Depending on the type of HVAC air conditioning or heat pump system it is and the efficiency range of the system will depend on the type of metering device the system has installed by the HVAC manufacturer of the air conditioner or heat pump system. Lower efficiency HVAC air conditioner and heat pump models have fixed orifice types while higher efficiency systems have thermostatic expansion valves installed in the HVAC air conditioner or heat pump system. The thermostatic expansion valve is far more complex than the fixed orifice metering device as the thermostatic expansion valve metering device modulates the refrigerant flow based on the temperature of the refrigerant temperature in the evaporator coil. This allows a specific amount of refrigerant, based on demand, to be metered into the evaporator coil while the fixed orifice metering device allows the same amount of refrigerant to enter the enter the coil no matter the conditions or 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 as it leaves the metering device and enters the evaporator coil.
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 is used in many air conditioning and heat pump applications including use in chillers for chilled water systems. 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 TXV bulb is remote from the TXV and is attached to the TXV via a capillary tube or cap tube. As the temperature increases and decreases the refrigerant inside the bulb responds by expanding and contracting based on the temperature pressure relationship of refrigerants. As the refrigerant expands and contracts causes a bellows to move in and out which causes a piston to open and close precisely based on the leaving temperature of the refrigerant in the evaporator coil. This allows the TXV metering device to feed the evaporator coil the precise amount of refrigerant it needs to maintain a specific superheat. Because the TXV precisely meters the refrigerant the TXV is used in many air conditioner and heat pump systems that need a higher efficiency. A properly engineered and installed system that uses a TXV will only give the evaporator coil what it demands and nothing more or nothing less.
- 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 that 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 manufacturers 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 they hole in the tube(s) is a fixed size.
- There are other non-conventional types of metering devices used for HVAC refrigeration including the float type and the electronic type however these are not commonly used in standard HVAC equipment.
There are other types of metering devices used in HVAC vapor compression refrigeration but 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 types are the next big step in refrigeration used for air conditioning and heat pump systems.