It is a known fact that if oil has become trapped in the evaporator, the warming of the evaporator may help return the oil to the compressor. This can easily be done with systems such as those in low temperature applications that have a defrost circuit,for example, technician can initiate a defrost cycle to warm the evaporator and aid in the return of the oil. Sometimes more than one defrost will need to be initiated. So whan you have a dirty evaporator you have poor heat transfer thereby reducing the boilong action of the refrigerant which decreases the velocity of the refrigerants' ability to return the oil to the compressor.
