The trend within the industry is to blame the compressor when a refrigeration system fails. However, according to Kevin Glass, engineering manager of Bitzer UK, compressor failure is a rarity and effective diagnosis is critical.
THE most recently available figures from Bitzer UK on compressor returns show that in 2007 less than 0.25% of total UK compressor sales were valid warranty claims. 95% of alleged warranty failures turned out to be caused by external influences from the refrigeration system itself - that is to say that the compressor was not at fault.
So actual compressor failures were a real rarity. Yet, still the trend within the industry is to blame the compressor when a refrigeration system fails. There is no justification for this, as Bitzer and other manufacturers will verify from their own analyses.
The compressor is one part of the refrigeration system that has mechanical moving parts and is therefore seen to be the component most prone to failure. It also has to work harder than anything else in a system - 24 hours a day at around 1500rpm.
In practice, a compressor and its associated refrigeration system would be lucky to get a full service once a year, yet the very small percentage of genuine compressor failures that are reported by manufacturers is evidence of how reliable compressors actually are.
Early diagnosis
Today's typical semi-hermetic reciprocating compressor has a minimum expected life of around 50,000 hours - that's just under six years running 24 hours a day. That is a remarkable life expectancy and compressor manufacturers continue to further improve product reliability to reduce warranty claims even more.
So if compressor failure is so rare, there must be other causes of refrigeration system breakdown. Some are the result of system users not being able to diagnose potential problems correctly or quickly enough. Early diagnosis must concentrate on key operational areas some of which are covered below.
The condition of the lubricating oil can have a great impact on the life of the compressor. The oil can be affected by a number of different scenarios, including refrigerant temperature and pressure or moisture.
Today's HFC refrigerants require the use of ester based oils, which under normal circumstances do not give us any concerns. However, these oils, being hygroscopic, will absorb moisture which can affect their ability to perform as intended. The effect of this will be an increase in wear on the bearing due to the reduction of viscosity.
Refrigerant temperature and pressure also have an effect on the oil. This is due to the miscibility and solubility of the refrigerant into the oil. This is understood by compressor manufacturers and the specification of the base viscosity of the oil takes this into account, so under normal operation there are no issues. However, outside of 'normal operation' a larger amount of refrigerant can be dissolved in the oil, which reduces the viscosity of the oil and can lead to early failure of bearings.
The refrigerant temperature and pressure has a large influence on how much of it is absorbed by the oil. Refrigerant dilution of the oil is greater with low temperature and high pressure so, for example, the dilution would be much greater for a system which has very low suction superheat, as a result of a faulty expansion device, than one with sufficient superheat.
Example: a screw compressor system with a condensing temperature of 45°C on R404A (20.5 bar) and a discharge temperature of 80°C would typically have an 87%/13% oil/refrigerant mixture. If the temperature were reduced to 50°C then the mixture would be 70%/30%. This effectively halves viscosity of the oil and bearing damage can be the result.
The majority of compressors in service are either hermetic or semi-hermetic, having a built-in drive motor. Another cause of failure on this type of compressor is electrical supply problems. Such problems as low voltage, missing phases (single phasing) or phase asymmetry have a disastrous effect on the compressor and along with incorrectly adjusted timers contribute to a large proportion of the total causes of failure.
Perhaps the largest surprise in the 2007 figures is the number of compressors returned under warranty which have no faults at all.
Diagnostic skills
The findings can point to many different reasons for a system failure and should also lead to corrective measures being taken in good time.
There is always pressure to keep refrigeration systems running and service technicians may not have sufficient time to investigate fully the root causes of the failure. The compressor takes the blame and it is felt that by replacing it the problem is cured. In actual fact, the replacement compressor is now also under threat from the real cause of system failure.
It is also questionable whether all operatives have the ability to undertake system diagnosis. Sometimes even basic skills, which in previous generations may well have been covered by traditional apprenticeship schemes, are not held.
So to keep compressor failures to a minimum, service engineers should be analysing the pressures and temperatures taken and then take the relevant actions to keep the system as well as the compressor operating efficiently and without risk of damage or failure.