What is a Fire Tube Boiler

A fire tube boiler is a type of boiler in which hot gases from a fire pass through one or (many) more tubes running through a sealed container of water. The heat of the gases is transferred through the walls of the tubes by thermal conduction, heating the water and ultimately creating steam.

The fire-tube boiler developed as the third of the four major historical types of boilers: low-pressure tank or “haystack” boilers, flued boilers with one or two large flues, fire-tube boilers with many small tubes, and high-pressure water-tube boilers. Their advantage over flued boilers with a single large flue is that the many small tubes offer far greater heating surface area for the same overall boiler volume. The general construction is as a tank of water penetrated by tubes that carry the hot flue gases from the fire. The tank is usually cylindrical for the most part—being the strongest practical shape for a pressurized container—and this cylindrical tank may be either horizontal or vertical.

This type of boiler was used on virtually all steam locomotives in the horizontal “locomotive” form. This has a cylindrical barrel containing the fire tubes, but also has an extension at one end to house the “firebox”. This firebox has an open base to provide a large grate area and often extends beyond the cylindrical barrel to form a rectangular or tapered enclosure. The horizontal fire-tube boiler is also typical of marine applications, using the Scotch boiler.Vertical boilers have also been built of the multiple fire-tube type, although these are comparatively rare; most vertical boilers were either flued, or with cross water-tubes.

Fire Tube boiler is the simplest form of the internal furnace, vertical fire tube boiler. Fire tube boilers are a portable boiler and it requires a small floor space.

The steam rating in fire tube boilers doesn’t exceed 2500 kg per hour and pressure is limited to 10 bars.

Construction:

• Fire tube boiler consists of a vertical, cylindrical shell, equipped with a fire box in the bottom, water space in the middle portion and steam space in the upper portion. The fire grate is placed at the bottom of the fire box and coal is fired at the fire box. An ash pit is located at the bottom of the grate to collect the ash of burnt coal, which is periodically removed.
• One or more cross tubes are either flanged or riveted to the water space are located in the fire box to increase the heating surface area to improve the water circulation. A short chimney is connected at the top of the fire box to discharge the waste flue exhaust gases at some greater height. Manhole and hand holes are provided for cleaning the interior of the boiler shell and cross tubes.
• The boiler consists of a pressure gauge, water level indicator, safety valve, steam stop valve and a manhole as mountings to provide safety and ease of working.

Working:

Fuel burns on the grate in the fire box. The resulting hot flue gases are allowed to pass around the cross tubes. The water surrounding the cylindrical fire box also receives heat byconvection and radiation. Thus steam is produced. The water circulation in the boiler depends on the density difference in water, created by the temperature difference in water.

Types of Fire Tube Boilers:

1. Cornish boiler
2. Lancashire boiler
3. Locomotive boiler
4. Scotch marine boiler
5. Admiralty-type direct tube boiler
6. Horizontal return tubular boiler
7. Immersion fired boiler
8. Vertical fire-tube boiler

Working Principle of Horizontal Return Fire Tube Boiler

Horizontal return fire tube boiler is most suitable for low capacity thermal power plant. The main constructional features of this boiler are one big size steam drum which lies horizontally upon supporting structures. There are numbers of fire tubes come from furnace and also aligned horizontally inside the drum. When the drum is filled with water these tubes are submerged in water.The fuels (normally coal) burnt in the furnace and combustible gasses move into the fire tubes, travel through these tubes from rear to front of the boiler drum and finally the gases come into the smoke box. The hot gasses in the tubes under water transfer heat to the water via the tube walls. Due to this heat energy steam bubbles are created and come upon the water surface. As the amount of steam is increased in that closed drum, steam pressure inside the drum increases which increase significantly the boiling temperature of the water and hence rate of production of steam is reduced. In this way a fire tube boiler controls its own pressure. In other words this is a self pressure controlled boiler.

Advantages of Fire Tube Boiler

1. Compact in construction.
2. Fluctuation of steam demand can be met easily.
3. Cheaper than water tube boiler.

Disadvantages of Fire Tube Boiler

1. Due to large water the required steam pressure rising time quite high.
2. Output steam pressure cannot be very high since the water and steam are kept in same vessel.
3. The steam received from fire tube boiler is not very dry.
4. In a fire tube boiler, the steam drum is always under pressure, so there may be a chance of huge explosion which resulting to severe accident.