The main boiler shell is an enclosed vessel where the hot steam is generated and collected. The boiler shell is subjected to very high pressure and for this reason, various types of boiler accesories and mountings are provided to ensure the safety of the boiler. A routine based maintanance is also required for firetube boilers, where the safety valves, fusible plugs, pressure gauge, water feed system, chimney etc are checked. The construction and working principle of firetube boiler is discussed as follows.
Construction of firetube boiler
The construction of firetube boiler is quite simple and this is the basic design of any modified version of firetube boiler. The main boiler shell is constructed with mettalic plates and all those plates are pivoted with each other to form a cylindrical structure. The boiler furnace is located at the bottom position of the boiler shell. After combustion of the coal, remaining ashes are stored into the ash pit. Several pipes or tubes are fitted into the boiler shell which all are connected with the furnace. The hot flue gases are passes through those pipes and after that they are released to the atmosphere via chimney. The steam control valve is mounted at the upper portion of boiler, from where the generated steam is fed for further uses. Various safety arrangements like pressure gauge, water level indicator etc are also mounted on the boiler shell. Sometime an additional water pump is fitted with the boiler to enhance the water feeding system.
Working Principle of firetube boiler
The working principle of firetube boiler is very simple. The name clarifies the opearing principle of firetube boiler, that is hot flue gases in the tube. Think about the basic need of a boiler. Imagine that you have a closed vessel and you are boiling some water inside it and collecting the generated steam. This operation of firetube boiler is based on this simple concept of conventional boiling. Here the boiler drum or boiler shell is treated like the vessel. Water is fed via the feeding pump into the boiler shell. The boiler pipes or tubes (here firetubes) are immersed into the feed water. The fuels are combusted into the boiler furnace. As those firetubes are connected with the boiler furnace, so the hot flue gases starts flowing throgh the pipes.
As soon as the hot flue gases are circulating through the firetubes, the surrounding water is heated up due to the natural heat exchanging process. After sometime the feed water reaches into the boiling point and starts evaporating. In this way water vapour or hot steam is produced inside the boiler vessel. Being lighter, those hot steams are stored at the upper portion of the boiler vessel. But this raw steam may contain some moisture, so those saturated steams are carried out via a superheater. Superheater dries the steam particles and produces the superhaeted steam. After that the steam is fed to the steam turbine via a steam controller valve. The feed water and steam both are stored into the boiler vessel, so rapid generation of steam leads to increase the pressure of the boiler shell. Thus, the temparature and pressure of the generated steam becomes very high. This is the basic working principle of any type of firetube boilers.
Types of firetube boilers
There are various types of firetube boilers. The classification is based on the use and construction of firetube boilers. All of them are the modified form of the basic firetube boiler. The types of firetube boilers are discuused as follows.
- Locomotive Boiler: As the name suggests, this type of firetube boiler is used mainly in locomotive engines, steam rollers etc. The boiler shell is kept horizontal and in locomotive boiler this shell is termed as boler barrel. Sometime the firetubes are made larger to accomodate the superheating elements inside the boiler barrel. The boiler furnace is located inside the boiler shell, that is why this type of boiler is termed as internally fired boiler.
- Cornish boiler: The Cornish boiler is a high pressure firetube boiler and this is the traditional model of firetube boiler. The boiler shell is kept horizontal and the structure is like a cylindrical drum. The size of this type of boiler is very large. An interesting thing about this boiler is, it contains only a single flue tube. This flue tube is commonly made with iron and a tall chimney is fitted with the boiler shell.
- Lancashire boiler: The design and construction of Lancashire boiler is inspired from the Cornish boiler. Here two flue tubes are fitted inside the main boiler shell. The fuel combustion system is more efficient in Lancashire boiler. The coal is supplied to the grate and the grate is connected with the flue tubes. Dampers are provided which efficiently controlls the flow of hot gases and in this way, the steam generation process is controlled.
- Vertical firetube boiler: As the name suggests, the boiler shell is kept in vertical position in this type of boiler. There are several flue tubes through which flue gases travelles. Actually, all the multitube boilers are the modified form of classical firetube boiler.
Advantages and disadvantages of firetube boilers
The modified forms of classical firetube boiler are used in several sectors like power generating units or industries. This firetube boiler posses some advantages over watertube boiler and they are discussed as follows.
- The firetube boiler is comparatively cheaper than watertube boiler so the foundation cost is lesser for this type of boiler.
- The construction is simple and the maintenance process is quite easy.
- This type of boilers are very easy to operate.
- The steam generation rate is quicker and constant.
- As the hot flue gases are confined into the firetubes so lesser chance of wastage.
- The replacement process of firetubes are very easy.
- The size is lesser, that is why they are also used in industrial applications.
The disadvantages are:
- The heat transfer process is not so good.
- Hot water and steam both are stored inside the boiler, so sometime the overall pressure of boiler shell approaches to a very high value.
- The capacity of steam generation is depended upon some factors.
- Firetube boiler is one of the historical boilers, based on which modern power industries are developing.
- In Scotch Marine boiler, multitube operation was started.
- According to the boiler safety rules, a firetube boiler must contain at least one safety valve, pressure gauge, water level indicator and steam control valve.