Compressors naturally run warm due to the heat from the Motor, from friction and from the Compression cycle. Providing the compressor runs within its design limits then it will give long and reliable service. When the compressor is run outside of its design limits or some other external situation exists that creates this problem then the lifetime of the compressor will be drastically reduced. Compressors are designed around evaporating and condensing conditions along with the various refrigerants available.
Compressors evaporating below -10°C may show frosting on the suction connection. This is quite normal providing the oil sump temperature is warm to the touch.
When the compressor is run outside of design limits, the first thing that happens is that the discharge temperature/superheat increases. This increase in temperature is first felt inside the cylinders and at the discharge valves. As the temperature increases, the lubrication to the cylinder and piston is literally cooked away causing the now dry rubbing surfaces to score and fine iron particles are scrapped from the cylinder wall and end up in the compressor sump oil and around the compressor motion gear. This overheated oil as it passes through the discharge valves causes the oil to coke or carbonise on the valve plate causing the valve to leak bye, elevating the temperature even higher. This gas leaking past the valves and pistons will elevate the compressor suction pressure and cause loss of capacity in the machine. As the oil in the compressor starts to break down due to the heat, acids and carbon are produced. These acids will, to a certain extent, be absorbed by the drier desiccant, but will eventually start attacking the motor winding insulation and cause either a spot burn or full motor burn out.
The weak acids that are produced will slowly migrate throughout the entire system and may cause other problems later on. As the oil continues to degrade, its ability to lubricate and support the bearing loads and stresses become compromised to the point where extensive wear to the machine takes place. With the discharge valve plate leaking bye, the upper side of the piston gudgeon pin bearing surfaces wear as the piston has compression loading on the up and down stroke. This will cause loss of capacity and also cause the piston to “slap” the valve plate causing a loud knocking noise.
The cylinder overheating will cause the gudgeon pin plastic caps to melt allowing the pin to grind into the cylinder wall cutting a deep elongated slot , thereby making the machine useless.
Overheating will cause the oil to darken as it becomes degraded (carbonised) .This can be seen by looking in the oil sight glass. The oil should look clear and transparent. Any darkening will be due to overheating. The temperature at the discharge pipe should be monitored and is normally around the 70-80°C mark. When discharge temperatures of 110°C plus are monitored the oil is starting to degrade. Discharge Safety thermostats or DGT’s normally trip around the 130°C mark. These stats will prevent the compressor from being totally destroyed providing preventative measures are taken to get the discharge condition back to normal, and are NOT left to short cycle under automatic control.
Remember these DGTs trip at the MAXIMUM worst condition and are a safety control, and should not be used as a running control point.
Typical causes of overheating: