RACA Journal August 2024 RACA_August_2024_digital | Page 41

Support it is imperative to have the tail end up . The reason for keeping the tail end up on vertical lines is to ensure that the refrigerant charge in the bulb stays in the bulb . This allows for a better reaction to changes in the suction line temperature . When clamping the bulb at the 4 or 8 o ’ clock positions on larger horizontal lines , keep the tail rotated so it is always down to keep the charge in the bulb .

Another important aspect is that of thermal contact . It is extremely important that the feeler bulb be installed with good thermal contact . I have often seen that the bulb is installed on dirty tubing or not correctly secured . The debris creates an insulating barrier preventing the bulb from accurately sensing the actual suction line temperature . This would result in the expansion valve opening , possibly allowing liquid to flood back to the compressor . This can and does result in compressor failure .

Always insulate the entire sensing bulb after installation . We need an accurate temperature reading of the suction line - and the suction line only . Ambient temperatures surrounding the feeler bulb will negatively impact on the accurate operation of the expansion valve .

Andre , you mentioned hunting applicable to thermostatic expansion valves . The definition of expansion valves that are hunting , is a valve that is cycling too frequently between open and closed . This condition may be observed when monitoring suction line pressure . Rapid changes in suction line pressure can be observed as the valve opens and closes .

Hunting thermostatic expansion valves can damage the compressor . A cause of hunting can be improper adjustment of a properly-sized TEV ie , superheat set too low . A low superheat setting can allow the superheat to fall to zero . This condition will cause the valve to close and the superheat will then quickly rise to well above its setting . This in turn can cause the valve to reopen wider than necessary and overshoot the superheat setting . This condition is called hunting . On one of the valve ’ s swings to the wide-open position , liquid refrigerant can flood the suction line and allow liquid to enter the compressor .

Another cause of hunting is an oversized refrigerant metering device . An over-sized valve dispenses liquid refrigerant into the low side too rapidly and then tries to correct by closing . This is then followed by the valve opening but again dispenses to much refrigerant . Again , the problem will be a valve that is hunting . It is also possible that dirt , debris , or even water in the refrigerant piping system enters the TEV which could cause hunting .

It should be noted that variations in the level of superheat of around 0.5 ° C or less are not considered hunting while variations greater than that amount should be addressed . The superheat must be allowed to change for the sensing bulb on the TEV to make corresponding adjustments to the valve opening while it maintains the set superheat .

Liquid slugging or poor system performance will also occur if a refrigerant metering device is not properly regulating refrigerant flow from the high side to the low side of the refrigerant system due to sticking or failure . One can remove the expansion valve and perform a bench test which will allow you to accurately set the expansion valve and test the operation .

Andre , I hope that this helps you with your expansion valve issues , a poorly installed or incorrectly adjusted valve can most certainly cost you a compressor . RACA