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 or oil temperature and/or pressure difference (p_C – p_O)
maximum and minimum discharge pressure
maximum operating current of the compressor
maximum evaporating temperature
minimum pressure difference (p_C – p_O)
minimum suction pressure (should be slightly higher than atmospheric pressure)
minimum refrigerant mass flow for motor cooling etc.
sufficient oil supply for sealing in profile area
sufficient additional cooling

These limits define the application limits and can vary according to frequency ranges and operating conditions.

Speed and frequency ranges

Permitted speed and frequency ranges of Bitzer screw compressors (also observe the application limits and the maximum current consumption of the motor)
Compressor	Frequency range (Hz)	Speed range (min^-1)	Remarks
HS.53 .. HS.85	20 .. 75	1200 .. 4400
HSNP74, HSNP85	20 .. 70	1200 .. 4100
HS.9573 .. HS.9583	20 .. 70	1200 .. 4100
HS.9593 .. HS.95103	20 .. 60	1200 .. 3500
CS.65 .. CS.105	25 .. 60	1450 .. 3500	On request larger ranges may be possible in certain cases. Version with integrated FI: CSV.
OS.53	25 .. 75	1450 .. 4500
OS.74 .. 85	25 .. 67	1450 .. 4000
OS.95	25 .. 67	1450 .. 4000
OS.105	26 .. 67	1500 .. 4000

Design at different supply voltages and supply frequencies

If power supply deviates from standard conditions (400 V/3/50 Hz), special voltage motors and adapted frequency inverter design are required (Compressor motors). More information is available upon request.

Application limits for operation with frequency inverter

The following figure shows exemplarily the application limits of HS. compressors operated at different frequencies and how they may change with evaporation and condensing temperature. Specific application limits for particular compressors, motors and refrigerants are given in the Bitzer Software.

Example for application limits for screw compressors HS.64 with frequency inverter and refrigerant R404A. The compressor may only be operated below the indicated frequency lines.to: evaporation temperature, tc: condensing temperature, Δtoh: suction gas superheatDotted grey lines (20 .. 35 Hz): Limits depending on motor temperature.Solid coloured lines (60 .. 75 Hz): Limits due to motor temperature or maximum current. — Example for application limits for screw compressors HS.64 with frequency inverter and refrigerant R404A. The compressor may only be operated **below** the indicated frequency lines.
t_o: evaporation temperature, t_c: condensing temperature, Δt_oh: suction gas superheat
Dotted grey lines (20 .. 35 Hz): Limits depending on motor temperature.
Solid coloured lines (60 .. 75 Hz): Limits due to motor temperature or maximum current.

Similarly, the following figure shows exemplarily the application limits of CSH compressors. Specific application limits, motors and refrigerants are given in the Bitzer Software. For CS. compressors, Bitzer has developed the special series CSV. with integrated frequency inverter, where compressor and FI complement each other optimally:

SB-160: Operating instructions Semi-hermetic compact screw compressors with integrated frequency inverter CSV.

Example for application limits for screw compressors CSH with frequency inverter (FU) and refrigerant R134a. The compressor may only be operated below the indicated frequency lines.to: evaporation temperature, tc: condensing temperature, Δtoh: suction gas superheatDark grey area top left: oil cooling required.Light grey area top left: additional cooling required.Dotted grey lines (25 .. 35 Hz): limits depending on motor temperature.Operation at 60 Hz according to individual design.White area bottom right: CR max. 75%. — Example for application limits for screw compressors CSH with frequency inverter (FU) and refrigerant R134a. The compressor may only be operated **below** the indicated frequency lines.
t_o: evaporation temperature, t_c: condensing temperature, Δt_oh: suction gas superheat
Dark grey area top left: oil cooling required.
Light grey area top left: additional cooling required.
Dotted grey lines (25 .. 35 Hz): limits depending on motor temperature.
Operation at 60 Hz according to individual design.
White area bottom right: CR max. 75%.

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".