Tips for preventing superheat hunting in TXVs

This article and its quiz should help you and your techs get a better understanding of a commonly overlooked cause of superheat hunting, its causes, and how the problem might be corrected.

Figure 1. A conventional balanced port thermostatic expansion valve and the three forces it responds to. F1: thermal bulb pressure times the diaphragm effective area; this force acts on the top of the diaphragm, which tends to open the valve. F2: evaporator pressure times the diaphragm effective area; this force acts on the underside of the diaphragm and tends to close the valve. This force is transmitted to the diaphragm through the valve body with internal equalized valves and through the external connection in external equalized valves. F3: superheat spring force which assists in closing the valve.

Just what is superheat hunting?

Superheat hunting is the result of the expansion valve (see Figure 1) excessively opening and closing in an attempt to maintain a constant operating condition.

Hunting can be seen indirectly by regular fluctuations in suction temperature, and in extremes, suction pressure.

Excessive hunting can reduce the capacity and efficiency of the system, resulting in uncomfortable conditions, loss of product, wasted energy, and ultimately, customer dissatisfaction.

Why TXVs hunt

• Oversized valve: The expansion valve may be oversized for the application or operating condition of the system.

  Valve capacity significantly exceeds the requirements of the system and when the valve attempts to adjust to system load, it overcompensates because it is oversized.

• Incorrect charge selection: The charge selected does not have the necessary control characteristics and/or dampening ability to stabilize operation.
•  Undercharged system: Intermittent loss of subcooling is causing loss of expansion valve capacity and resulting intermittent high superheat.
•  Poor bulb contact: Loss or delay of temperature signal to the bulb, causing erratic and unpredictable operation.
•  An imbalanced heat exchanger (multi-circuit coil): An imbalance in the heat load on each circuit creates a false temperature signal to the expansion valve bulb and results in erratic operation.

  Since this problem is commonly overlooked in the field, a closer examination and a possible solution are the focus of this article.
  
  

  

  Figure 2. At the expansion valve outlet, flow is divided into two or more paths (circuits) at the inlet of the evaporator by the distributor. These paths recombine as they exit the evaporator into the suction manifold.

  TXVs on multi-circuit heat exchangers

  TXVs respond to the temperature of the suction line. (They respond to pressure too, but this is not the concern of this article.)

  At the expansion valve outlet, flow is divided into two or more paths (circuits) at the inlet of the evaporator by the distributor; these paths then recombine as they exit the evaporator into the suction manifold. (See Figure 2.)

  Ideally, each circuit is equally loaded and absorbs an equivalent amount of heat. If one assumes the refrigerant flow rate and heat load through each circuit is equal, then the superheat condition exiting each circuit will be equal and when all of the flow streams recombine, the result is a “true” average condition of the evaporator suction gas.

  When one or more circuits has a lighter heat load, some refrigerant from that circuit remains unevaporated when it exits the coil. When this unevaporated liquid refrigerant combines with the other superheated flow streams, the recombined suction flow no longer represents an average condition.
  
  The suction temperature where the bulb is mounted will be lower than the “true” average of the circuits if they were all properly superheated.

  Sensing a “cold” suction condition will cause the valve to close down because it is sensing a condition that is not superheated enough; when the valve closes down, it restricts flow to all circuits and eventually “dries out” the circuits which are flooding.

  By this time, the remaining circuits have become highly superheated due to the reduced flow rate. At the point the “flooding” circuit(s) begin to be superheated, the suction temperature rises rapidly because there is no more liquid present to falsely reduce the suction temperature.

  Sensing a now “warm” suction condition, the valve opens to decrease superheat and the lightly loaded circuit begins to flood into the suction manifold again. Suction temperature drops rapidly again, the valve closes down again, and the whole sequence repeats in a cyclical fashion.
  
  

  Why circuits get loaded unevenly

  Again, the ideal situation is to assume each circuit is equally loaded and absorbs an equivalent amount of heat; however, this situation does not always occur.

  There are several reasons why circuits can become unevenly loaded.

  • Poor heat exchanger design: In this case, each circuit is not of equal length and loading.
  •  Poor refrigerant distribution: This problem occurs due to the wrong choice of distributor or feeder tubes, partially blocked passageways of feeder tubes, unequal feeder tube lengths, and/or kinked feeder tubes.
  •  Uneven airflow: Airflow across the evaporator is reduced in some areas while increased in other areas. Dirty coils or damaged coil fins can have a similar effect on airflow.
    
    

    Diagnosing a hunting problem

    Diagnosing a hunting problem due to an imbalanced heat exchanger requires measuring the exit temperature of each circuit upstream of the suction manifold.

    To perform this process, average the temperatures of all of the circuits upstream of the suction manifold and compare this average temperature to the actual temperature of the suction manifold close to where the bulb is mounted.

    If the average value of the circuit exit temperatures exceeds the actual suction temperature value by more than 2°F, then there is probably one or more circuit(s) which are not completely superheated (flooding).

    A closer examination of the individual circuit temperatures and the associated suction pressure should reveal which circuit(s) are causing the problem.

    One simple rule to remember is that the valve’s response will favor the circuit that is flooding. Because of this favorable response, a heat exchanger can be operating at a reasonable exit superheat but still have a significant loss in capacity, because the expansion valve is responding to one or more flooding circuits while the other circuits remain highly superheated, and thus highly inefficient.
    
    

    Correcting the problem

    This can be a difficult task. First, the service tech must recognize the cause of the problem. If not, the problem can only be compensated for and this could mean a reduction in system performance.

    Here are some tips for correcting or compensating for an imbalanced heat exchanger:

    • If possible, examine and correct any problems with airflow, coil circuitry, and distribution so that the circuits are more evenly fed and loaded.

      The goal is a more consistent circuit exit temperature on all circuits. One lightly loaded circuit may be tolerable if there are, for example, eight circuits. However, this is probably not the case if there are only three.

    •  Adjust the superheat of the valve to a slightly higher value.

      Attempting to control an evaporator near to or lower than 5°F operating superheat can exceed the sensing capability of most expansion valves and result in hunting and subsequent intermittent flooding.

    •  If practical, move the bulb farther downstream on the suction line.

    Better mixing of the refrigerant prior to the bulb can smooth out the valve response, although capacity and efficiency may not improve significantly.