2205Q2JE-HO-S6-N_2020.01.
Chapter 2 Compressor Specifications and Structure
SCV-series Screw Compressor 2.4 Compressor Structure and Mechanism
2-16
Conversely, if the same compressor is used for a high compression-ratio application (an application
with large difference between suction and discharge pressures), the discharge port will open before
the refrigerant gas pressure has risen to the necessary level. This would cause the refrigerant gas
in the outlet piping to flow back through the discharge port as shown in the lower left graph of Figure
2-13 unless the flowing-back gas is overcome by driving the compressor using extra power.
In summary, the greatest benefit provided by properly setting the Vi value is that the rotor driving power
(brake horse power) is made optimum for the load. The refrigeration capacity generally changes little
even if the Vi is varied. However, the efficiency of the brake horse power becomes the maximum and
the loss of power is minimized if the Vi is optimally adjusted.
The performance curves in below Figure 2-12 show the relationship between the refrigeration capacity
and the brake horse power. The curves indicated by thick solid lines in the graph represent the brake
horse powers most efficient for achieving the specific refrigerating capacities. In a compressor with a
fixed Vi value, each brake horse power curve leaves away from the ideal curve in some refrigerating
capacity areas. On the other hand, it is possible for a variable Vi compressor to set the brake horse
power in the vicinity of the ideal value.
As for how to change the Vi settings, refer to Section 4.1 in this manual Chapter 4.
Figure 2-12 Refrigeration Capacity of Screw Compressor
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