Btu Buddy 111: Burned Hermetic Compressor - Part 2
Now, Bob’s company has promoted him to help train a new employee, right out of a school specializing in HVAC, just like Bob was. Bob is now Tim’s Btu Buddy. Tim is anxious to travel with Bob. Tim realizes that he is right out of school, with the theory and lab work that he accomplished in school, but still needs help. He knows that he worked with many of the components of the systems in the school, under ideal conditions with good light and air conditioning. Now it is into the field, sometimes under the house with poor lighting, or out on the rooftop in the sun, where the real action is. He is naturally and normally reluctant, but he has Bob to help guide him.
Bob and Tim have gone back to the job with the grounded compressor. They changed the compressor yesterday and ran the unit for a few minutes, then shut the system down, leaving the crankcase heat on so they could start it up later.
Tim asked, “What do you want to do first? We don’t have any clues as to why that compressor burned yesterday.”
Bob said, “We need to start the system up. Our gauges are still on the system. Then we need to check the superheat at the condensing unit suction line. This system uses a TXV (thermostatic expansion valve) so we need to see if it is maintaining the correct superheat.”
Tim said, “I know that the superheat should be 8° to 12°F at the coil. What should it be at the condensing unit?”
Bob responded, “We know it shouldn’t be below 8° and something above 12° would be acceptable. The suction line goes from the evaporator below up to the condenser with about 10 feet of insulated line across the roof so a guess would be that it should not be over 15° of superheat.”
They started the unit and fastened a thermister type temperature lead to the suction line to observe the readings. After the unit had run for about 20 minutes, Tim said, “The unit uses R-410A and the suction pressure is 135 psig, which corresponds to 47° evaporation temperature. The reading of the suction line temperature is 47°, so the superheat is 0° here at the condensing unit. That is not good.”
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Bob said, “We may have discovered the problem. Let’s go to the evaporator and look at that expansion valve and see if we can spot anything.”
They went to the evaporator coil and the expansion valve looked normal, and Bob said, “Cut the insulation off of that expansion valve control bulb and let’s see how it is mounted.”
Tim cut away the insulation and said, “This bulb looks like it is mounted down tight on the line. No, wait a minute. There is a coupling under one end of the bulb and the bulb is mounted on the top of the line. The bulb is not down tight on the line.”
Bob said, “It has run for at least a year like that, but we better not take any chances on it. I am going to shut the unit down until we can do something about the superheat.”
Bob explained, “The bulb is supposed to be in good contact with the line so that it will become the same temperature as the line. The line will be the same temperature as the gas flowing in the line. This is a 1-3/8-inch line and the bulb should be firmly mounted about 45° off of the bottom (Figure 1).”
Tim asked, “Why not mount it on the bottom of the line?”
Bob answered, “There is almost always oil traveling in the bottom of the line and it would act as an insulator and the bulb would not get the true gas temperature.”
Tim said, “Little things mean a lot in this business. I guess you really have to know the details and pay attention to them.”
Bob added, “Some member of the installation crew may not know exactly how to mount the sensing bulb. I wonder if our company did this installation. If so, we need to pass this service ticket to the installation department for future reference.”
They re-mounted the bulb correctly and started the unit back up and observed the readings. The superheat was running 12° at the condensing unit and Bob said, “Well, this unit should give long years of service now.”
Tim said, “Not so fast, how did that cause the compressor to fail?”
Bob said, “I am glad you asked that. A good technician always wants to know why. A constant flow of liquid refrigerant back to the compressor will dilute the refrigerant oil in the crankcase of the compressor. This would cause the oil to become thin and it would not lubricate correctly. The oil is supposed to form a film between the metal surfaces, such as the rod to crankshaft, or the wrist pin to the piston, and the pistons to the cylinder walls (Figure 2). When it is diluted, the film barrier is not there and there is metal to metal contact. Since the oil is thin, there is no telling how long a compressor will run like that. Sooner or later, one of the parts will fail due to friction. The chances are that is what happened and the part hit the motor windings and caused the ground.”
Tim asked, “Shouldn’t someone have noticed something at startup time?”
Bob said, “When the unit was started up, it was probably hot and the startup technician just felt the suction line and noticed that it was cold and kept moving. A proper startup should require a closer look. A superheat check at startup would have prevented this. Look at the rust on the old compressor’s housing. That should tell a good technician that the compressor had been sweating. I wonder if anyone has looked in on this unit since it was started up. I doubt it.”
Tim said, “I guess that is one good reason that a yearly inspection is good. Some people do not want to pay for it and think that it is not necessary, but I see the need.”
Bob said, “Good point, the next time a customer says it isn’t worth it, you have a story to tell.”
Publication date: 6/18/2012
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