How Thermostatic Expansion Valves (TXV) Work

Learn how thermostatic expansion valves work in HVAC systems.

Since the minimum efficiency regulation changed to 13 SEER in January 2006, most OEM systems now incorporate a thermostatic expansion valve (TXV) style metering device as the standard for air conditioning systems. It is now extremely important for the HVAC technician to understand the design and operation of this type of valve.

The thermostatic expansion valve (TXV) is a precision device, which is designed to regulate the rate at which liquid refrigerant flows into the evaporator. This controlled flow is necessary to maximize the efficiency of the evaporator while preventing excess liquid refrigerant from returning to the compressor (floodback).

One of the design features of the TXV is to separate the high pressure and low pressure sides of an air conditioning system. Liquid refrigerant enters the valve under high pressure via the system’s liquid line, but its pressure is reduced when the TXV limits the amount of this liquid refrigerant entering the evaporator.

The TXV – What It Does Not Do

The thermostatic expansion valve controls one thing only:  the rate of flow of liquid refrigerant into the evaporator. Contrary to what you may have heard, the TXV is not designed to control:

  • Air Temperature
  • Head Pressure
  • Capacity
  • Suction Pressure
  • Humidity

Trying to use the TXV to control any of these system variables will lead to poor system performance – and possible compressor failure.

How the TXV Controls the System

As the thermostatic expansion valve regulates the rate at which liquid refrigerant flows into the evaporator, it maintains a proper supply of refrigerant by matching this flow rate against how quickly the refrigerant evaporates (boils off) in the evaporator coil. To do this, the TXV responds to two variables: the temperature of the refrigerant vapor as it leaves the evaporator (P1) and the pressure in the evaporator itself (P2). It does this by using a movable valve pin against the spring pressure (P3) to precisely control the flow of liquid refrigerant into the evaporator (P4):

TXV Pressure Balance EquationTXV
P1+P4 = P2+P3
P1 = Bulb Pressure (Opening Force)
P2 = Evaporator Pressure (Closing Force)
P3 = Superheat Spring Pressure (Closing Force)
P4 = Liquid Pressure (Opening Force)

Energy Transfer in the TXV

Here is a closer view of the TXV in operation. The flow of the liquid refrigerant is restricted by the valve pin. As the flow is restricted, several things happen:

  • The pressure on the liquid refrigerant drops
  • A small amount of the liquid refrigerant is converted to gas, in response to the drop in pressure
  • This “flash gas” represents a high degree of energy transfer, as the sensible heat of the refrigerant is converted to latent heat
  • The low pressure liquid and vapor combination moves into the evaporator, where the rest of the liquid refrigerant “boils off” into its gaseous state as it absorbs heat from its surroundings.

The pressure drop that occurs in the thermostatic expansion valve is critical to the operation of the refrigeration system. As it moves through the evaporator, the low pressure liquid and gas combination continues to vaporize, absorbing heat from the system load. In order for the system to operate properly, the TXV must precisely control the flow of liquid refrigerant, in response to system conditions.

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148 thoughts on “How Thermostatic Expansion Valves (TXV) Work

  1. if you go to a job site to install a two ton condenser with a tow ton coil. but when you get to the site you realize that the coil is good for three ton. what we should do? can we use txv valve with three ton coil?

  2. Need more info and explanation. When the system is off is the valve open or closed? Now the compressor comes on because of a call for cooling. Does the valve open or close? As the system runs what happens to make the valve maintain a constant superheat? What happens in the system to make the supoerheat change that it has to be maintained?

    • Flow control, or metering, of the refrigerant is accomplished by use of a temperature sensing bulb, filled with a similar gas as in the system, that causes the valve to open against the spring pressure in the valve body as the temperature on the bulb increases. As the suction line temperature decreases, so does the pressure in the bulb and therefore on the spring causing the valve to close.

    • The TXV controls superheat by utilizing the system pressure and temperature at the exit of the coil (suction) to regulate how much refrigerant is fed at the entrance of the coil. If the superheat is above the setpoint, the valve will open wider. If the superheat is below the setpoint, the valve will start to close. It does this through a balance of different pressures acting on the valve.

      The two controlling pressures are the equalizer and bulb pressures. The equalizer line is a direct line from the valve into the suction line of the coil (on an externally equalized valve), so the system suction pressure is transmitted directly to the valve as a closing force (under the diaphragm). The bulb pressure is actually transmitted as a function of the suction line temperature at the same general location as the equalizer line (exit of the coil). As the bulb temperature increases, the pressure inside increases and vice versa, and this pressure is transmitted to the top side of the diaphragm as an opening force. The bulb is filled with refrigerant/gas that mimics the properties of the system refrigerant, and in some cases, it is simply filled with the same refrigerant as the system. Because of this, it has the same pressure/temperature profile as the system refrigerant, and the resulting valve movement will have a constant relationship to the boiling point of that refrigerant.

      The third pressure acting on the valve is the spring pressure, which can be adjusted (in an adjustable model). The spring pressure is applied as a constant closing force on the valve (like a thumb on the scale toward closing the valve) and is adjusted to increase or decrease the superheat setting. As more spring pressure is applied, more force is applied in the closing direction of the valve, which increases the superheat.

      Typically, when a system shuts off, the suction line pressure immediately increases because the compressor is no longer pulling. This increase in suction pressure results in a decrease in superheat, which causes the valve to close off. Functionally inside the valve, the equalizer pressure will increase (which is the same as suction pressure). The equalizer pressure is a closing force.

      When the system kicks back on, the opposite happens, but even more so. Not only does the suction pressure immediately drop when the compressor starts pulling, but the suction line temperature has been slowly rising as the system has been off, and the load or space the system is cooling has also been absorbing heat. All of these factors result in a high superheat, and the valve immediately opens to lower the superheat level. This is typically known as the “pulldown”. The valve will typically be wide open until the system superheat drops to near the desired level, or setpoint, then the valve will begin closing off to try to maintain that setpoint.

      After a running for a while, the system will become more stable, and thus the TXV becomes more stable because it moves when system temperatures and pressures move. The valve will settle around the balances of the system pressures, temperatures, and spring pressure being applied, which is your superheat setting.

      Other than start-up and shut-down, there are other system changes that can disrupt this balance, such as a blocked coil/fan, loss of refrigerant, increase/decrease in heat load, and changes in the ambient/outdoor conditions.

  3. Is it possible to determine if valve is “getting old” and should be replaced as a preventative maintenance measure?

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