature fluctuations due to faster regulation, higher average effective evaporation temperature and thus higher efficiency
- Regulation with frequency inverter, right third: very steady room resp. coolant temperature (± 0,5 K possible) due to stepless control, higher average effective evaporation temperature and thus higher efficiency as well as e.g. lower dehumidification of unpacked foods and raw material
With frequency inverter operation, the average evaporation temperature can be raised e.g. from -7 to -4.5°C. Rising the evaporation temperature by 1 K increases system efficiency by up to 3%.
Refrigerating capacity as a function of the load is illustrated in the following graph. The frequency inverter has advantages especially in part load.
Dotted line: CR control.
Solid line: FI capacity control.
Operating conditions: R449A, tO: -10°C / tC: 45°C / ΔtOH: 20 K
While the optimum capacity of the CR control is always at 100%, with FI it is achieved in the part load range and can be adjusted by compressor design and control range. Due to the conflicting goals of high full load COPr/EER and high part-load efficiency, a compromise is necessary.
Also observe the following technical documents
- KB-100: Operating Instructions Semi-hermetic reciprocating single stage compressors
- KB-120: Operating Instructions Semi-hermetic reciprocating compressors for subcritical R744 applications
- KB-130: Operating Instructions semi-hermetic reciprocating compressors for transcritical R744 applications
- KB-150: Operating Instructions Semi-hermetic reciprocating two stage compressors
- KB-520: Operating instructions Open drive reciprocating compressors
- CB-110 and CB-111: Operating instructions Varipack - external Bitzer frequency inverters
- AT-640: Use of ammonia (R717) in Bitzer compressors
- AT-744: Application guide for the use of R744