Guide to Thermostatic Expansion Valves

Learn how thermostatic expansion valves work in HVAC systems.

Originally published on June 24, 2013

Understanding TXVs

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.

Understanding the Function of the TXV

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.

Understanding 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)

 

Understanding How the TXV Transfers Energy

Here is a closer view of the TXV in operation. The valve pin restricts the flow of the liquid refrigerant. 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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247 thoughts on “Guide to Thermostatic Expansion Valves

  1. With vehicles using R134a refrigerant and with the understanding that outside ambient temperature and humidity effects these values, approximately what is the pressure drop (difference) on the liquid line just before the TXV compared to just exiting the TXV (prior to entering the evaporator) on the liquid line. Also, what is the line temperature increase from the liquid line entering the evaporator compared (difference) to the vapor (suction) line exiting the evaporator. Lastly, what is the temperature drop (difference) on the discharge (vapor) line just before entering the condenser compared (difference) to the liquid line exiting the condenser. I realize are not exact values but on a new, well working system, what are the approximate or range that I can expect from these three questions. I appreciate any answers from folks with experience in this field since I’m new to using a K-Type Thermometer in addition to my long used manifold gauges to achieve more accurate diagnosis. Again, thanks in advance!

  2. Hello James,

    There are many variables that influence both pressure and temperature in a refrigeration system, chief among them are:
    1. Correctly sizing and matching system equipment and components
    2. Proper installation of equipment and associated components
    3. Selecting the proper air speed for indoor and outdoor heat exchangers (evap & condenser)
    4. Cleanliness of the heat exchangers
    5. Ensuring that the refrigerant is free of non-condensables and is the proper refrigerant for the system
    6. Ensuring that the liquid refrigerant does not pick up heat after heat rejection in the condenser
    7. Failed components in the refrigeration system

  3. Hi Geoffrey,

    Not sure what you mean by typical; typical system efficiency, typical fan speed, typical outdoor air temperature, typical indoor heat load. In what mode is the system operating – heating or cooling? There are too many variables in refrigeration to give general answers.

    It’s best to refer to the installation instructions for the system, there is usually a charging chart that will indicate the condensing pressure/temperature for a number of variables.

  4. What condition will cause the sub cooling and super heat to fluctuate as well as temperature and pressures,Paige low side goes from 124 to 131,high side Paige goes from 288 to 301,this happens every 23 seconds,bulb is correctly positioned and insulated,2stage compressor heat pumps,airflow is good and charge was weighed in,we are stumped,thanks

  5. What is a typical temperature for the refrigerant to enter the evaporator in an air-to-water heat pump?

  6. Hello Barbara Fox,

    If the TXV is “cracked”, I assume that it is leaking refrigerant. Any refrigeration system will work properly for only so long, if there is a refrigerant leak. Refrigeration systems are considered “closed” systems, which means that there should never be leaks that allow refrigerant to escape into the atmosphere.

  7. Hello Emory,

    The two measurements that indicate a correctly operating TXV are: Subcooling and superheat. Subcooling reveals if a full column of liquid refrigerant is being delivered to the inlet of the TXV. Superheat reveals if the TXV is properly delivering the correct amount of refrigerant into the evaporator.
    It’s also important to take the measurements after 10-15 min. of run time for pressures and temperatures to balance and stabilize.

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