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Air Conditioning Basics: A/C 101
The Thermal Expansion Valve: Superheat
After leaving the Filter/Dryer, the
refrigerant/oil mixture then flows to the Thermal Expansion Valve (TXV),
again through leak-tight plumbing lines and fittings. The TXV, generally
fitted directly to the Evaporator, is merely a more sophisticated
version of the Expansion Valve shown in Figure 1. It is called a
"Thermal" expansion valve because the device senses the temperature of
the chilled air or refrigerant associated with the Evaporator and
automatically and continually adjusts its internal orifice size (and,
therefore, refrigerant flow rate) to maintain optimum Evaporator
effectiveness and also to protect the Compressor from ingesting
un-evaporated liquid.
Specifically, the job of the TXV is to maintain as much of the
internal surface of the Evaporator in contact with boiling (evaporating)
cold refrigerant as possible, but also to make sure that all of the
refrigerant has evaporated into vapor form before it leaves the
Evaporator. Further, as a safety factor, the TXV must control the flow
of refrigerant to the Evaporator so that at the very end of the
evaporation process, there is enough "warm" evaporator surface to heat
the departing refrigerant gas above its boiling point by, say, 10 Fº.
This vapor-heating process is known as "superheating" and is important
primarily to insure that liquid refrigerant does not enter the
compressor and cause hydraulic damage through attempting the compression
of a liquid. In the most simple terms, the TXV avoids complete
"flooding" of the Evaporator so there is some room left for superheating
the refrigerant gas.
Specifically, the TXV functions by moving a small tapered needle stem
in and out of a companion passage in response to a differential
pressure. The differential pressure arises from both the pressure
exerted by the Sensing Bulb (which contains refrigerant) and the
pressure within the Evaporator itself. This differential, in addition to
commands from the Control, is what determines the physical size of the
metering orifice and, therefore, the actual amount of flow of
refrigerant to the Evaporator.
As indicated earlier, when the refrigerant flows through the
Expansion Valve (here, the TXV), it undergoes a very significant
reduction in pressure. This process results in some of the warm liquid
refrigerant actually bursting into the vapor phase ("flashing") as it
passes through the valve. As this occurs, the remaining and now low
pressure and liquid refrigerant becomes quite cold. This cold and mostly
liquid refrigerant then flows directly into the Evaporator.
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