A superheater is a device found in steam boilers that is used to convert wet, saturated steam into dry steam. Superheaters are a very beneficial part of the steam cycle, because dry steam contains more thermal energy and increases the overall efficiency of the cycle. Not only that, dry steam also is less likely to condense within the cylinders of a reciprocating engine or the casing of a steam turbine. Boiler superheaters can be found in three varieties: radiant superheaters, convection superheaters and separately fired superheaters.
Radiant superheaters are located directly within the combustion chamber of the boiler itself. This arrangement allows for the burner from the boiler to heat both the boiler tubing and the superheater tubes, making radiant superheaters highly effective devices. These are most commonly found in steam power plants and also were widely used in steam automobiles. In steam automobiles and power plant boilers alike, the superheater tubes — sometimes known as vaporizer coils — were located directly on top of the burner. Steam usually is run through the superheater after it has been admitted through the throttle.
Convection superheaters are most commonly found on locomotives. Much like a convection oven, this type of superheater utilizes the hot gases from the burner to reheat the steam. A convection superheater can be extremely efficient, because most of the thermal energy is given only to the boiler tubing, and what would normally be exhaust instead heats the superheater tubes. These are also found in power plants, but they were mostly implemented in steam locomotives.
On steam locomotives, convection superheaters are slightly different from what one would find in stationary steam power plants. Both use the same principle of utilizing hot gases, but locomotive superheaters usually are somewhat separate from the boiler itself. Instead, they are positioned in front of the boiler, and hot gases from the boiler tubing go through tubing of the superheater.
Superheaters have many advantages, the most notable being reduced fuel consumption and increased efficiency. The only disadvantage is increased maintenance costs, but superheaters still generally are considered to be worth any extra costs. Another disadvantage of superheaters is found almost exclusively on locomotives using fire-tube boilers. Without proper and regular maintenance, the tubing within the superheater is prone to rupture. If the tubing was to rupture, high pressure steam could escape through the opening and into the firebox of the locomotive, and such an event would put any personnel in the engine cab in serious danger.