OMM WCT
Centrifugal Chillers
D-EOMWC01302-16EN - 7/84
,
Figure 3, Two-Stage Vapor Compression Refrigerant Cycle (valid for each circuit)
2
nd
Stage
Expansion
Valve
Condenser
1
st
Stage
Evaporator
Economizer
Subcooler
Capacity Control System
The motor-driven
inlet guide vanes (IGV) located at the entrance to the compressor first stage impeller
control the quantity of refrigerant entering the impeller thereby controlling the compressor capacity in
the first-stage. There is a motor driven variable diffuser geometry or discharge diffuser control (DDC )
in the second stage. Both IGV and DDC is well-related to keep wide range of efficient operation without
surge.
The operating range is from 20% to 100% of the specified nominal capacity depending on system water
temperatures.
The major components of the two-stage system are evaporator, condenser, expansion valve, economizer
and compressor.
There are two compression stages in a single compressor by two impellers mounted on a single common
shaft within a single casing. The discharge of one stage feeds the input of the next stage.
The driveline of each of the WCT is made up of one two-stage compressor, gear train and a 2 pole, 3
phase squirrel cage induction semi-hermetic motor.
Condenser/Subcooler
The condenser is a shell-and-water tube heat exchanger with the refrigerant in the shell side, where
gained heat from evaporator and compressor is rejected out of chiller system.
Starting at the inlet of the condenser, the high-pressure, high temperature refrigerant vapor is forced into
the condenser and as it passes through the condenser shell, it gives up its latent heat of condensation,
heat of compression and other heat absorbed to the cooling tower water flowing inside the condenser
tubes i.e.
Isobaric/Isothermal ( constant pressure, temp ) Heat Rejection process.
The decrease in the
refrigerant’s latent heat content equals the increase in the water’s sensible heat.
This heat removal
changes the phase of the refrigerant and it becomes liquid at constant pressure and temperature.
As the condensed refrigerant liquid enters the condenser’s internal subcooler at the bottom of the
condenser, just before it leaves the condenser, it loses its sensible heat further and becomes subcooled
liquid at a lower temperature due to heat transfer to the water in the subcooler tubes. Allowing any
refrigerant vapor to enter the subcooler decreases the efficiency of the subcooler because the rate of
convection heat transfer in the vapor phase is much less than in the liquid phase. Further, allowing