OMM WCT
Centrifugal Chillers
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vapor to enter the subcooler may allow vapor to leave the condenser, thereby decreasing the efficiency
of the system. Therefore, the liquid level must extend far enough above the subcooler entrance to
prevent vapor within vortex, which is typically formed at high flow rates, from entering the subcooler.
When the chiller is operating at load, the most reliable source of liquid refrigerant is the condenser.
Liquid refrigerant in the evaporator will be boiling.
Expansion Valve
The liquid refrigerant travels through the liquid line to the expansion valve where the pressure is
reduced and part of the refrigerant flashes into vapor creating of a two-phase refrigerant mixture
downstream of the expansion valve. The vapor absorbs the liquid’s latent heat of vaporization and
lowers the liquid temperature flowing to the economizer. Therefore, the net latent heat content of the
refrigerant is unchanged or no heat loss to the outside due to this heat exchange between the liquid and
vapor.
This is the first expansion process in the two-stage cycle. Since the decrease in the refrigerant liquid’s
sensible heat content equals the increase in the refrigerant vapor’s latent heat of vaporization, the total
enthalpies before and after expansion are the same. This part of the refrigeration cycle is called the
Isenthalpic – constant enthalpy Expansion process.
Economizer and Two-stage Compressor
An economizer is a flash tank consisting of baffles to separate the refrigerant vapor from liquid and
mechanical float-type expansion valve(s) for liquid control. Economizer separates any vapor, mainly
generated at primary expansion valve, and preventing such vapor feed it into evaporator. Vapor in the
economizer may no longer work for cooling water in the evaporator, but 1
st
compression stage of
compressor must work for the vapor once it is provided into evaporator which cause loss of
compression power. Thus economizer promises efficient chiller operation than without economizer
system, which is one of the most advantage of 2-stage with economizer system.
The expanded liquid-vapor mixture from the expansion valve enters the economizer where vapor and
liquid separate from each other.
The liquid, being denser than the vapor, accumulates at the bottom of the economizer and the vapor
bubbles through liquid refrigerant to the top of the economizer. As the second stage impeller of the
compressor exerts suction and draws vapor from the economizer, it reduces the pressure of the
economizer. As the pressure is lowered, so is the temperature or boiling point of the refrigerant in the
economizer to a temperature corresponding to the suction pressure of the second stage. This vapor is
piped to the inlet of the second stage impeller, thereby maintaining the economizer at interstage
pressure.
As the liquid level rises, it lifts the float valve and opens it. The liquid exiting the float valve opening
expands second time and its pressure drops further and more latent heat is absorbed by the vapor from
liquid – a zero heat loss from the refrigerant to the ambient, and lowers the refrigerant liquid’s
temperature further flowing into the evaporator.
Because of this additional refrigerant liquid temperature drop, there will not be a capacity penalty to the
chiller even though the mass flow of the refrigerant is less than a single-cycle system compared with the
same capacity. The shrinkage of refrigerant mass, on the other hand is, due to the refrigerant vapor
separating in the economizer and leaving the economizer to the inlet of the second stage suction of the
compressor where it mixes with and desuperheats the discharge vapor from the first stage impeller. The
two incoming refrigerant vapor streams mix together in the interstage elbow before flowing into the
second stage compression ( Isentropic ( constant entropy ) compression process ).
As the economizer vapor decreases the temperature of the first stage discharge vapor entering the
second stage, it reduces the required compression energy input by utilizing energy that would otherwise
be wasted which gives an efficiency advantage to the two-stage over single-stage.
