At the sea level, with an atmospheric pressure of 29.922 in. of mercury in the barometer - in other words at a pressure of 15 lbs. on the square inch - water boils at a temperature of 212° F. (100° C). Above this level, the layers of atmosphere become less dense and consequently exert less pressure. The boiling point is, as a result, reduced several degrees below 212° F. With an increase of atmospheric pressure, as found in a deep mine, the reverse takes place, and water requires the application of several degrees of heat above 212° F. before it actually boils.

This variation of the boiling point of water under different pressures is taken advantage of in many manufacturing processes by the use of the vacuum pan. Under a reduced pressure, produced by mechanical means, liquors can be evaporated and concentrated in the vacuum pan without injury to the active ingredient they contain. By working under a low pressure, clarified sugar juices, food extracts, glycerin, dyewood, gelatin, and other liquors can be concentrated to any desired extent without injury. If such liquors were heated to a temperature of boiling water for any prolonged period, as would be necessary were they evaporated in an open pan, their nature or constitution would to a greater or lesser extent undergo a change and they would be spoiled. For vacuum evaporation, a pump is necessary, first for exhausting the air and the steam from the vacuum pan and then for sending both to a vessel called a condenser where the vapors are condensed. One of the most practical devices is called the multiple effect system. This device consists of four simple vacuum pans so connected that the steam from the boiling liquid of the first is made to pass through the others. In this way the heat of the steam of the first pan is sufficient to heat the liquid of the second to the boiling point, the heat of the steam of the second raises the temperature of the third, and so on.