The health of a refrigeration compressor is fundamental to the efficiency and longevity of the system. The manufacture will provide what’s called the ‘Operating Envelope’ and its crucial that the parameters remain within these conditions. If they start to operate outside of this envelope, then the compressor will begin to fail. Some of the reasons why this occurs could be incorrect selection of the compressor or other system components during the design phase, through to poor service, maintenance and commissioning. Lets take a look at these in more detail now.
Compressor burnouts are often caused when they overheating.
Why compressors overheat
Compressors generate heat. All compressors are designed to tolerate normal thermal gains from compression, motor windings, friction, and normal super-heat. All this heat can be measured on a running system simply by taking the discharge line temperature about six inches or less from the compressor. If a discharge line temperature exceeds 120°C, the temperature inside the compressor at the discharge valve or valves is 150°C or more. At that high a temperature, oil and some refrigerants begin to break down. Carbon and sludge will form. Overheated compressors suffer suction valve failure, worn pistons, sludge and carbon formation, and since proper lubrication is impossible, the compressor will eventually seize up.
Corrective action needs to be taken or the compressor will fail.
On the refrigeration side, the most common causes of overheating are the improper setting of controls. A TXV, EXV, EPR, hot gas bypass, unloaders, pressure control switches, any or all of them improperly set, can contribute to overheating of the return gas superheat. Piping problems involving the suction line can cause high temperature return gas. Semi and full hermetic compressors are cooled by suction gas. Cooling the motor windings is a function of gas flow and the gas temperature. Each compressor manufacturer sets limits. The most common high limit is 18°C with some 24°C. This temperature is the actual maximum temperature of the entering suction gas, not the superheat. Suction gas superheat is usually limited to 4°C to 8°C. It is always best to check the manufacturers specifications to obtain these limits, and not exceed them. Heat is added to suction gas as it flows from the evaporator to the compressor.
This heat may be excessive if the suction line is abnormally long, uninsulated, or installed in hot spaces. High compression ratios are a cause of overheating.
A refrigeration system’s compression or pressure ratio is defined as the absolute discharge pressure divided by the absolute suction pressure. Calculating this ratio can be a big help when it comes to troubleshooting a system.
Absolute pressure = Gauge Pressure + Atmospheric Pressure
Discharge pressure = 10BarG
Suction pressure = 0.35BarG
Compression Ratio = (10BarG + 1.1Bar) / (0.35BarG + 1Bar) = (11.1BarA) / 1.35BarA) = a ratio of 8.2:1
This means the discharge pressure is 8.2 times the magnitude of the suction pressure.
The compressor manufacturer determines maximum compression ratios. Determining the compression ratio that a compressor is running at is easily done. If a compressor is running with a compression ratio outside its design limits, the reason or reasons must be found and corrected or the compressor will burn out. The gauge readings taken will guide the service technician to a starting point.
HIGH CONDENSING PRESSURES
High head pressures with an air cooled condenser may be caused by:
- a dirty or blocked condenser
- fan problems
- high ambient air temperatures
- non-condensable’s (air or nitrogen mixed with the refrigerant) in the system
- misadjusted, or malfunctioning fan controls
- refrigerant overcharge
- high heat load on the evaporator
On water-cooled equipment:
- look for scaled-up condensers
- high inlet water temperature
- low water quantity.
LOW EVAPORATOR PRESSURES
Usually, low suction pressure is the more common problem when too high a compression ratio is encountered. The following can all contribute to low suction pressure:
- low load
- misadjusted, or improperly sized expansion device
- mismatched or defective evaporator
- poor suction pipe sizing
- poor installation
The problem may be poor system design, making it necessary to operate the compressor outside its design conditions.
If no other reason can be found for high compression ratio, recalculate the size of a system using all the known parameters to see if the present system should be able to handle the now accurately known load. Chances are, it won’t be able to handle the load. More refrigeration will need to be added. Perhaps the original design engineer was not notified of the entire load, or more load has been added to the original design.
Electrical failures are a major cause of compressor motor failure. A common cause of Three-phase motor compressors failing is from voltage and current imbalance. And this imbalance causes overheating. Single phasing, where one leg of the three phases is lost, is the ultimate imbalance. Failure is rapid. There are many inexpensive devices that detect phase loss, imbalance, too high or too low a voltage, and quickly take the compressor offline before it can be badly damaged. Normal voltage tolerances for most compressors are +/- 10% of nameplate rating. Voltages outside these minimum/maximums should be corrected.
A three-phase compressor is a big, expensive item. Every protective device available ought to be used to prolong its useful life. Single-phase motor compressors may employ a complete set of starting components, or at least, a run capacitor. Should any of the components fail, the compressor will draw locked rotor current (LRA) when trying to start. Even though an internal or external overload may open and take the compressor offline, overheating has occurred, and repeated attempts to start should be avoided as this will result in motor winding failure. Always check the start and run components and replace any defective items with the manufacturer’s recommended devices. A compressor takes a few minutes of run time to get rid of the excess heat generated by the locked rotor current drawn from normal starting. Short cycling systems do not allow all the heat from previous starts to be removed. A build-up of heat will occur and the compressor will burn out. Re-adjust control settings to prevent short cycling and use anti-short cycling time delay devices.
The substantial number of compressors are often diagnosed as having failed but were found to be perfectly good and this must be addressed. The service person may have jumped to a conclusion without making a thorough investigation. Perhaps that person did not have the proper tools, instruments, or training to make a correct diagnosis. The manufacturer doing the teardown followed up on many of the returned good compressors to attempt to find out the reasons why many compressors were misdiagnosed.
They found the reasons for the errors were:
- Not allowing the compressor to cool off long enough to permit any internal overloads to close
- Not using a good Voltage/Ohm meter or insulation tester to check the motor windings
- Using the wrong scale on a meter
- Not knowing what meter readings mean
- Not checking contactors, wires, and terminals
- Not checking startup voltage at the compressor terminals
- Not checking starting and running components on single-phase units
- Not waiting for pressure to equalize on cap tube type systems
- All others, such as simply a blown fuse, defective control, etc
When replacing a compressor, do not take shortcuts. Shortcuts may save time and even get a system running, but are not worth the chances of repeat compressor failure. Always follow good cleanup procedures. Before leaving any compressor replacement job, a good technician will identify any problems and correct them, or at least bring them to the attention of the service manager who can recommend corrective action to the customer. If a compressor fails and the cause of failure has not been determined and eliminated, the replacement compressor will quickly fail.