The metering device performs two important functions in a refrigeration system. It matches the incoming flow rate of refrigerant to the rate the refrigerant boils off and it provides a pressure drop between the high and low sides of the system.
Heat is removed from the air passing over the evaporator because the evaporator is at a lower temperature than the air.
The metering device, located between the condenser and evaporator, converts the high-pressure, high-temperature refrigerant from the condenser into a low-pressure, low-temperature mixture of liquid and vapor refrigerant.
In direct expansion (DX) evaporators, the refrigerant mixture entering the evaporator is about 75% to 80% liquid refrigerant and 20% to 25% vapor.
Larger systems use flooded evaporators, where only liquid enters the evaporator.
The metering device must allow just enough liquid refrigerant to enter the evaporator and completely boil off before it exits the coil. No liquid refrigerant should leave the evaporator.
If all liquid refrigerant does not boil off it can flood-back to the compressor, diluting the lubricating oil and causing increased compressor wear.
If liquid enters the compressor cylinders, a condition known as slugging, it can severely damage the compressor.
Expansion valve metering devices can vary the amount of refrigerant entering the evaporator based on load variations. Fixed metering devices are less responsive to changes in the cooling load.
Fixed metering devices include capillary tubes and fixed-orifice metering devices. The amount of refrigerant flowing through the orifice depends on head pressure.
On hot days, higher head pressure forces more refrigerant through fixed metering devices resulting in lower superheat. On cooler days, the opposite occurs.
One drawback of fixed metering devices is that they do not stop liquid refrigerant from migrating to the evaporator during the off cycle. This can lead to refrigerant flood-back and slugging.
Another drawback is that fixed metering devices cannot adjust to changes in load or other variables
The capillary tube consists of a length of small-diameter copper tube.
The amount of refrigerant flowing through the tube is based on its inside diameter and length.
If the capillary tube is too short or the inside diameter too great, too much liquid refrigerant will pass through, potentially damaging the compressor.
Capillary tubes quickly equalize system pressure after compressor shutdown, reducing the need for compressor start-assist components.
Capillary tubes are delicate and must be handled with extreme care.
Cap tubes must not be cut with tubing cutters or a hacksaw. Use a special cap tube cutter or a file.
A fixed-orifice metering devices consists of a body and removable piston. Their main advantage over capillary tubes is size and ruggedness.
For split-system applications, the correct metering piston is shipped with the condenser and must be field-installed in the evaporator. Multiple metering orifices are sometimes used in packaged equipment.
Fixed-orifice metering devices can be taken apart easily for cleaning or replacement.
Fixed-orifice metering devices are widely use in heat pumps because they eliminate the need for check valves.
Expansion valves increase or decrease the flow of refrigerant into the evaporator coil based on the cooling load.
A manual expansion valves uses a hand valve to adjust refrigerant flow.
Manual expansion valves are effective on systems with a constant design load. However, they must be monitored for changing conditions.
High-side float valves are installed on the high side of the system and feed refrigerant to the evaporator based on the level of liquid in the float chamber.
As the cooling load increases, more liquid leaves the condenser, raising the liquid level in the float tank. The float rises, opening the orifice and sending more liquid to the evaporator.
Low-side float valves are installed on the low side of the system and used with flooded evaporators.
Automatic expansion valves use a spring-loaded diaphragm and valve to maintain a constant evaporator pressure in small capacity equipment with fairly constant loads.
When suction pressure rises due to a load increase, the automatic expansion valve reduces refrigerant flow to the evaporator enough to maintain a constant suction pressure.
The automatic expansion valve cannot prevent inadequate of excessive refrigerant flow because it responds only to pressure changes and not temperature changes.
The thermal expansion valve, commonly called a TXV, is a metering device that senses the temperature at the evaporator outlet with a sensing bulb.
The temperature sensed at the evaporator outlet represents evaporator superheat. The valve responds by adjusting refrigerant flow to match the load.
Refrigerant vapor is superheated in the evaporator coil. The TXV senses the superheat at the outlet of the coil and adjusts refrigerant flow entering the coil to maintain a desired superheat value.
The valve is in equilibrium when pressure from the sensing bulb and a combination of spring pressure and evaporator inlet pressure balance each other across a diaphragm.
If the cooling load increases, increased superheat will increase pressure in the sensing bulb. Increased bulb pressure will move the diaphragm, opening the needle valve to admit more refrigerant.
The fluid in the sensing bulb is often the same as the system refrigerant. Expansion valves used in residential systems are typically selected to maintain a 15°F to 20°F superheat.
Today’s TXVs use a balanced-port design that keeps excess head pressure from acting as an opening force on the piston.
Internally equalized expansion valves work well with small evaporator coils that have little pressure drop. The small pressure drop does not affect superheat.
An externally equalized expansion valve can solve the problem of evaporator coil pressure drop by sensing pressure at the evaporator outlet, allowing pressures across the diaphragm in the valve to be equalized.
The TXV sensing bulb contains a refrigerant charge that expands and contracts in response to temperature changes in the suction line. This allows the TXV to adjust superheat.
The sensing bulb is replaceable on some TXVs in case it loses its charge.
A thermal-electric expansion valve uses a thermistor to sense suction line temperature at the evaporator coil outlet.
A change in thermistor resistance changes current flow through a bimetal element in the expansion valve. As the bimetal heats and moves, it opens or closes a needle valve, regulating refrigerant flow.
An electronic expansion valve uses a DC stepper motor to provide up to 760 discrete steps of orifice size control. Suction line temperature is sensed with a thermistor.
Pulsing-solenoid EEVs have two orifices. A fixed orifice is always open and an adjustable orifice modulates open and shut in response to signals from an electronic control module.
As system temperatures and pressures change, expansion valves will sometimes overcompensate by allowing too much or too little refrigerant to flow to the evaporator in a condition called “hunting”.
Hunting can be minimized by selecting the correct expansion valve for the job and by making sure the superheat is properly adjusted.
Oversized expansion valves are very susceptible to hunting.
A distributor is used to make sure that refrigerant leaving the expansion valve is evenly distributed to the different circuits of the evaporator coil.
Distributors are typically made of precision-machined metal to assure equal distribution of the liquid/vapor mixture.
Expansion valves must be sized to system capacity. For example a 3-ton system must use a 3-ton TXV. Slight oversizing is permissible, but a grossly oversized expansion valve may cause slugging of the compressor.
Install the expansion valve as close to the evaporator inlet as possible, following the manufacturer’s installation instructions.
Best performance is obtained if the valve feeds vertically up or down into the distributor.
If the valve must be brazed during installation, protect it from overheating with a wet cloth and use a nitrogen purge to prevent copper oxide contamination.
Location of the sensing bulb is very important for correct operation. Installation on a downward-pitched horizontal section of the suction line is preferred.
On lines up to 5/8” O.D., the bulb should be installed on top of the suction line.
On lines with a 7/8” O.D. or larger, the bulb should be clamped near the bottom of the line.
Install a short trap if a vertical riser follows the horizontal section where the sensing bulb is installed.
External equalizer lines must be attached to the suction line immediately downstream from the sensing bulb.
Expansion valves are set at the factory and normally do not need adjustment. However, cases may arise where an adjustment of the superheat setting is required.
Metering devices are often blamed for system problems that can be caused by any number of other faults.