When refrigerant flows into a direct exchange evaporator, it is mostly saturated liquid, with some vapor.  As refrigerant travels through the evaporator, it absorbs heat from the air.  As it absorbs heat, it vaporizes.  If the system operates according to design, the refrigerant will be 100% vapor as it nears the exit of the evaporator.  Before leaving the evaporator, the vapor continues absorbing heat, becoming superheated.  Any temperature gains the refrigerant has as a vapor is superheat. There must be at least a small amount of superheat present to ensure that only vaporized refrigerant enters the compressor.
How much superheat should there be at the exit of the evaporator?
The amount of superheat that should be present before exiting the evaporator depends on the system’s design and application. In general, low temperature applications require 4° to 6°F of superheat, and a medium temperature system requires 6° to 8° of superheat.
What causes improper superheat values?
The main cause of improper superheat is low air flow and insufficient heat exchange. This problem is usually caused by a dirty or iced up evaporator or condenser coil (or both).  An inoperable evaporator fan motor could also cause the low air flow.
Another cause is a metering device that is dirty, defective, or out of adjustment.  If a metering device feeds either too little or too much refrigerant into the evaporator, improper superheat values will result.
To learn more about evaporators, superheat, or a large variety of other topics, subscribe to the Ignitor Labs Membership and the Tech Foundations Program. Check out our full Interactive Training Course on the HVACR Foundations.