In order to generate steam fast enough to supply the engine, most boilers have tubes, either fire tubes or water tubes. These provide a large area of heating surface. Early boilers and engines had to change about 30 pounds of water to steam every hour to supply one horsepower. Modern types do better, altough efficiency varies widely according to type. Fuel consumption can be reduced by preheating water, and air for the fire, with heat from exhaust steam.
Considerable advantage is obtained from superheat.
Since increased temperature is needed to produce steam when the overlying pressure is high, early boilers could barely keep high-pressure steam saturated, that is, free from condensed water vapor, as it expanded and cooled while working trought the engine. By passing steam trought tubes surrounded by fire, it can be superheated several hundred degrees. Then its expansion while working the engine will not cause condensation ad loss of power. Since most water contains minerals which form scale in or around boiler tubes, a water purifier may be needed. This usually has a spring which holds a valve closed tightly against normal steam pressure.