Furnace here mainly refers to the water tube boiler furnace for fossil fuel, used in steam generating stations of modern steam power generating utilities (excludes supercritical type). The key part of a water tube boiler furnace is a boiler drum and associated tubing forming the walls of the furnace in which water is boiled into steam, commonly under pressure. Inside the furnace, combustion takes place using an oxidizer such as air where some type of fossil fuel is burnt to heat the water in the tubes.

Types of fuel[]

The usual and simplest oxider used is air.

Fuel to burn for heat generation may include the following:

  1. pulverized coal
  2. fuel oil
  3. natural gas


The word furnace generally refers to either a smoke tube boiler or a water tube boiler. In a smoke tube boiler, smoke flows inside the tubes to heat water outside, but in a water tube boiler, water flows inside the tubes to be heated by fire or flue gas on the outside of the tubes. The smoke tube boiler is only used for small capacities and is almost out of vogue today. In modern utility steam power stations the water tube boilers are invariably used because of larger and larger capacities demanded.

A typical construction is described here with a single furnace. Manufacturers design may vary depending on their own design requirements based on various factors.

Furnace walls in these are made up of tubes known as generating tubes because they generate steam from water they carry. All these wall tubes, forming the walls of furnace, are open to a ‘boiler drum' from top headers. These top headers are hung from adjustable tie rods from the structure. This drum is a cylindrical one, hung and fixed on the top of structure allowing for longitudinal expansion. The tubes emanating from it form the walls of furnace and terminate also in headers at lower level. Thus the tubes and the bottom headers will be hanging from the top headers. Some of these wall tubes form the roof of the furnace.

Boiler walls construction[]

Present day boilers are made of paneled walls (at factory itself) as compared to tube by tube welding at site. Each tube is placed very closely to the next one to avoid furnace leakage and to give more space for maximum number of tubes to give maximum heating surface area.

The furnace walls outside are covered with something like mineral wool which acts as thermal insulation and on that some metal sheeting to avoid air ingress and mechanical damage.

The water for the boiler, known as feed water, before entering the boiler drum, passes through a feed heater, known as an economizer, located in a second pass of the furnace, where it is preheated by flue gases coming out from the furnace and going to the stack (chimney).


The feed water then enters the drum through a top surface perforated feed pipe, at the bottom portion of the drum, throughout its length, through which water is distributed below drum water level. The feed water then passes through the tubes where it is converted into steam and returned to the drum above water surface for separation of water particles. Inside the drum, in the top mounted separators, the outlet steam passing through them become dry and then exit at the top of the drum. The drum has rows of tube holes with welded stub ends at factory for site welding of tubes.


The steam from the top of the drum is carried through tubes to a superheating zone in another part of the furnace. These superheater tubes are called pendent superheaters as they would be vertically hanging from headers located at top of the furnace.

These superheater coils are devided into two sections, the primary and secondary. All these coils terminate in headers on top of boiler furnace. These tubes are placed inside the furnace, just at the outlet of furnace gases, where the hind wall tubes of the furnace form a nose, projecting inside the furnace and also sloping towards the furnace. The primary superheater ends in a header at the top of the furnace. To this header feed water is sprayed controlled by the temperature of steam at the secondary superheater outlet, thus maintaining the rated temperature at the boiler outlet.

The secondary pendent superheater starts after this controlled steam temperature and the outlet header of this secondary superheater ends in a pipe to carry steam to turbine. This pipe provides necessary space for the isolating valve for the boiler.

When more than one steam turbine is used in series, the steam moving between the turbines may go back through the furnace to be reheated in a series of reheater tubes in the furnace.


The furnace thus formed allows the following mountings for its normal operation.

1. Isolating, safety and vent and drain valves on the drum and headers.
2. Burners on walls.
3. Soot blowers on walls and
4. Instruments and controls, and other safety devices at required points.

Ash hopper[]

At the lowest portion, isolated from furnace walls and supported on the ground, is a separate hopper for ash impounding. The top of this hopper and the generating tube headers are separated with a water seal so they are completely isolated for independent vertical movement. Water is also allowed to flow continuously through these seals for cooling and sealing purposes. The outside air also cannot enter the furnace chamber, which works under a small vacuum.


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