Batch pan evaporators consist of spherical shaped, steam jacketed vessels. The heat transfer per unit volume is small requiring long residence times. The heating is due only to natural convection; therefore, the heat transfer characteristics are poor. Batch plants are of historical significance; modern evaporation plants are far-removed from this basic idea. The vapors are a tremendous source of low-pressure steam and must be reused. Batch pan systems are used in some jam processes. Soluble solids are increased to 65% or above in a jacketed vacuum kettle. Evaporation of liquid in open pan evaporator is achieved by use of internal coils, direct heaters, or by means of jackets. When heat sensitive products are to be processed, the heating vessel is connected to a vacuum system so that boiling of the product occurs at lower temperatures.