2205Q2JE-HO-S6-N_2020.01.
Chapter 2 Compressor Specifications and Structure
SCV-series Screw Compressor 2.4 Compressor Structure and Mechanism
2-15
Figure 2-11 Variable Vi Mechanism of SCV-series
2.4.3.3 Why the Vi Needs to Be Changed?
The advantages that can change the Vi settings of the compressor are as follows:
[1] You ca set the Vi that is suitable for use refrigerant.
Internal volume ratio Vi is expressed as follows with the compression ratio.
Pd: discharge pressure, Ps: suction pressure,
κ: ratio of specific heats (a constant specific to each refrigerant).
As seen from the formula, the Vi corresponding to a certain pressure ratio varies with the type of
refrigerant gas. If you can change the Vi settings, you will be able to apply to a variety of refrigerant
using a single compressor.
[2] You can operate the compressor most efficiently under varying conditions.
Compressors of the same model will be used for a variety of applications whose load conditions are
different, e.g. air conditioning, cold storage, and freezing. The compressor, however, can be
operated most efficiently when the Vi is matched with the load condition.
For example, if a conventional M port compressor with the fixed Vi is used for a low compression
ratio application (an application with small difference between suction and discharge pressures), a
pressure higher than the necessary pressure will be reached before the discharge port opens as
shown in the lower right graph of Figure 2-13 in next page. This means that power will be used
wastefully for unneeded compression.
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