Bernoulli Law of Velocity: states that with an increase of velocity there will be a decrease in pressure. In the center of a refrigerant pipe with an increase of velocity, the vapor pressures drop. Vapor pressure is less in the center of the refrigerant pipe because of a higher velocity. At the same time, there is a higher pressure and lower velocity on the walls of the refrigerant piping due to friction loss. Liquid may be in the center of the refrigerant piping at a lower pressure and a higher velocity. With the pressure drop in the center of the refrigerant pipe, liquid can be present until .5ºF to 2ºF of superheat is attained.

Superheat is the measurement of how full the evaporator is of liquid refrigerant. High superheat means the evaporator is empty. Low superheat means the evaporator is full. Superheat should never fall below 4° degrees or a compressor failure will occur. Never add refrigerant when the compressor is at full load amp (FLA) or running load amp (RLA). High superheat at (FLA) or (RLA) means high heat load on the evaporator or condenser.