Btu Buddy 86: Flooded Compressor Discussion

May 24, 2010

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.

Bob and Btu Buddy have gotten together for breakfast to discuss their last service call, which concerned a compressor that had liquid refrigerant flooding back to it.

Btu Buddy said, “Bob, you asked two questions yesterday while we were leaving the job. The first was: Why did liquid flooding back to the compressor not destroy the compressor? The other question was: Why didn’t the flooded coil affect the other two coils in this multi-evaporator system?”

Bob said, “Yes, that compressor looked like it had been flooded for awhile. It looks like that would have done major damage.”

Btu Buddy explained, “You have to think about what was going on inside the compressor. There were not great quantities of liquid getting back or the compressor would have been making noises. The season is mild and the liquid may have been getting back to the compressor for quite awhile. We cannot tell what damage was occurring inside the compressor. If this were a critical job, it would probably pay to open that compressor during an off time and examine it for problems.”

Bob asked, “What kind of problems would you look for?”

Btu Buddy said, “Probably only saturated refrigerant vapor was getting back to the compressor with small amounts of liquid.”

Bob said, “I don’t understand.”

Figure 1. This is a refrigerated air drier that removes moisture from air that is used for pneumatic controls. This air must be moisture free. (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 responded, “Let’s suppose we explain it like an air compressor. The air compressor pulls air out of the atmosphere and compresses it. Suppose the air was saturated, 100 percent humid. It would be like pulling fog into the compressor. Would it do damage?”

Bob said, “I never thought of it, but I guess not.”

Btu Buddy then said, “You are correct. The heat from the compression would probably boil the saturated air to a vapor. The air leaving the compressor would be full of moisture. This is why you oftentimes see refrigerated after coolers on critical air systems, such as air that is used for pneumatic controls (Figure 1). Control air has to be very dry because if there is any free moisture, it will plug the tiny passages in the controls. Now back to the refrigerant system. If the suction gas is 100 percent saturated, it will be like a fog returning to the compressor.”

Bob said, “It looks to me like the compressor would boil that saturated vapor off, like the air compressor example.”

Figure 2. Refrigerant gas would enter this compressor from the right and have to pass over the motor windings. If there were liquid refrigerant returning, it would be heavier than the vapor and drop into the lower motor housing and drain back with the oil to the crankcase and dilute the oil. (©Delmar Cengage Learning.)

Btu Buddy remarked, “You are correct. The compressor could probably handle saturated vapor because it has to pass across the motor windings and into the cylinders (Figure 2). But let’s take it a step further. A system like you worked on yesterday had three different coils on three different floors. Each is supposed to control its own refrigerant flow. The one on the second floor had a loose sensing bulb on the suction line. That valve was letting more than saturated vapor move towards the compressor. Remember, oil and refrigerant are circulating in the system. The liquid that was moving down the line was staying in the oil in the bottom of the suction line. This liquid will reach the compressor and pour out in the motor housing and some of it will stay with the oil as it moves to the compressor crankcase by gravity. The refrigerant that is in the oil will do the damage in this case.”

Bob then said, “I don’t understand why or how.”

Btu Buddy explained, “We have determined that not enough liquid refrigerant has returned to the compressor for liquid slugging. We would hear the compressor hammering, trying to compress it, which it cannot do.”

“True,” said Bob, “tell me more.”

Btu Buddy then explained, “Let’s use a car engine for this explanation. What weight oil do you use in your car?”

Bob said, “I use 20W-40 oil. How does this figure into a refrigerant compressor?”

“Well,” said Btu Buddy, “a car engine is much like a compressor. It has to have the proper lubrication. Suppose you changed the oil in your car and decided to mix the crankcase oil with fuel oil. They are both oils, they just ignite and burn at different temperatures. They will both lubricate.”

Bob said, “I would think that the oil would be too thin to lubricate and that would cause engine wear.”

Btu Buddy said, “You have the picture now. The system would continue to run, but with marginal lubrication. Where do you think that would show up in a compressor?”

Bob said, “I am getting it now. It would show up where the most stress is occurring, at the cylinder walls, the pistons and rod connections, and the crankshaft.”

Btu Buddy then said, “If you opened that compressor, that is exactly where you would look. You would feel the surfaces for how smooth they were and use instruments to check to make sure the dimensions were correct. If that compressor ran for a short while it is probably good to go. Every hour that it runs in that atmosphere it is doing some damage. In this case, all sounds good so it is best to just let it run.”

“Now for the other question: Why did this only effect one of the three coils?” Bob said.

Btu Buddy said, “There is no doubt that the coil that was flooding affected the common suction pressure. The saturated gas from the coil on the second floor mixed with the superheated gas from the other two coils. This helped boil off some of the returning liquid, but not all of it. Remember the liquid mixed with the oil is much harder to boil to a vapor. The suction pressure in the coil on the second floor was high enough to affect coil performance and the coil outlet air temperature went up enough that it wouldn’t properly cool that floor.”

“Boy,” exclaimed Bob, “there is still a lot to learn.”

Btu Buddy agreed, “There really is a lot to learn. Your education never stops in any technical field. The good part about it is you don’t have to learn it all at once. Some of the best help you can get comes from good books, manufacturers, and the people behind the parts counter, particularly the ones with grey hair.”

For additional articles on troubleshooting, visit The NEWS’ HVACR In-Depth page.

Publication date: 05/24/2010