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:

1. 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.
2. 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.
3. 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.
4. 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.
5. 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.

Thermostatic Expansion Valves - How These Metering Devices Work

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

Resource: The Home Comfort Book: The ultimate guide to creating a comfortable, healthy, long lasting, and efficient home