By Grant Laidlaw
Many people ask for assistance in the understanding of theoretical and practical aspects of the industry. I will endeavour to enlighten.
Sarel asks: Hi Grant, some time ago I ran into information with regards to expansion valve feeler bulb location not always in the 12 o’clock position. We have just lost a compressor due to the expansion valve feeler bulb incorrectly installed. What about evaporators with multiple expansion valves? Could you help on this as well as expansion valves hunting. Thanks.
Thermostatic expansion valve (TEV) bulb location
Hi Sarel. The thermostatic expansion valve sensing bulb that controls the thermostatic expansion valve is clamped to the refrigerant suction line where it monitors the temperature. The TEV should be installed as close as possible to the evaporator coil inlet.
The expansion valve body is installed in the liquid refrigerant line ahead of the evaporator coil. The sensor bulb is to be fastened to the suction line as close to the evaporator as possible.
For TEVs that use external pressure equalisation, the equalising pressure line must be connected to the suction line immediately after the temperature sensing bulb that operates the TEV. In addition, it is good practice to connect the equalising line to the top (twelve o’clock position) of the suction line. This prevents oil and possible liquid refrigerant ingress into the equalising line which may cause erratic operation of the expansion valve.
As far as location is concerned, one manufacturer differs from the next. Some state the twelve o’clock position while others the one o’clock position. Always follow the manufacturer’s specification. The twelve and one o’clock positions are typically for small suction lines up to 19mm in diameter.
This contrasts with a system that that uses a larger diameter line (more than 19mm in diameter). In this case the thermostatic expansion valve sensor bulb is clamped at the four or eight o’clock position on the lower portion of the suction line.
Some manufacturer’s specifications provide more detail such as: “The bulb is best mounted on a horizontal suction line tube and in a position corresponding to between one o’clock and four o’clock. The location depends on the outside diameter of the tube.”
This is to give a more accurate reading, since, should any liquid refrigerant be present in the suction line exiting the evaporator, this liquid refrigerant will most likely be found just above the oil flowing on the bottom of the tubing on its way back to the compressor.
The intent is to place the TEV’s sensing bulb close to but above the level of liquid refrigerant in the line. For larger diameter refrigerant tubing the two, three, or four o’clock position may be specified whilst on small diameter tubing the twelve, one o’clock position is used.
The reason we don’t place the sensor bulb on the bottom of the suction line is that oil travelling along the bottom of that tubing can act as an insulator to prevent the sensor bulb from accurately sensing the temperature of the refrigerant.
It then stands to reason that one should never attach the feeler bulb, or any sensor for that matter to the bottom of the refrigerant tubing (six o’clock position). In addition, never place the bulb downstream of a P-trap. Bulb placement before a P-trap (upstream) is recommended.
In some instances, one can install the feeler bulb in the vertical position, however this is not ideal and only should be done when one has no other option.
When putting the bulb on a vertical line it is imperative to have the tail end up. The reason for this 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.
Sarel, if multiple expansion valves are installed, each thermostatic expansion valve has to be installed on the refrigerant suction line corresponding to that particular portion of the evaporator to control conditions in each individual evaporator or portions thereof. Never place any of the feeler bulbs on the common header of the suction line fed by multiple evaporators.
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. 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 flood back to the compressor. This can and does result in compressor failure.
The suction line tubing should therefore be thoroughly cleaned to ensure good thermal contact. Of course, following on this, it is imperative that the bulb be properly secured. I have seen plants where the expansion valve feeler bulb is not in contact with the suction line. Again, this will result in the expansion valve opening, possibly allowing a liquid flood back to the compressor, and again this can and does result in compressor failure.
Always insulate the entire sensing bulb after installation. An accurate temperature reading of the suction line and the suction line only is needed. Ambient temperatures surrounding the feeler bulb will negatively impact on the accurate operation of the expansion valve.
Installation mistakes to avoid
A TEV sensor that is too loose or has a poor thermal contact to the suction line can send incorrect signals to the TEV and can cause liquid refrigerant flood back damage to the compressor.
The sensing bulb must also not be exposed to external sources of heat or cold such as a warm air current. Insulation should be applied to protect the bulb from any external influence.
Most installation guides require that the system has nothing installed on the refrigerant tubing that extends between the TEV and the evaporator coil, except in the case when installations have a refrigerant distributor installed. In this case an externally equalised expansion valve is used.
The temperature sensing bulb should never be placed at the bottom or under-side of the suction line, a situation that will cause improper reading of the line’s temperature, thus causing inaccurate TEV operation.
A poorly installed or incorrectly adjusted valve can most certainly cost you a compressor.
Avoid where possible, installing TEV temperature sensing bulbs on vertical piping.
Do not install the sensor bulb on a common suction manifold.
Follow the manufacturer’s instruction as where to install the feeler bulb, as a general rule: 12 o’clock for suction lines 19mm and smaller and four o’clock for larger than 19mm.
On an installation it is also important that the TEV be installed in a location allowing access to adjust the device.
Hunting thermostatic expansion valves
The definition of hunting expansion valves is a valve that is cycling too frequently between open and closed. This condition may be detected by observing 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 such as 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.
Yet 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 rapidly and again dispensing too much refrigerant. Again, the problem detected 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 this undesirable situation.
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.
An erratically operating or failed thermostatic expansion valve:
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.
Sarel, when it comes to expansion valves all that you need is understanding and some attention to detail. 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.
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