Vacuum Pan, a closed vessel, from which a portion of the air and steam can be removed from the surface of a boiling liquid. Heat is usually supplied to the vacuum pan by steam coils, which are placed in the lower part of the vessel. The upper part is connected with an air pump, which exhausts the air and then the steam as fast as it is formed. Vacuum pans are used for evaporating syrup in sugar refineries and in the manufacture of other substances, where evaporation at a low temperature is required. The principle upon which they work is that the boiling point of a liquid is lowered as the pressure upon its surface decreases. See BOILING POINT; VACUUM.