KT-420-4 // PDF Download
37
DANGER
Safety reference to avoid an imminently hazardous situation which may result in death or serious injury.
In addition to the safety references listed in this document, it is essential to observe the references and residual
risks in the respective operating instructions!
3
Operation with frequency inverter
3.1
Cooling capacity and system efficiency
Mechanical capacity control
The cooling capacity of a reciprocating compressor can be mechanically adapted to the cooling demand of the sys-
tem, e.g. by:
• blocked suction port
• blocked suction-side gas flow to individual cylinder banks
• in compound systems also by switching individual compressors on and off
The compressor is operated at a constant speed, the motor speed correlates directly with the mains supply fre-
quency. This results in the following nominal speed for 4-pole asynchronous motors:
• 1450 min
-1
at 50 Hz and
• 1750 min
-1
at 60 Hz.
Capacity control with frequency inverter
The average load torque at the compressor shaft mainly depends on operating conditions and refrigerant proper-
ties. Thus, it remains approximately constant over a wide range of speed / frequency. Cooling capacity and power
consumption therefore vary approximately proportional to the speed (see graph below), cooling capacity can be
steplessly adapted via speed control. Permitted speeds / frequencies for BITZER compressors are given below
(Speed and frequency ranges of BITZER reciprocating compressors).
0
140
80
40
20
100
60
0
10
20
30
40
50
60
120
f [Hz]
Q
0
[%]
1 0
6
0
0
250
850
550
1450
2050
n [min
-1
]
1150
1750
2350
70
80
Fig. 3: Typical graph of the cooling capacity Q
0
depending on the speed and frequency of reciprocating compressors
