KT-420-4 // PDF Download
40
Application limits
The following figure shows exemplarily the application limits of a reciprocating compressor operated at different fre-
quencies 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.
10
60
50
40
30
20
20
140
120
100
80
60
40
p [bar]
C
p [bar]
o
t
= 10 K
oh
t [°C]
o
20
15
10
5
0
-10
-20
-30
50 Hz
60 Hz
70 Hz
Fig. 4: Example for application limits for reciprocating compressor 4MTE-10 with refrigerant R744 (CO
2
) and frequency inverter at 50, 60 and 70
Hz (limits due to motor temperature or maximum current). The compressor may only be operated below the indicated frequency lines (other-
wise, select a larger inverter for a wider range of the application limit).
t
o
: evaporation temperature, t
c
: condensing temperature, Δt
oh
: suction gas superheat
Dark grey area: mind operating conditions.
Vibrations
Compressor vibrations and pressure pulsations are normally very low by design. However, they can evoke reson-
ance frequencies in piping and heat exchangers (i.e. meet the natural frequency of the system) which result in sys-
tem sound, vibration, and potentially in piping fatigue and leakage. Possible sources of vibrations are:
• gas pulsations in the discharge 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 com-
pressor, both during the system design development activity and also at commissioning of each individual system
(
).
4
Selection
4.1
Selection with the BITZER SOFTWARE
Information
At present, the BITZER SOFTWARE offers calculations with frequency inverter only for semi-hermetic com-
pressors.
