a chamber surrounding the housing of a heat exchanger, or the cylinder of a steam engine, through which the heating steam passes. The steam jacket in heat exchangers maintains the wall temperature of the housing at a constant level. The steam jacket of a steam engine maintains the temperature of the cylinder walls at an approximately constant level, close to the temperature of live steam; this reduces the heat loss resulting from initial condensation, which occurs when steam entering the cylinder comes into contact with the cooler cylinder walls. Steam jackets are particularly important for engines that operate on saturated steam, since they substantially increase efficiency.