Service & Maintenance / Extra Edition

Btu Buddy 77: Starved Evaporator, TXV

August 24, 2009
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Bob is a service technician who is well trained and nationally certified. However, he sometimes suffers from the same confusion that all technicians occasionally do - the facts that he gathers may or may not point to the obvious cause of the problem or the best solution. But Bob has something that no one else has. He recalls his long-time HVACR mentor and imagines him accompanying him as “Btu Buddy,” someone who reminds him to take time to stop and think before rushing to judgment, helping keep him on the right track, even with facts that are confusing.

This service call was for a routine checkup on an old 7-½ ton unit using R-22. The call was going well until the owner mentioned to Bob that the unit had never cooled well. He said that late in the afternoon the unit would run all the time and the store temperature would begin to rise. The manager just never mentioned it to the service people before, but he asked Bob to look into this and see if the system was performing up to capacity.

Bob had checked the system out, changed the filters, and oiled the motors. He had given the system the touch test and everything seemed fine. The suction line seemed cool enough and the liquid line felt warm, but not hot.

Bob then decided to fasten gauges to the system and check it out all the way. He fastened a temperature tester to the suction line and to the liquid line.

The readings for the R-22 system were:

Suction pressure - 64 psig, evaporator temperature 37°F
Suction line temperature at the outdoor unit - 60°
Superheat - 23° (60 – 37 = 23)
Outdoor ambient temperature - 96°
Head pressure - 226 psig, condensing temperature 110°
Liquid line temperature - 95°
Subcooling - 15° (110 – 95 = 15)

The condensing temperature should really be about 126° (96° outdoor temp + 30 = 126) for a head pressure of 278 psig.

Bob was sitting down and looking over the figures and studying them when Btu Buddy showed up.

Bob said, “I must be in trouble. You always show up when I am confused or have a problem.”

Btu Buddy said, “I am not sure you have a problem. Tell me what you know.”

Bob then explained that the suction pressure was low, with a high superheat. The head pressure was low; it was 226 psig when it should be about 278 psig. The subcooling temperature was about right, so the refrigerant charge must be about right. The only thing Bob could think of was that the evaporator coil must be starved of refrigerant, causing a low capacity.

Btu Buddy said, “Bob, you are on the right track. What can you do to increase the superheat?”

Bob said, “I will have to see what kind of expansion device it has.”

Btu Buddy said, “Let’s go and look.”

The indoor unit was in a closet in the back of the store. When they got there, Bob remarked, “The unit has a TXV (thermostatic expansion valve). This should be easy. It is adjustable and I can just decrease the superheat and it will be good to go.”

Figure 1. This example shows how a TXV sensing bulb should be mounted in relation to the external equalizer line. (Figures are from Refrigeration & Air Conditioning Technology, 5th Edition, by William Whitman, William Johnson, and John Tomczyk, published by Delmar Cengage Learning.) (©Delmar Cengage Learning.)

Btu Buddy then said, “Look around before you make an adjustment. It is very seldom that a TXV is out of adjustment. They come from the factory adjusted correctly and they shouldn’t drift out of adjustment.”

Bob looked over the system and said, “The valve does have an adjustment. Maybe the valve is defective and should be changed.”

Btu Buddy then said, “Check the way the bulb is mounted and the location of the external equalizer.”

Bob looked at the bulb and said, “It is under the insulation, but it seems like it is tight on the line.”

Btu Buddy then asked, “Do you see anything different about the external equalizer line?”

Bob said, “It is mounted right under the expansion valve bulb and it is really sweating.”

Btu Buddy then said, “You are really on the right track now. The external equalizer line should be mounted after (down stream of) the bulb (Figure 1).”

Figure 2. This figure shows a leaking TXV that is allowing a small amount of refrigerant liquid to influence the TXV metering. The small amount of liquid is not enough to flood back to the compressor and cause harm. (©Delmar Cengage Learning.)

Btu Buddy went on to explain, “If the valve has an internal leak, liquid refrigerant will be pushed into the low side and it will be right on the bulb, the sensor for the valve. This will cause the valve to throttle back because it thinks there is liquid in the suction line (Figure 2).”

Bob then said, “All we should need to do is move the bulb upstream like it should be.”

“That is correct,” said Btu Buddy. “We have to make sure there is room for the bulb on the line. Remember, the bulb should be mounted on a straight section of the line, several inches from any elbow.”

Bob said, “There is plenty of horizontal line between the external equalizer and the first elbow.”

Bob then shut the unit off and moved the bulb back toward the evaporator and cleaned the line with sand tape and cleaned the bulb element and mounted it on the suction line with approved strapping (Figure 3).

Figure 3. This illustration shows the correct positioning and mounting for a TXV sensing bulb. (©Delmar Cengage Learning.)

When all was complete, Bob turned on the power and started the unit. After about 15 minutes of running time, he took the following readings:

Suction pressure - 69 psig, evaporator temperature 40°
Suction line temperature - 52°
Superheat - 12° (52 – 40 = 12)
Head pressure - 278 psig, condensing temperature 125°
Liquid line temperature - 115°
Subcooling - 10° (125 – 115 = 10)

Bob then said, “I would say that all is well with this system.”

Btu Buddy said, “I would say that you are correct. A call back to the customer late this afternoon should confirm that the system is performing up to capacity.”

Bob said, “It really pays to get the system running correctly. I am surprised that someone else hadn’t found this out.”

Btu Buddy then said, “Probably the owner never mentioned it to another technician, or maybe the other technicians just never decided to look into it, like you did. Good job.”

Btu Buddy added, “There is help everywhere. Some technicians just don’t take a real interest in being professional and learning all they can. The better technicians will always be in demand.”

Publication date: 08/24/2009

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