Originally Posted by chemi-cool
Common Failures in Hermetic Compressors.
Most people involved in the refrigeration & air conditioning business know that many hours of talk and a huge quantity of papers have dealt with this topic during years. But we also know that these common failures still happen daily, so I find it is time to add another few papers to the already existing pile of papers.
What is wrong with these hermetic compressors, since we see them fail in such a quantity? From my point of view, nothing is wrong with the compressors.
Today's manufacturing technology has made it possible to improve the quality to a level never seen before, so the compressors should have a long and trouble free life span.
However, the life span depends on quality of installation, operating conditions and service.
When a compressor fails, one has to ask oneself: "Why did it fail"? The true answer has to be found inside the compressor, which means the compressor has to be cut open for inspection.
From the inspection the answer to the question is normally found, so one has to ask oneself the next question: "What can be done to prevent this failure to happen again?"
Seized Piston
It is very essential to take action according to the failure found from the inspection, before a replacement compressor is installed. Statistics show that replacement compressors failure rate is much higher than in new applications. This clearly indicates that the root problem still remains in the application.
Seized connecting rod bearing
The most common problems seen in hermetic compressors over the past years are:
Lack of lubrication / low oil level.
Overheating.
Flooded starts.
Slugging Moisture
Burnt motor windings
In this edition of RACA I will deal with the "lack of lubrication / low oil level" topic.
It is very important for a compressors life span that the bearings are supplied sufficiently with clean oil, which has the right viscosity to create a sufficient oil film on the bearing surfaces.
The two terms "lack of lubrication" and "low oil level" may look confusing, as lack of oil causes lack of lubrication, but lack of lubrication does not necessarily occur due to low oil level only.
Lack of lubrication is possible even with correct oil level in the crankcase housing. It may happen due to:
The oil does not reach the bearing surfaces due to low oil level or blocked oil channels.
The oil reaches the bearing surfaces, but is mixed with refrigerant or other contaminants and does not lubricate effectively.
The oil reaches the bearing surfaces, but is decomposed due to overheating, oxidation, moisture etc. and has lost lubrication performance.
The oil reaches the bearing surfaces, but the viscosity is too high to lubricate efficiently due to high oil temperature. Oil temperature higher than 95° is considered critical.
Low oil level occurs when the oil leaves the compressor and does not come back at all, or does not come back quickly enough or in sufficient quantity to maintain a correct oil level in the compressor.
Oil return to the compressor must be equal to oil leaving the compressor.
Conventional compressors will always carry-over some quantity of oil to the system. Therefore, it is very important to monitor the oil level in the compressors sight glass during the first hours of running in a new system, as it may be necessary to add oil to the system until a balance is obtained. To get a quick and sufficient oil return during operation is just not to install an oil separator. The most important factor for efficient oil return is the pipe design. When going to site, how often does one see horizontal suction lines with a slope of approx. 0.5% ( 0.5 cm/m) in the flow direction? How often does one see a U-trap in the suction line, when the compressor is situated at a higher level than the evaporator? How often does one see U- traps (risers) for vertical suction lines?
Seized Crankshaft Bearing
How often does one see double risers for compressors in parallel or compressors with capacity regulation? The velocity in horizontal suction lines is recommended to be minimum 4m/s and in vertical suction lines 8 to 12m/ s, but how often does one see vertical suction lines with a smaller diameter than the horizontal line? Short cycling may also cause low oil level, as the compressor does not run long enough to return oil sufficiently. Especially in applications with long pipe runs or vertical pipes with risers oil return will be critical.
Loss of refrigerant charge may allow some oil to leak out of the system. However, more important is that lost refrigerant charge causes reduced refrigerant flow through the system and oil may be trapped and not returned.
Especially in low temperature systems low oil level can be a problem even with a proper pipe design. The problem is related to the frequency and duration of the defrost cycles, which has to be carefully planned to ensure that trapped oil in the evaporators is returned to the compressor before low oil level occurs. Please, note that in low temperature systems the sight glass may show low oil level during normal operation, but when starting up after a defrost cycle the increased refrigerant velocity will return the oil from the evaporators.
Operation with part load means reduced velocity in the system and the risk for insufficient oil return will occur. A hot gas by pass installation can eliminate this problem.
The conclusion to eliminate system problems which causes lack of lubrication must be:
Correct system design.
Clean and careful installation.
Proper component selection and operation.
Scheduled inspection and maintenance.
Chemi:)
