Wheeler On HVACR: The Role Of The Metering Device In An Air-Conditioning System
As I pointed 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 evaporate. 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 discuss the differences between these different types of refrigerant metering systems.<
Capillary TubesFor years, capillary tubes were the standard metering type used on bottom-of-the-line (builder-model) air conditioners. Actually, during the 1960s and 1970s, Lennox had a unique system where the refrigerant liquid line that supplied the evaporator on split systems (where the condenser section and the evaporator section are connected with long refrigerant lines) was precisely sized to serve as a single, long capillary tube. Of course, the pressure drop across this line meant that it was cold near the evaporator connection, so the liquid line had to be insulated (like the suction line).
The advantage that capillary tubes have over orifices is that they do a better job of distributing the liquid refrigerant to each of the evaporator's several parallel circuits, so each circuit is served by its own capillary tube. However, when capillary tubes get clogged (generally caused by varnish accumulations from compressor overheating), the clog runs the length of each tube. And changing capillary tubes for any reason is just out of the question, because the process is so difficult.
Metering OrificesMetering orifices, on the other hand, are pretty easy to clean, and they have other advantages. They can be drilled into pistons which can serve as check valves for heat-pump applications, so the refrigerant can flow both ways and each cycle (heating or cooling) can have its own precisely-sized metering piston. Also, metering pistons can be easily changed so that they can be precisely matched to different evaporator designs.
However, metering pistons have their drawbacks, because properly distributing the refrigerant to multiple evaporator circuits becomes tricky at best. And some manufacturers have done a very poor job of this in the past, meaning that their systems didn't really work at rated capacity. There is a special problem with proper refrigerant distribution when evaporator coils are designed for installation in several positions. Gravity tends to affect which way the heavier liquid flows and which way the lighter flash-gas flows, and this can create problems.
Thermostatic Expansion ValvesTXVs are the Cadillac of metering-device designs for smaller systems. They have a sensing bulb that is strapped to the line leaving the evaporator. This input regulates the refrigerant flow to keep the evaporator temperature working at optimum conditions while providing maximum compressor protection. A TXV senses the refrigerant pressure in the evaporator (its saturated temperature) and the refrigerant-filled bulb senses the temperature of the superheated refrigerant leaving the evaporator. So a TXV ensures that there is always about 15 degrees F of superheat on the suction (return) line.
TXVs also have drawbacks. Refrigerant has to be properly distributed to the individual evaporator circuits, yet manufacturers have always done a better job of designing refrigerant-distribution circuits when using TXVs. In addition, while capillary tubes and metering orifices allow refrigerant pressure differences to equalize inside the system on shutdown, TXVs usually don't allow for such pressure equalization. As a result, most TXVs require special hard-start circuitry to be used with single-phase compressors.
Another consideration when using TXVs is that the internal refrigerant pressure leaving it doesn't always represent the actual saturation temperature of the evaporator. This is because there is often a significant pressure drop across the device that distributes the refrigerant to the evaporator circuits. So TXVs for HVAC systems are usually externally equalized - that is, they have an extra line which runs to the end of the evaporator to sense the true pressure there.
Electronic Expansion ValvesElectronic Expansion Valves have made headway into commercial-product designs over the past 20 years. Their cost, which usually includes sophisticated digital circuitry, is usually too high for use in smaller systems. However, the precise refrigerant flow that they afford and their extremely rapid reaction to changing conditions make this type of metering the best and most efficient available.
EXVs, as they are often called, use precise temperature sensors (one for saturation and one for superheat) to ensure that the evaporator is always filled with the maximum amount of saturated refrigerant, while still affording precise compressor protection. In fact, some designs put the superheat sensor inside the compressor itself. In addition, since commercial systems are usually supplied by 3-phase power, which doesn't require hard-start compressor circuitry, the metering can also be designed to serve as a solenoid valve to prevent refrigerant from draining to the compressor intake during shutdown. So, other than the fact that there are more things to go wrong and that they are expensive, EXVs don't have many drawbacks. They are perfect for any job.
So, which of the above do I recommend? Each has its own place and best use. Of course, everyone would like to have their system come with a TXV or an EXV, but they don't usually want to pay the extra price. Yet, when you do pay the premium for a top-of-the-line system, you should expect a premium metering device.
And when it comes to the choice of metering pistons vs. capillary tubes, each has its own drawbacks. So, I would choose whichever one the manufacturer recommends to meet its rated specifications. And no, for that reason, I don't recommend mismatching evaporator and condenser brands, because then only God knows at what efficiency the resulting system conglomeration will operate.