FUNCTION OF EXV in a system with flooded evaporator.
Most technicians are familiar with the application of an expansion
valve (mechanical or electronic) where the expansion valve is
controlling the quality of the suction gas to the compressor.
Nominally, the expansion valve is set to maintain a certain level of
superheat in the suction gas. The valve monitors suction gas temperature and compressor suction pressure, using this relationship to adjust expansion valve opening as appropriate to maintain proper suction gas conditions at the compressor.
Conventional direct expansion systems must be set up to maintain
something like 7 to 10 degrees C (13 to 18 degrees F) of superheat in
the suction gas, in order to make absolutly sure that no liquid
enters the compressor. The superheat level is that heat above
the saturated temperature of the gas at the working suction
pressure. More superheat reduces capacity and efficiency of the
system. On an operating system, due to variations in load on
the evaporator a superheat of the level indicated is necessary to
make sure that, as the load varies and the valve modulates to adjust
to the varying load, the superheat does not go to ¨0 ¨ and incur
the risk of a liquid slug to the compressor which could, in turn,
cause serious damage to the compressor. The design of the
system is normally such that, once the liquid passes through the
expansion valve, the volume enclosing the 1iquid is increasing,
causing the liquid to expand, changing to a gas and absorbing heat
in the evaporator coil. This expansion process takes place in
the evaporator, absorbing heat from the air (or liquid as the case
may be).
In the case of a Turbocor compressor chiller with a flooded
evaporator, the function of the expansion valve (normally an
electronic expansion valve) is to maintain the correct liquid level
in the evaporator.
In the case of an air cooled unit, there is a liquid level sensor
on the evaporator and the controls use a liquid level setpoint in
combination with the actual liquid level being measured to operate
the expansion valve. Actual liquid level higher than the
setpoint results in closing the valve to reduce liquid flow into the
evaporator, lowering the liquid level. Actual liquid level
lower than the setpoint results in opening the valve to increase
liquid flow to the evaporator, raising the liquid level. The
controller normally has a control algorithm that the action of the
expansion valve is ¨smoothed¨ to prevent large swings in the
liquid level.
In the case of a water cooled unit, the expansion valve controls
the level of the liquid in the condenser which, indirectly controls
the liquid level in the evaporator. When the chiller is correctly
charged and the liquid level setpoint of the condenser correctly set
and maintained, the liquid level in the evaporator will be correct
for the operation of the system.
With a flooded evaporator, the heat exchanger tubes are only in
the bottom half of the shell. The top half of the shell is
open, frequently with baffles of some type. In operation, the
liquid refrigerant fills the bottom half of the shell, just covering
the heat exchanger tubes. This design is such that liquid
refrigerant droplets will not be pulled off the surface of the
liquid refrigerant and into the suction of the compressor. The
change of the liquid refrigerant to a gas occurs at the tube surface
as heat is transferred from the liquid being cooled (normally the
chilled water), to the refrigerant. The gas bubbles formed at
the tube surface rise within the liquid refrigerant in the
evaporator, leaving the surface of the liquid refrigerant as gas.
In
the Turbocor compressor based chiller, the suction superheat is
precisely controlled by the compressor itself, by adjusting
compressor rotation speed. In this manner, suction superheat
can be maintained at less than 1 degree C, significantly increasing
both capacity and efficiency of the system.
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