The Compressor Mystery Solution
At the conclusion of the article, "The Mystery of the Oil Safety Trip," readers were encouraged to enter a contest by submitting their answers to the Contractor Connection at www.emersonclimatecontractor.com. Our winner is Paul Catrino, Pennsauken, N.J., of United Refrigeration. He wrote:
"What jumped out at me was the high discharge temperature. Copeland uses 225 degrees F as a critical discharge temp for their compressors. This system is very close to that. Also, the amps are very high. I believe 23 or 21 amps are what we would like to see for this compressor. Your net oil pressure is 18. Your ohms are good.
"Is it possible the high oil level in the oil sight glass is really refrigerant? Either by slugging or migration, refrigerant is getting to the crankcase. Is there a pumpdown system on this case?
"My thoughts are that there is refrigerant in the crankcase; when it starts you hear a rumble, it finally settles down, but as you pump out the refrigerant, you pump out the oil, washing the oil off of the mechanical parts. This will cause the high discharge temp.
"Your compressor superheat is good. Your evaporator superheat is 6. I am leaning towards either no pumpdown or a leaking solenoid valve. Keep putting these questions in your magazine. It keeps us thinking."
The ProblemAccording to Copeland's Dave Bell, "The protector trips from a high amp draw. Even though the operating amps were 5 amps below the RLA, the system had a high amp draw.
"Operating amps should have been 23.0 at 218 V under current operating conditions. Oil pressure is low, although it is not at the trip point.
"With low oil pressure and high amp draw, one would suspect worn internal components like rod journals or bearings. This compressor has an internal overload, so when it trips the control circuit stays closed.
"With the compressor cycled off on overload and the control circuit closed, the oil protection device tried to sense proper oil pressure. With the compressor off, there is no differential pressure so the oil control trips."
What caused the problem? "If we have worn bearings," Bell said, "I would suspect some wear on the rods also. Do a quick head inspection for any upper-end wear or broken components. Look at the cylinders. In this case, I don't believe we would see major upper-end wear other than maybe some cylinder wear.
"Next look at the lower end and crankcase. Pull the oil screen, look at the oil. Look for debris that has the oil screen restricted. If there is bearing wear, the oil will probably be gray and the screen will be at least partially restricted.
"Compare operating amperage to suction and head pressures applied on the compressor, not the RLA. The RLA is derived from a mathematical formula and has nothing to do with the compressor's running or operating amperage.
"The high oil level was a result of off-cycle liquid migration. The noise heard on startup was liquid refrigerant being cleared from the compressor, a flooded start, which in itself can cause erratic bearing wear.
"In this case, it was the result of the oil trip and off cycle that followed. A severe flooded start can result in broken internal compressor components. Take care clearing the liquid to prevent damage, then verify the oil level after the liquid refrigerant is cleared out.
"The oil pressure readings are also misleading because you are only looking at the gross pressure coming from the oil pump. Net oil pressure is the pressure from the oil pump minus the crankcase pressure.
"Also, this compressor is rated for medium-temp applications and operating here in a low-temp freezer. This is a marginal application; the restrictions need to be followed.
"Sooner than later, the compressor will be replaced. However, once we confirm the worn bearings, we need to think about what caused the wear.
"Maybe it was a loss of oil, liquid refrigerant returning to the crankcase, or high discharge temperatures. Find the root cause and remedy the problem to prevent future failures for your customer."
Publication date: 11/22/2004