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 6 of 61
1.2 Explanation of Vi (Internal Volumetric Ratio)
In the case of a reciprocating compressor, the volume of the refrigerant sucked into the cylinder
decreases and the refrigerant pressure increases as the piston ascends. When the pressure exceeds
the discharge side pressure plus the force of the spring on the discharge plate valve, the refrigerant in
the cylinder pushes open the valve and passes to the discharge side.
In the case of the screw compressor, a volume of refrigerant is sucked into the groove between the
rotors and the volume decreases while pressure increases as the rotors rotate. The process up to this
point is the same as for a reciprocating compressor. When the volume is decreased to the designed
Vi, the groove is linked to the discharge port and the refrigerant is pushed out. The groove is linked to
the discharge port according to the volume of the groove and is not dependent on internal pressure.
Vi (internal volumetric ratio) is used to represent the value of the decreased volume of suction
refrigerant when the groove aligns with the discharge port (or is discharged).
This can be expressed as follows:
Vi =
Volume of suction refrigerant when compression begins
Volume of same quantity of refrigerant at discharge port
In other words, Vi is the ratio of the groove volume after competition of suction to the volume when the
discharge port opens.
Conventional screw compressors have three fixed Vi values, that is 2.63, 3.65 and 5.80, termed “L
port,” “M port” and “H port,” respectively.
The relationships are:
Vi = (P
d
/P
s
)
1/k
or Vi
k
= P
d
/P
s
Consequently, the Vi corresponding to the compression ratio changes according to the refrigerant
used.
For approximate values, refer to the graph given in Fig. 12. The new V-Series, Maximizer Series
Screw Compressors, are designed so that the Vi can be adjusted on site according to operating
conditions.
1.3 Reasons for Adjusting Vi
Operating conditions of refrigeration systems are not always constant. As well, the same model of
compressor may be operated under a variety of pressure conditions, e.g., air conditioning, cold
storage and freezing applications. In the case of air conditioning and cold storage, the conditions will
vary depending on the need for cooling, heating, low and high temperature.
Needless to say, compressors must be operated at maximum efficiency under various conditions. The
drawback of the conventional compressor is that a fixed Vi is established for the compressor during
production. This Vi can later be changed by machining the compressor but is limited to change from a
higher to a lower value only. Variable Vi screw compressors in the Maximizer Series were developed
as an answer to this drawback. Many compressors of this type are used in special reefer carrier
applications, but because of the sophisticated structure and relatively high cost, they have not been
popular for general applications.
The V-Series, which incorporates a variable Vi, has consequently been developed for these general
applications.
The Vi of the V-Series can be readily changed between L, M and H at the installation plant according
to operating conditions.
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