Expansion Valve Hunting Guidance on automatic and electronic expansion valves. Hunting describes a situation where a cyclic action of the expansion valve causes the evaporator to be overfed and underfed with liquid refrigerant. This results in a cyclic swing of refrigerant pressure and saturation temperature within the evaporator, which in turn causes the average refrigerating capacity of the system to be severely reduced due to a generally lower suction pressure at the inlet to the compressor. Under extreme conditions, excess liquid refrigerant supply to the evaporator may lead to liquid flooding of the compressor with subsequent compressor failure. Hunting is caused by a number of possibilities:

1. High refrigerant velocity within the evaporator due to undersized tubes for the application cause the liquid refrigerant to pass through the coil in slugs which can pass through the evaporator into the suction line. The expansion valve bulb experience a sudden drop in temperature as this liquid evaporates at a temperature well below the normal superheat value and causes the valve to quickly err toward the closed position. The reduced refrigerant flow that then occurs causes superheat to rise considerably, which then forces the valve to open widely with a sudden increase in refrigerant flow.
2. A time lag occurs between the change of valve position and the effect at the evaporator outlet (suction line) since the revised refrigerant flow rate will take some time to travel through the tubes of the evaporator coil before reaching the outlet where the bulb of the expansion valve can sense the new condition. There will also be a time lag between a change in temperature of the outer surface of the suction line and the change in the expansion valve overall bulb temperature leading to the increase/decrease in pressure within the bulb and the action upon the diaphragm within the valve. The time lag brought about by the combination of the above will lead to undershooting and overshooting of the desired condition with similar results on system efficiency as described in 1 above. A solution to this problem is to increase the mass, and thus the thermal response, of the sensor bulb, which will damp the rate of temperature change and the valve’s ability to respond. This will either reduce or eliminate hunting altogether. Thermal damping can be applied to air conditioning and refrigeration electronic control systems where outdoor fan speeds must be varied to maintain a specified liquid line temperature at the outlet of condenser coils for Low Ambient Control, particularly as the response rate of the electronic control system is virtually instantaneous, yet the effects of fan speed changes are considerably time-lagged through the condenser coil and at the liquid line. The control software can also be written in a way that overcomes this problem.
3. Over-sizing of the expansion valve will also cause hunting since an excess amount of liquid refrigerant passing through the valve will cause sudden chilling of the bulb at the evaporator outlet, thus leading to a sudden temperature drop at the bulb causing a rapid closing action at the valve. The much-reduced refrigerant liquid supply, after time lag, causes excessive superheat, which eventually causes the valve to open, thereby supplying too much refrigerant due to excess over-sizing. Ensure that the valve is selected for the actual capacity rating rather than a nominal capacity value. 4. Incorrect sensor bulb positioning can also lead to expansion valve hunting. If the bulb is located on a vertical outlet header connecting two or more evaporator coils, liquid may overflow a coil from above thereby chilling the bulb. The bulb should always be located on the horizontal outlet pipe from the coil to ensure this situation cannot arise as illustrated in Figure 1. Locate the bulb as close to the evaporator outlet as possible. The bulb must not be located on the underside of the suction line. The manufacturer’s recommendations must be followed and these may specify the 3 or 9 o’clock position or 45 degrees between the side and underside of the suction line. Insulate the bulb/suction line with high quality insulation that is impervious to water vapour, particularly on low temperature systems where the presence of moisture will lead to freezing and incorrect bulb response

      A cross-charged sensor bulb will also reduce the effects of hunting (see previous bulletins for details of cross-charged expansion valves). Charge Migration The refrigerant liquid passing through the expansion valve experiences a dramatic drop in pressure and flash gas is produced as part of the liquid evaporates. This causes the body of the valve to be continually chilled. Heat is conducted from the diaphragm assembly above the valve body leading to a drop in temperature of the refrigerant charge within the diaphragm housing. Under certain circumstances, this housing can fall to a temperature that is lower than the temperature at the sensor bulb. If this situation arises with a gas charged valve, the majority of the charge will condense within the diaphragm and control of the valve will then be transferred to this point rather than at the bulb. These conditions will give rise to severe restriction of refrigerant flow to the evaporator coil and may also lead to complete shutting of the valve since the very low and almost equal temperature/pressure on both sides of the diaphragm will allow the spring to close the valve. Gas charged valves should therefore only be used where there is a reasonable pressure drop between the valve outlet and the suction line leaving the coil. The presence of a distributor will ensure this is the case as will a coil of high pressure drop. Under these conditions, the valve body will be at a higher temperature than the bulb because the higher pressure at the valve will bring about a higher saturation temperature than that at the coil outlet by the bulb. Any migration will be toward the bulb, which will thereby retain control over the valve. The next bulletin of this series will focus on distributors and flooded evaporators. Do you require refrigeration or air-conditioning  training check out our  air conditioning & refrigeration courses by clicking here