What is Fire Tube Boiler:
A boiler in which flames and hot gases produced during combustion pass through the tubes and water flowing outside the tube called fire tube boilers. Fire tube boiler is the most predominant boiler used for heating, commercial and industrial applications. These boilers are suitable for clean fuels such as oil and gas.
Fire tube boiler types are normally under external pressure. Therefore, they will collapse instead of bursting. It has a tube end exposed to the combustion products. It has flat surfaces that require stay bolts to resist the pressure imposed on it. Boiler tubes must be rolled and beading to prevent the burning of tube ends due to hot gases. Beading also increases the heat transfer rate near the tube sheet. Smaller size fire tube boilers are vertical and larger sizes are horizontal. These boilers are popular in the process industry for small steam requirement even with lower thermal efficiency and poor safety inherent to the design.
Fire tube boiler working:
The simple vertical boiler is a fire tube boiler and easy to understand the working principle of fire tube boilers. Simple vertical fire tube boiler produces steam with low pressure and in small quantities. It uses for low power generation or for limited space. It consists of a cylindrical shell surrounding a nearly cylindrical firebox/ furnace. Grate locates at the bottom of the firebox. Firebox contains two or more inclined cross tubes. This inclination of tubes provides to increase heating surface and also improves circulation of water. Manhole provides at the top of the boiler for cleaning purpose. Mudhole locates at shell bottom to remove mud.
Space between the shell and firebox filled with water to be heated. After combustion of fuels in firebox, hot gases produced in a furnace. These produced hot gases pass through the fire tubes and water circulates around the fire tubes. Hot gases/ flame transfer their heat to the shell side water through thermal conduction and then shell side water converts into steam.
As the steam increased in boiler shell/ drum, the steam pressure inside the shell increased. With the increase of steam pressure inside the shell, shell side water boiling temperature also increases and causes to reduce the steam production rate. Therefore, fire tube boiler controls own pressure and that’s why also called self-pressure controlled boilers.
Types of fire tube boiler:
Fire tube boiler classified as:
- Simple vertical boiler
- Horizontal return tubular (HRT) boiler
- Locomotive boiler
- Scotch Marine boiler
- Cochran boiler
- Lancashire boiler
- Cornish boiler
- Velcon boiler
- Economic or firebox type boiler
The HRT boiler represents only 5% of the fire tube boilers for different purposes. The locomotive boiler used for both portable and stationary purposes. Railway engines are examples of locomotive boilers. Lancashire boiler is a stationary fire tube (internally fired) boiler with two flue tubes. Cornish boiler is similar to Lancashire boiler but it has one flue tube. Scotch marine boiler is the dominant fire tube boiler using for both heating and industrial processes. This is available about 50,000-lb/hr capacity. Above this capacity, water tube boilers are used.
Advantages of fire tube boiler:
- Requires less space or compact size
- Easy to use, operate, clean and maintain
- Steam fluctuation demands can easily be met
- Comparatively cheaper as compared to water tube boiler
Disadvantages of fire tube boiler:
Loosening of tubes in tube sheet is the biggest disadvantage of the fire tube boiler and directly affects the boiler operation. Cracking, burning and corrosion of tube ends also occurs which increase maintenance cost. Waterside corrosion and pitting leading to leakage in the boiler. Fireside corrosion and pitting leading to leakage or pulling out of tube sheet. Waterside scale deposition also causes overheating, sagging and loosening of the tube sheet.
Not suitable for high pressure and a large amount of steam
Low thermal efficiency