Application range
For a safe compressor operation with frequency inverter, the following limiting factors must be strictly observed:
- minimum and maximum frequency (see below)
- maximum motor temperature
- maximum discharge gas temperature and pressure difference (pC – pO)
- maximum and minimum discharge pressure
- maximum operating current of the compressor
- maximum evaporating temperature
- minimum pressure difference (pC – pO)
- minimum suction pressure
These limits define the application limits (see below) and can vary according to frequency ranges and operating conditions.
Speed and frequency ranges
Minimum frequency is 35 resp. 44 Hz: This has been demonstrated as the minimum safe speed required to deliver adequate lubrication to the compressor bearing system. Running at lower speeds risks loss of lubrication and bearing failure.
The maximum frequency is due to the increasing centrifugal forces which affect the mechanial stability of the scrolls.
Compressor | Frequency range (Hz) | Speed range (min-1) |
|---|---|---|
ESH7 | 35 .. 65 | 2000 .. 3800 |
ELH7 | 35 .. 65 | 2000 .. 3800 |
ELA7 | 35 .. 65 | 2000 .. 3800 |
ELV21 | 44 .. 140 | 2500 .. 8000 |
ELV52 | 44 .. 130 | 2500 .. 7500 |
ORBIT | ||
GED8 | 35 .. 60 | 2000 .. 3500 |
GSD6 .. GSD8 | 35 .. 75 | 2000 .. 4400 |
GSU6 .. GSU8 | 35 .. 75 | 2100 .. 4500 |
ORBIT tandem / trio | see below (Tandem and trio operation) | |
Please consult Bitzer for external VSD application guidelines.
Application limits
In general, compressor models approved for operation with frequency inverter have undergone complete load cycling testing at minimum and maximum speeds. Normally, the application limits correspond to the fixed speed limits published e.g. in the Bitzer Software. However, some special considerations may require restrictions at certain speeds.
The following figure shows an example of application limits for an Orbit compressor at different frequencies. The compressor may only be operated below the indicated frequency lines ‒ otherwise, select a special voltage motor (Special voltage motors).
to: evaporation temperature, tc: condensing temperature, Δtoh: suction gas superheat
Grey area: limitations for frequencies < 50 Hz possible.
Specific application limits for particular compressors, motors and refrigerants are given in the BITZER Software or available on request.
Vibrations
Compressor vibrations and pressure pulsations are normally very low by design. However, they can evoke resonance frequencies in piping and heat exchangers (i.e. meet the natural frequency of the system) which result in system sound, vibration, and potentially in piping fatigue and leakage. Possible sources of vibrations are:
- pressure pulsations in the discharge gas line
- torque vibrations acting on the compressor mountings or on the flanges of line connections
- resonances with the economiser line (for screw and scroll compressors)
The frequency of these vibrations is related to the compressor operating frequency, which can vary over a wide range. Compared to single speed systems (without frequency inverter), this problem is intensified in variable speed applications: Even if the piping is adequate at a given compressor speed, it may not be so at other speeds set by the frequency inverter. For this reason, piping vibrations must be checked throughout the speed range of the compressor, both during the system design development activity and also at commissioning of each individual system (Configuration of the frequency inverter).
Data from sound measurements for each compressor is provided in the Technical Information AT-340 and in the Bitzer Software, tab "Technical data".