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After the refrigerant leaves the condenser, (covered in an earlier post), the refrigerant is delivered, via the liquid line, to the metering device.
The metering device has two jobs, provide the necessary pressure drop, and provide the correct amount of refrigerant to the evaporator to handle the design load.
‘Design’ is in bold italic because the metering device, no matter the type, is matched to the load conditions and will only perform properly in a small operating window.
I like to lump the metering devices into two categories, fixed metering devices, and dynamic metering devices.
Fixed metering devices operate in a very small range and are extremely susceptible to operating conditions. The basic design of a fixed metering device is based on a restriction of the flow causing a pressure drop in the line while maintaining enough of a through put that some refrigerant makes it into the evaporating coil.
Fixed metering devices are inexpensive solutions but also inefficient and have limited capability. You will likely see a decline of fixed metering devices as the demand for more energy efficient design rises.
Fixed metering devices do not respond to the load per say, only to the differential pressure between the condenser and the evaporator.
Some fixed devices rely on an orifice, or ‘seat’ and an adjustable ‘trim’ which is a tapered stem that, when held at the seat, creates a restriction that is adjustable.
Another type of fixed metering device is the capillary tube, a very common type of metering device in small appliances. The capillary tube is a long length of tubing with a small diameter bore inside. The small bore and length, the bore determines how much refrigerant will pass, while the length, determines the pressure drop. Word of caution, if you cut a capillary tube, its ruined, the length and continuous, unobstructed bore, are critical! Patching it back together will change the characteristics of the tube, usually resulting in a very different performance.
A third type of fixed device is an orifice tube, commonly found in cars. The orifice tube is a metal tube that sometimes contains a removable inlet screen and a plate with a very small hole in it (the orifice), the very small hole controls the amount of refrigerant and causes the needed pressure drop. Due to the itty-bitty hole, the orifice tubes plug up easily. Best way to spot this is to look for frost just after the tube or temperature below the expected evaporator temperature. (Assuming the unit has the proper charge in it.)
A fourth, yes there’s another, is the piston orifice, the piston contains the small hole just like the plate in the orifice tube, but the piston is free floating in a large close coupled nut assembly. The piston moves forward during operation, then relaxes when flow stops, this action allows for the system to equalize the high and low side pressures lowing the compressor starting torque. Warning, these assemblies are usually brass brazed onto copper tube, they will gal, strip, cross thread, and twist the tube if not properly serviced, never force it together, keep it straight, do not over-tighten, and ALWAYS use a backup wrench to hold the swivel nut!
Below are some sketches of the basic configurations of fixed metering devices:
Dynamic Metering Devices try to adapt to changes in load by one of several means, pressure, level, or temperature.
Pressure controlled metering devices are called automatic expansion valves. The valve is an adjustable spring loaded diaphragm against the evaporator pressure with a tension that corresponds to the middle of the expected range of operation. When the evaporator pressure fluctuates with load, the valve can open or close to compensate within a narrow range. These were found a lot in old window, PTAC, and purge units, and look like right-angle valves with a silver tank on top, the adjustment was usually a slotted screw sticking out the top. Note; if it leaks around the screw, its bad.
Metering devices that respond to level changes are used only in flooded evaporator arrangements and are usually configured with a float to respond to liquid level changes. (There were some electronic level sensors but they never became very popular.) The float is installed in a ‘side’ chamber of the evaporator, one for serviceability, and second, for level stability. As the level went up or down the float would move up or down and either feed refrigerant in, or cut it back. Floats could also be used to send a signal to a pneumatic or electronic controller that could actuate a remote valve to achieve the same purpose. The pivot pins in these designs wear very quickly and periodic replacement is a must. Most side chambers could be isolated and re-built, but some units must have the entire charge removed to work on the float.
Temperature controlled metering devices are called thermostatic expansion valves and are used in air-conditioners, spot coolers, window units, chillers, refrigerators, freezers, etc. The most common temperature controlled expansion valve (TXV) is controlled via a sensing bulb full of a captured refrigerant charge. The bulb senses the temperature of the suction line. An increase in suction temperature opens the valve and increases the refrigerant flow to the evaporator, a decrease in the suction temperature cuts back the valve due to the expansion and contraction of the captured refrigerant in the bulb and power assembly. There are both balanced (pressure compensated) and unbalanced (spring only compensated) valves.
EXV, or electronic expansion valves, are thermal expansion valves with motors attached instead of power assemblies. They have a controller that senses the super-heat and adjust the valve accordingly. EXV valves can modulate the refrigerant flow very accurately and are the most efficient solution for many applications.
I worked for over thirty years in the HVACR industry. I have designed, installed, serviced, and trouble shot units of various types throughout the years. The posts here are information based on that experience, I hope you find them useful. If you have a different experience, please comment.