Many refrigeration compressors break down simply because they overheat. There are two main reasons why overheating can have such a devastating effect on compressors:
Loss of the lubricating properties of the refrigeration oils
Chemical decomposition of the refrigerants and/or oils, which can occur suddenly
The oil breakdown in the refrigeration system has plenty of negative side effects. Sludge and solid particulate matter can plug the oil inlet screen in the compressor sump or the lubrication passages in the crankshaft—loss of lubrication can quickly result in failed bearings. Oil-breakdown deposits can also line the internal surfaces of the refrigeration system, especially the inner walls of the piping, compressor and control valves, plugging up thermostatic expansion valves (TEVs) and other control valves.
All these problems can be the result of a dirty condenser, which results in a higher condensing temperature. According to EPA research, a heat transfer coil with a meager 0.042-inch-thick film of dirt on its surface can result in nearly a 21-percent loss of heat-transfer capacity. A higher condensing temperature, combined with increased suction temperature (from underfeeding TEVs), can result in excessive discharge temperatures that accelerate oil decomposition.
Before they can fix the problem, technicians must know what caused the excessive discharge temperatures in the first place. The four most common root causes are listed below:
High suction superheat—common system conditions that cause increased suction temperatures are high thermostatic expansion valve superheat settings, ineffective or missing insulation and restricted thermostatic expansion valves.
Reduced condenser capacity—typically the result of poor maintenance, this occurs when the condenser fins become restricted with dirt and the airflow necessary to deliver the condenser’s rated capacity is reduced.
Lowering the suction pressure—this is important to operate the system with the highest possible suction pressure. When system suction pressure is reduced, instead of fixing the true issue, the compression ratio increases creating higher discharge temperatures.
Refrigerant characteristics—because R-22 is subject to higher compression ratios, which can stress bearings and reduce compression efficiency, R-22 can be problematic as a “refrigeration” refrigerant, particularly in low-temperature applications.
In many cases, system problems and compressor failures are directly related to high discharge temperatures.
Condensers should be cleaned regularly to keep them operating at their rated capacity. The suction vapor temperature should be kept within acceptable limits by setting TEV to superheat appropriately and insulating the suction line properly. Compressors should not be allowed to operate below design suction pressures, because this will lead to higher discharge temperatures.
With some refrigerants and applications, other solutions (for example, a compressor body cooling fan motor) may be required to solve high discharge temperature issues. A temperature-responsive expansion valve, which responds to discharge temperature, may also be used. By injecting saturated liquid/vapor into the suction line temperature-responsive expansion valves will reduce the temperature of the superheated suction vapor, which in turn reduces the excessive discharge temperatures.
Article contributed by Sporlan Division, Supermarket Refrigeration, Parker Hannifin
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