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.
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.
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.
Since this problem is commonly overlooked in the field, a closer examination and a possible solution are the focus 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When this unevaporated liquid refrigerant combines with the other superheated flow streams, the recombined suction flow no longer represents an average condition.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.
There are several reasons why circuits can become unevenly loaded.
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 2Â°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.
Here are some tips for correcting or compensating for an imbalanced heat exchanger:
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.
Attempting to control an evaporator near to or lower than 5Â°F operating superheat can exceed the sensing capability of most expansion valves and result in hunting and subsequent intermittent flooding.
Better mixing of the refrigerant prior to the bulb can smooth out the valve response, although capacity and efficiency may not improve significantly.