Types of metering device in refrigeration

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What is a Metering Device - Refrigeration Components
What is a Metering Device – Specific Types of Metering Devices used in HVAC Refrigeration:

The metering device is the component in an air conditioning system that drastically reduces the temperature and pressure of refrigerant coming from the condenser before that refrigerant enters the evaporator.

A metering device can be as simple as the capillary tube, which is a two-foot long spiral of copper tubing that is narrower than the liquid line feeding into it. The capillary tube is commonly used as a system’s metering device because of its simplicity and low cost.

A more sophisticated type of metering device is the thermal expansion valve (TXV or TEV). The top of the TXV has a sensing bulb that is attached to the suction line (the line connecting the evaporator with the compressor). The sensing bulb contains a substance (usually the same as the system’s refrigerant). Temperature and pressure drop in the sensing bulb is translated to pressure drop in the TXV, which releases tension against an internal spring that is designed to restrict refrigerant passing through the TXV. This behavior optimizes the flow of refrigerant into the evaporator, which is necessary to maintain proper heat transfer within the evaporator, and also to prevent liquid refrigerant from entering the compressor.

References

Hand, Martin. “AC Evaporator, Condenser & Related Components”. MDH Motors.

“Thermal Expansion Valve”. Wikipedia.

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.

We will answer the question, “What is a metering device”. We will define the various types of devices and how they work.
How the metering device is involved in the refrigeration cycle
A comprehensive list defining the metering devices commonly used in HVAC.

What is a Metering Device - Refrigeration Components

TXV Metering Device for a Heat Pump

Depending on the type of HVAC system and the effective efficiency range will depend on the type of metering device. Metering devices installed by the manufacturer of the HVAC system partially define the efficiency of the equipment. Lower efficiency HVAC systems have fixed orifice types. The higher efficiency systems have thermostatic expansion valves. TXV’s installed in the HVAC air conditioner or heat pump system gives higher efficiency.

The thermostatic expansion valve is far more complex than the fixed orifice metering device. The thermostatic expansion valve metering device modulates the refrigerant flow. The modulation is based on the temperature of the refrigerant in the evaporator coil. This allows a specific amount of refrigerant, based on demand, to be metered into the evaporator coil. The fixed orifice metering device allows the same amount of refrigerant to enter the coil no matter the demand.

As the liquid refrigerant enters the metering device it changes temperature and pressure. A partial amount of the liquid refrigerant flashes into a refrigerant gas or vapor. The refrigerant does this as it leaves the metering device and enters the evaporator coil.

What is a Metering Device – Specific Types of Metering Devices used in HVAC Refrigeration:

Thermostatic Expansion Valve - Also referred to as the TEV or TXV for short. The thermostatic expansion valve is used in many HVAC applications including use in chillers for chilled water systems. More on TXV’s below.
The other type of expansion valve used in HVAC is the fixed orifice. The fixed orifice is simple and can be either a piston-type or utilize a distributor. The distributor feeds capillary tubes which terminate in the evaporator coil. The piston-type comes in various sizes and is interchangeable with the capacity or tonnage of the condensing unit. Always follow the manufacturer’s instructions for piston size if using the fixed orifice for HVAC refrigeration.
Capillary tubes (cap tubes) are also used to meter refrigerant to the evaporator coil. These are considered fixed orifice since the hole in the tube(s) is a fixed size.
The electronic expansion valve. These metering devices are used in the newer VRV (Variable Refrigerant Volume) and VRF (Variable Refrigerant Flow). These are used in air conditioner and heat pump systems that have inverter duty compressors that modulate. They are only in the higher-end models with high efficiency.
There are other non-conventional types of metering devices used for HVAC refrigeration. These include the float type, however, these are not commonly used in standard HVAC equipment.

Related Link: Read about superheat and subcooling (opens in a new window)

TXVs respond to the temperature of the refrigerant leaving the evaporator coil or evaporator barrel. The TXV has a sensing bulb that holds a slight refrigerant charge inside the bulb. The bulb is remote from the TXV and is attached to the TXV via a capillary tube. The bulb is attached to the suction line where the superheat leaving the evaporator coil causes the bulb to react. As the temperature increases and decreases, the refrigerant inside the bulb responds. It expands and contracts based on the temperature-pressure relationship of refrigerants. As the refrigerant expands and contracts it causes a bellows to move in and out.

This causes a piston to open and close precisely based on the leaving temperature of the refrigerant in the evaporator coil. This TXV feeds the evaporator coil the precise amount of refrigerant it needs to maintain a specific superheat. Because the TXV precisely meters the refrigerant it is used in many HVAC systems that need higher efficiency. A properly engineered and installed system that uses a TXV will only give the evaporator coil what it demands. Nothing more or nothing less in refrigerant volume except what is needed based on demand.

Conclusion

There are other types used in HVAC vapor compression refrigeration. However, these are the most common types are used in HVACR in both the commercial market and residential markets. As systems grow more sophisticated and complex that is likely to change. Electronic Expansion Valve (EEV’s) types are the next big step in refrigeration used for air conditioning and heat pump systems. These are used in the new variable capacity refrigeration systems now out or just on the horizon.

What is a Metering Device