The word “cold” does not exist in scientific terms; it is recognized and measured by the lack of heat. Refrigerators work by pumping heat out of the cabinet interior. If you wave your hand over the back of your fridge, you’ll feel a little warmth. However, if it feels decidedly hot, the compressor pumping refrigerant through the system may be on the point of failure – and that will leave you with a fridge full of spoiled food. If you suspect overheating, call your refrigerator technician immediately.
Compressor failure occurs when its lubricating oil is heated beyond 135 degrees Celsius. At this point the oil breaks down chemically and loses its lubricating properties. In addition to causing compressor failure, the entire system suffers collateral damage. Compressors generate heat naturally via friction, compression, and electrical motor heat. These factors are taken into account during design; therefore, if compressor temperature is kept within design limits, no harm results. Overheating is caused by one or more of four main system problems.
High compression ratio
The primary cause of compressor overheating and failure is a high compression ratio. This results in operating the compressor at too low a suction pressure, too high a discharge pressure or a combination of both. Therefore, the service technician must adjust the system to run at the lowest possible compression ratio.
High return gas temperature
High return gas temperature is caused by high discharge pressure cycling through the system. This can result from a variety of system problems, such as a blocked or undersized condenser, undersized discharge line, lack of condenser air circulation, inoperative fan motor and the overcharging of refrigerant gas in the system.
Inadequate motor cooling
Air cooled compressors require at least 650 feet per minute of air velocity across the body. If the condenser fan is faulty it must be replaced. On the other hand, if it does not meet these requirements, the technician must install an auxiliary fan to compensate for the lack of air flow.
Ask the technician to check the type of refrigerant in use if the system has been previously recharged. The use of R12 refrigerant usually presents no problem. On the other hand, R22 is a poor refrigerant to use for low temperature applications and is marginal for use in medium temperature below minus 12 degrees Celsius. While R502 refrigerant works under normal conditions, it can be susceptible to problems caused by excessively low suction pressure or high return gas temperature.
Many contributing factors may cause overheating, such as undersized evaporators or oversized compressors, or plugged driers that prevent proper circulation of refrigerant via the thermostatic expansion valve. To correct this, the technician must set the valve to a minimum practical evaporator superheat as recommended by the manufacturer. He must also keep the pressure drop in the suction line to a minimum while maintaining sufficient velocity for proper compressor oil return.
Check for overheating by measuring discharge line temperature. Apply a strip of calibrated heat-sensitive tape to the discharge line six inches from the discharge service valve to act as a visual check. Failure occurs at 135 degrees Celsius. The danger zone lies at 121 degrees. The compressor temperature is within design limits at 107 degrees or less. If all else fails or if redesigning the system is impractical, ask the technician to install a de-superheating expansion valve on the suction line, at least two elbows away from the evaporator. This injects liquid refrigerant into the system which cools the suction gas to a safe temperature.