In a boiler, the saturated or slightly wet steam is made to pass from the steam drum to a separate set of tubes inside the boiler where the steam receives more heat than is generated in the furnace.

The superheated steam is collected in another drum from which it passes on to the turbine room; in some instances, the drum is omitted and the superheated steam passes directly from the superheater to the turbine room.

In the superheater, the temperature of the steam is thereby increased but the pressure remains the same, or drops slightly on account of the friction in the superheater tubes and piping to the turbine room.

The particular section of a boiler in which the saturated steam from the steam drum is heated to a higher temperature (or to a superheated condition) is known as the superheater. Superheaters are broadly classed according to the source of heat:

1. Convection type—This is usually placed in the gas passages of the boiler where the heat is transmitted by convection.

2. Radiant type—This is situated in the boiler walls which receive radiant heat.

The degree of superheat is largely determined by the position of the superheater, the amount of superheating surface, and the velocity of the steam through the superheater tubes. In the case of radiant types, the temperature is affected by furnace temperatures.

The radiant type has the advantage that it can be added to existing installations.

Often both types of superheaters are used in conjunction with each other as the combination of the two will generally result in a greater utilization of the heat from the burning fuel and in more efficient overall operation of the boiler.

Since there is no water inside the superheater tubes, they will be subjected to more severe service than the other tubes in a boiler. Special attention is given to the materials used in their design, manufacture and installation.

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