ACHRNEWS

Btu Buddy 64: Charging an Overcharged System

July 21, 2008

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.

The day was starting off all wrong for Bob. He received a call from the dispatcher that a system was not cooling properly. It was a small office with a 4-ton unit that used a capillary tube metering device. When he arrived, the office thermostat was set to control at 72°F and the thermometer was reading 76°. The office felt warm and humid. The office manager told Bob, “Back in the spring, we had a service company check the system and they found a leak and repaired it, then added refrigerant. For some reason I did not have confidence in that company or technician. His truck looked like it needed to be sent to the landfill and dumped. The back end was full of old wire and pipe and junk. Not a very professional look.”

Bob went to the outdoor unit and felt the suction line and it was really cold. This seemed like it should cool the place. He thought the filters were restricted so he went to the air handler and removed the filter and discovered that it was clean. He used his flashlight to examine the coils and they looked really good.

Bob then went back to the condensing unit with gauges and fastened them to the suction line. The temperature outside was 94° so the unit really had a load on it. The low-pressure gauge read 85 psig and the high-side gauge read 375 psig; the refrigerant was R-22. These readings were much too high.

Figure 1. This compressor is sweating all over because liquid refrigerant or saturated vapor is entering the compressor. (Figures are from Refrigeration & Air Conditioning Technology, 5th Edition, by William Whitman, William Johnson, and John Tomczyk, published by Delmar Cengage Learning.) (Click on the image for an enlarged view.)

Bob then removed the cover to the compressor compartment and discovered the compressor was sweating all over (Figure 1). It was evident that the system had too much refrigerant. The last technician had charged the system in mild weather without taking the low outdoor temperature into consideration.

Bob went to the truck and got his temperature tester and recovery setup to remove refrigerant from the system.

An hour later, Bob was standing there scratching his head when Btu Buddy appeared and asked, “What is the problem, Bob?”

Bob then said, “I have a problem charging this system and you showed up - what a timely visit. I need help. I have been removing refrigerant for an hour, a little at a time, and cannot seem to get the charge correct, according to the superheat method.”

Btu Buddy then explained, “It is almost impossible to charge a system correctly by removing refrigerant. You either take out too much or too little. I am going to recommend that you use the recovery unit and remove refrigerant until you are sure that the charge is short. Then add vapor back to the system to arrive at the correct charge based on the superheat.”

Bob then said, “I was removing liquid from the system and I guess it really would be hard to get that just right.”

Figure 2. These two systems are being charged on a mild day. The line lengths are used to decide the amount of superheat for the different systems. Charging a system on a 95°F day is the ideal time to use this procedure. When the temperature is less than 95°, the technician can cover the condenser to cause the head pressure to rise and simulate the conditions of a 95° day. (Click on the image for an enlarged view.)

Bob removed refrigerant until the suction pressure was about 65 psig and the head pressure was reduced to 250 psig. Then he started adding vapor back to the system and watched the superheat. The system had a line set of about 20 feet, so he charged the system to a superheat of 12° (Figure 2 shows how this was calculated).

Bob went to the manager’s office and reported what he had done, and the manager said, “I can already feel the difference. Thanks.”

Bob then asked Btu Buddy a question, “When I checked the system using the touch test, the suction line was really cold. It seemed like the system should have been really cooling. What exactly was the problem that it was not cooling?”

Btu Buddy answered with a question, “What was the coil boiling temperature of the evaporator when you checked the pressure?”

Bob then said, “I asked you a question and you answered with a question. I guess you are trying to get me to think again. The suction pressure was 85 psig. That equated to a suction boiling temperature of about 55°.”

Btu Buddy then asked, “What should the boiling temperature be for an air conditioning coil?”

Bob then said, “I see what you are getting at. The suction boiling temperature should be about 40°. The coil was not cold enough, but the suction line was really cold.”

Btu Buddy said, “It was actually 50° because there was 0° superheat, about the temperature you had with a 40° coil with 10° superheat. The big difference in the feel of the line was that with 0° superheat it had liquid in the line. When there is liquid in a suction line, it will remove heat from your hand much faster than vapor. With vapor in a line at 50°, you can feel it change temperature when you grip it very tight. If there is liquid in the line and you grip it tight, it will actually be so cold that it seems to hurt your hand.”

Bob then said, “That is why you have always told me to grip the line tight, to see if I can detect liquid that conducts heat faster than vapor.”

Btu Buddy said, “That is correct. Now, there was one other sign that you didn’t seem to notice about this system. There was a large puddle of water under the condensing unit. Where did it come from?”

Bob said, “You are right. I should have noticed that without having to remove the compressor compartment door. The compressor sweating created the puddle. In this hot weather, the suction line would sweat up to the compressor, but it shouldn’t create a puddle like this unit had. I could have probably diagnosed an overcharge of refrigerant from the outside, if I had been thinking.”

Btu Buddy said, “That was a sure sign of overcharge. If you walk by a unit in hot and dry weather and see a big puddle, you should expect an overcharge on a capillary tube system. I can remember one time I was asked to quote doing the maintenance for an office building. I walked up on the roof and there were about 50 condensing units on the roof. The building manager was with me. I looked around and there were empty refrigerant cylinders all over the roof. Also, there were about 10 compressors that had been changed and were just sitting on the roof. I could tell that some very poor service had been performed on the place. As we walked around, I felt each suction line and observed the units. Many of them had large puddles of water under them and it was hot and dry. I explained to the manager what was going on and told him that before I could take over the maintenance, my technicians would need to get the system charges correct. It took two men four days just to regulate the charges to the correct charge. All of the preliminary diagnostics were done with looking and touching the system components.”

Bob said, “Boy, that seems like a lot of time.”

Btu Buddy said, “It was, but I was not willing to take over 50 systems that were out of order. The first thing that would have happened is our company would start getting the blame for the problems. I believed that the systems should be brought up to standard before we assumed the responsibility.”

Bob then said, “You didn’t mention what the liquid refrigerant getting back to the compressor could do.”

Btu Buddy said, “I was getting around to that. The liquid refrigerant that was flowing back to the compressor was probably wet gas. By that I mean there was not a steady stream, but just a trickle of liquid or a saturated gas mixture. If you were looking at it like it were water, you will notice that you cannot see water vapor in the air until it becomes saturated and then we call it fog. The small amount of liquid getting back to the compressor was mostly boiled to a vapor by the warm motor windings. If any of the liquid trickled back as real liquid, it would drop down in the oil. When the oil is mixed with liquid refrigerant, it will dilute the oil and it will not be a good lubricant. Long running times would eventually score the bearing surfaces due to poor lubrication.”

Bob then said, “You can really learn a lot about the system by looking and touching. The gauges will verify what you see and are used to tune the system to the best performance.”

Btu Buddy then said, “You are learning a lot from these calls. Keep up the good work.”

Publication date: 07/21/2008