Welcome to the solutions page

By Grant Laidlaw

Many people ask for assistance in the understanding of theoretical and practical aspects of the industry. We are going back to basics as I have questions coming in that indicate that the basic understanding necessary to work in the industry is not in place.

Grant Laidlaw is currently the owner of the Air Conditioning and Refrigeration Academy (ACRA) in Edenvale. He holds a Bachelor of Business Administration and an associate degree in educational administration. He has a National Technical Diploma and completed an apprenticeship with Transnet. He has dual-trades status: refrigeration and electrical. He has been involved with SAIRAC for over two decades and served on the Johannesburg committee as chairman and was also president between 2015 and 2018. Currently he is the SAIRAC national treasurer.

Hi Andre, what we are dealing with here forms part of refrigerant controls – thermostatic expansion valves are still common in industry with electronic versions freely available for some time now. Assuming that we all understand what is, the effect of, the importance of, measuring and setting of superheat we can look at the thermostatic expansion valve.

Let us begin with how to select the correct thermostatic expansion valve. In most cases your supplier will be of assistance but in the field, you may have a situation where labels are missing, and information is not readily available.

You will need the following information:                     

• Type of refrigerant
• Maximum evaporator load
• Evaporating temperature
• Maximum and minimum condensing pressure
• Maximum and minimum liquid temperature
• If pressure limiting is required
• If an external equaliser is required

Procedure:

1.Determine pressure difference across the valve as follows.
  Subtract evaporating pressure from highest condenser pressure
  Subtract evaporating pressure from lowest condenser pressure
  Use the average of the two to determine pressure difference between evaporator and condenser
  From this value deduct the other possible pressure differences, for example:

• Liquid line and accessories such as solenoid valve, filter drier, shut-off valves, heat exchanger etc. (use 100 kPa if exact loss is not known)
• Heat exchanger friction losses, if applicable
• Pressure loss (gain) due to vertical lift (drop):
• Deduct 12 kPa per meter that the TEV is above the condenser
• add 12 kPa per meter that the TEV is below the condenser
• Pressure drop across the distributor:
• Deduct 100 kPa if a distributor is used (in the case of an externally equalised TEV)

2.Determine the liquid temperature leaving the condenser (condensing temperature minus sub cooling). Reduce this temperature further if a long liquid line is used to determine the liquid temperature entering the valve.

3.Determine the correction factor for the temperature of the liquid entering the valve.

4.Calculate the corrected valve capacity required by dividing the maximum evaporator load by the liquid correction factors.

5.Select the valve from the appropriate capacity table for the evaporator temperature and the pressure drop available (corrected capacity vs. pressure drop at the correct evaporating temperature).

6.The valve capacity should equal or slightly exceed (max. 5%) the design rating of the system. A valve with a capacity rating up to 10% below the full load condition may be selected if the system is to operate at reduced loads for long periods of time, and if a slightly higher than normal superheat can be tolerated at full load conditions.

7.Select the proper thermostatic charge based on the type of refrigerant in the system and the evaporating temperature.

8.Check whether an externally equalised valve is required. Always use an externally equalised valve when a refrigerant distributor is used.

9.Determine the appropriate connections required.

10.Determine the length of capillary tube required.

11.Determine the order number according to the manufacturer’s instructions and select the valve.

The following alternative to steps 3, 4 and 5 above may be used:

3.Select the valve from the appropriate capacity table for the evaporator temperature and the pressure drop available (maximum evaporator load vs. pressure drop across valve at the correct evaporating temperature).

4.Determine the correction factor for the temperature of the liquid entering the valve.

5. Calculate the corrected valve capacity by multiplying the valve capacity according to the catalogue by the liquid correction factors.

These factors include corrections for liquid refrigerant density and net refrigerating effect and are based on an average evaporating temperature of –18ºC. However, they may be used for any evaporator temperature from –40ºC to +4.4ºC since the variation in the actual factors across this range is insignificant.

Andre, let us move on to thermostatic expansion valve (TEV) bulb location and installation. The thermostatic expansion valve sensing bulb that controls the thermostatic expansion valve is clamped to the refrigerant suction line where it monitors the temperature.

For TEVs that use an 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 feeler bulb location is concerned, we find some differences between manufacturers. 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 is in contrast to a system 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 manufacturers specifications give more detail stating that: “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 as 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 traveling 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 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.

Diagram from a manufacturer with clear of placement recommendations in accordance to pipe sizes. (Courtesy of Danfoss)

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.

REFERENCES:

1. ACRA
2. Danfoss