What is the difference between a water tube and fire tube boiler?
A water tube boiler and a fire tube boiler are two types of boilers used for generating steam. The main difference between them is the way the heat is transferred to the water in order to generate steam. This article explains most major differences between water tube and fire tube boilers; it covers their design, working principles, applications, advantages, and disadvantages.
Water Tube Boiler and Fire Tube Boiler Compared
Good to know – ‘fire tube’ boilers are also spelt ‘firetube’ or ‘fire-tube’, but all terms mean the same thing. Similarly, ‘water tube’, ‘watertube’ and ‘water-tube’ are just similar way of spelling the same thing. This article purposely uses all terms to increase the reader’s familiarity with them.
Fire Tube Boiler Tube (left) and Water Tube Boiler Tube (right).
Design and Working Principle
Watertube Boilers
Watertube boilers have water flowing through tubes that are externally heated by combustion gases (exhaust gases). The water absorbs heat from the combustion gases as it moves through the tubes, which causes it to turn into steam; this steam is then collected in a steam drum or header. Watertube boilers can operate at higher pressures than firetube boilers due to the water's containment within the tubes and the smaller resultant pressure boundary.
Watertube Boiler
Good to know – produced steam is usually gathered in headers then discharged to a steam drum, but this depends upon the watertube boiler design.
Firetube Boilers
In a firetube boiler, hot gases from the combustion process pass through a series of tubes immersed in a water-filled vessel. As the water surrounding the tubes increases in temperature, it changes state to steam, which then accumulates at the top of the boiler shell.
Firetube Boiler
Applications
Watertube boilers are generally used for high-pressure applications, such as in thermal power plants, due to their capability to operate at high pressures and temperatures (steam turbines require both high pressure and high temperature steam to operate effectively). Conversely, firetube boilers find use in lower-pressure applications like heating and providing process steam for small to medium-sized industrial facilities. A typical power station watertube boiler may operate at 3,200 psi, 932⁰F (220 bar, 500⁰C ) or more.
Power Station Watertube Boiler
Watertube Boiler Advantages and Disadvantages
Water-tube boilers are capable of handling higher pressures and have a more significant rate of steam production (higher rate of evaporation) due to their numerous thin-walled tubes. They offer greater flexibility in responding to load changes and are considered safer due to a lower risk of catastrophic failure. However, their complex design leads to higher initial costs and maintenance requirements.
Firetube Boiler Advantages and Disadvantages
Simpler in design, firetube boilers are easier to operate and maintain, and they are usually less expensive to purchase and install. Their limitations include a restricted pressure and steam capacity, a slower response to load changes, and lower efficiency when managing fluctuating demands.
Firetube Boiler Cross-Section
Good to know – a fire tube boiler has a single large cylindrical shell. The shell is manufactured from a flat steel plate that is rolled and then welded together at the seams. Fire tube boilers are restricted in working pressure range because the boiler shell acts as a large pressure boundary. Increasing the strength of the shell requires thicker plate, which becomes uneconomically expensive and difficult to manufacture as the size of the boiler increases. For this reason, fire-tube boilers are ill suited to operate at high pressures.
Firetube Boiler Shell Production Process
Fire Tube and Water Tube Boiler Comparison
Watertube boilers are appropriate for large-scale, high-pressure, high temperature applications, and are more capable of catering for rapid load changes.
Firetube boilers are appropriate for low to medium pressure applications and lower temperatures; they are also more simplistic in design, easier to install, and easier to maintain.
Additional Resources
https://www.sciencedirect.com/topics/engineering/fire-tube
https://www.sciencedirect.com/science/article/pii/S2214157X2100410X
https://www.iqsdirectory.com/articles/boiler/water-tube-boilers.html
https://ivypanda.com/essays/fire-tube-boiler-the-case-study/