An internal heat exchanger is simply a heat exchanger that uses the cold gas leaving the evaporator coil to cool the high-pressure liquid that is headed into the beginning of the evaporator coil via an expansion device. The gas is used to chill a chamber that normally has a series of pipes for the liquid running through it. The superheated gas then proceeds on to the compressor. The subcooling term refers to cooling the liquid below its boiling point. 10 °F (5.6 °C) of subcooling means it is 10 °F colder than boiling at a given pressure. As it represents a difference of temperatures, the subcooling value is not measured on an absolute temperature scale, only on a relative scale as a temperature difference.