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Here are
descriptions and
comparisons of the
different types of
refrigerant
metering systems.
Appointed out in the first of this series of articles on air-conditioning systems, the metering device is just a restriction in the system. It creates the necessary pressure differences to allow the refrigerant on the system's high-pressure side to condense into a liquid, and yet is low enough on the low-pressure side to allow it to evapopate. That means it must keep the evaporator saturation pressure above the freezing temperature, but still be low enough to provide proper cooling and dehumidification.
The metering device also has the job of making sure that there is enough refrigerant flowing to the evaporator to ensure that it is almost fully filled with saturated refrigerant, while protecting the compressor by preventing liquid refrigerant from returning to its intake. And that is a pretty tall order, considering all the variations in indoor and outdoor conditions that the system will see during its lifetime.
Now, you would expect the metering device to be pretty complicated, considering how important a role it plays in an air-conditioning system, but this is the first place where manufacturers usually look to cut costs. As a result, most residential and light commercial systems have metering devices that are no more than an orifice, a precisely drilled hole in a brass piston. Or they may use capillary-tube metering, which is just a fancy description for a piece of thin copper tube that is cut to the proper length to provide the best average restriction.
However, better metering devices are also available, and they are almost always used on higher-efficiency installations. The long-time standard of these has been the Thermostatic Expansion Valve (TXV or TEV). And in larger, more expensive systems it is now quite common to see electronic expansion valves. So now let's...