REFRIGERATION DIVISION
V SERIES
SCREW COMPRESSOR HANDLING MANUAL
Supersedes all previous version. This information is for reference use only and subject to change without notice
Revision 2 (June 05,2000) Page 7 of 61
With the fixed Vi of a conventional compressor, maximum efficiency can only be obtained when the
system is operating at a pressure equivalent to the designed Vi. Unnecessary power is consumed,
however, when pressure conditions diverge from the designed value. For example, if low compression
ratio (high compression pressure or low discharge pressure) operation is carried out using a
conventional M port compressor (designed for a medium compression ratio), compression will exceed
discharge pressure and power will be wasted.
Fig. 8 Internal Volume Ratio Fig. 9 Relationship between design and
operating conditions
Conversely, if the same M port compressor is used under high compression conditions (high suction
pressure or high discharge pressure), the discharge port opens before internal pressure has
increased sufficiently, allowing refrigerant to flow back from the discharge port. Power is also wasted.
Obviously, if a compressor is to be operated for an extended period under varying conditions, a
variable Vi design is preferable to a fixed Vi type. For a conventional compressor with a high Vi, the
discharge port can be machined to lower the Vi but a unit with a low Vi cannot be changed to a high
Vi type. If a higher Vi is needed, the compressor must be replaced with a new one.
1.4 Variable Vi Mechanism
The Vi of a conventional screw compressor is determined by the combination of the axial discharge
port of the rotors on the bearing head and the radial discharge port of the shaft (radial discharge port
on the unloader slide valve). In the case of a conventional model, the axial and radial elements are
combined to exhibit particular characteristics at partial load. In the case of V-Series compressors, the
Vi can be changed by altering the size of the radial port while maintaining the axial port at Vi 5.10.
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