Fire Tube Boiler vs Water Tube Boiler: A Detailed Comparison

Industrial boilers are broadly classified based on the relative position of water and flue gases (hot burned gases produced as a result of coal or oil-firing in the boiler furnace). Under this differentiating aspect, steam boilers are principally categorized as fired-tube and water-tube boilers worldwide. They are designed using international engineering standards such as ASME Boiler and Pressure Vessel Code (BPVC).

All boilers, whether fire-type or water-type, are steam generators, which means they are designed and constructed to generate steam at required temperatures and pressures. The capacity of a boiler to produce steam at a given temperature in unit time is called its evaporative capacity or, more simply, steam-generation capacity. It is measured in kg/hr.

There are certain components that both fire-tube and water-tube boilers share. Both include a furnace, combustion chamber, metal tubes, metal drum/s, shell, stack or chimney, and so on.

Other common features include fuel to be burned, water to be steamed, as well as control valves, proportional valves, pressure relief valves, back pressure gauges, and other boiler tooling and instrumentation depending upon the design and configuration of the boiler in question.

Nevertheless, there are principle differences between the two types, which are well explained in the tabulated form immediately below.

Fire-tube BoilerWater-tube Boiler

The relative position of water and hot gases is such that the latter surrounds the former. It signifies that the hot (flue) gases circulate inside the tubes whilst water remains outside these tubes and heat transfer occurs via. convective surfaces.
The relative position of water and hot gases is such that water circulates in the water-tubes (which are called the generating tubes, risers, and downcomers) while the hot-gases pass over these tubes in the shell.
It produces steam with low to moderate evaporative capacity.It produces steam with extremely high evaporative capacity.
It generates steam up to temperatures that are required for small-scale applications on an industrial level. It generates steam at superheated temperatures that are used to run high pressure (H.P)  large steam turbines.
Its design is simpler, and the structure less stronger.It is larger in size and volume and, therefore, occupies comparatively more square per foot.
Water circulation is somewhat slower, thereby causing different water temperatures at different points in the boiler. Such thermal differentiation (or temperature gradient) causes thermal stresses in the boiler’s body.The water-circulation is faster and more rapid. It helps maintain the overall boiler temperature at a constant value and, thereupon lower thermal stresses.
Steam generation is relatively slower as water-exposure to the hot flue gases is inherently limited.Steam generation is quicker as water is divided into smaller portions in the thin generating-tubes which are fully exposed to hot flue gases rising from the furnace and are directed radially towards the water tubes.
The direction of water-circulation is not well defined.The direction of water-circulation is well-defined.
Water circulation is somewhat slower thereby causing different water temperatures at different points in the boiler. Such thermal differentiation (or temperature gradient) causes thermal stresses in the boiler’s body.The water-circulation is faster and more rapid. It helps maintain the overall boiler temperature at a constant value and, thereupon lower thermal stresses.
Its life is relatively shorter due to suboptimal clearances between boiler different components and supports.Its life is longer than its fire-tube counterpart as there are provided sufficient clearances for tube-metal expansion due to which there is least disturbance in geometry as well as in overall boiler construction.
Its transportability is bit difficult a task due to non-sectional shell design.Its transportability is easier due to the sectional construction.
Its safety-checks are more attention-seeking. Any trouble with the flue-pipe or any accidental rise in pressure can be catastrophic.It is called a safety boiler as any damage to a water-tube does not cause any boiler explosion as such.
The heat transfer surfaces are less effective due to their under-developed engineering design.The heat-transfer surfaces are optimally designed in the form of multiple water-tube designs, their positioning, quantity, configuration, and so forth.
It is run with water whose quality is not so relevant. Muddy and impure water can be run with a fire-tube boiler.It is run only with good-quality water. If it contains scale-causing foreign particles, it may cause deposits in the tubes and subsequently overheat or a tube-bust due to a rise in pressure and temperature.
Its maintenance cost is relatively cheaper.It costs more to maintain the boiler’s all-time optimum condition due to its rather complex water-tube circuitry.
Its testing and physical inspection are somewhat easier and do not require critical improvisation.Its testing, evaluation, and inspection are a bit difficult and critical tasks due to the sophistication of its components and tooling.
Its applications include running heavy-duty engines, marines, passenger liners, large vessels, oil-tankers and so forth.Its applications include running heavy-duty engines, marines, passenger liners, large vessels, oil-tankers and so forth.
Its examples include the Cornish Boiler, Lancashire Boiler, Cochran Boiler, Locomotive Boiler, and so on.Its examples include marine boilers such as Scotch-marine boiler, Babcock and Wilcox Boiler, Yarrow boiler, Stirling Boiler, Lamont Boiler, as well as Thornycroft Boiler, Foster Wheeler Boilers, such as D-type, DSD type, ESD I, ESD II, ESD III, and so on.
Its thermal efficiency is lower due to less surface-area for heat transfer.Its thermal efficiency is way-higher than a fire-tube boiler due to large convective surfaces and improved combustion performance.
It produces low-quality steam which is one of the reasons for its costlier maintenance due to corrosion and so on.It produces low-quality steam, which is one of the reasons for its costlier maintenance due to corrosion and so on.
Its design is somewhat rigid, close, and non-modular.It is designed in a modular fashion which means it is flexible in accepting engineering modifications pertinent to the new operational needs.
It is advantageous in regions where water-scarcity is not an issue.It requires less water because water keeps circulating in the tubes due to its thermal (natural) flow.
Its running cost is a bit higher in terms of the higher mass of fuel to be burned.Its running cost is bit lower in terms of lower mass of fuel required for an equivalent steaming rate.
It causes more emissions and soot and is therefore not very environmentally friendly.Its combustion and emission control technologies have evolved over some time, due to which it complies more closely with environmental regulations.
It is compatible with a limited number of fuels. Mostly, fire-tube boilers are coal-fired or oil-water boilers.It is versatile in terms of firing a number of fuels such as natural gas, oil, biomass, waste-materials, and so on.
It is protected with refractory-lining in the furnace or combustion section only.Few boiler designs, such as Foster Wheeler ESD IV, have excessive use of refractory materials all over in the boiler construction for the protection against extremely high temperatures.
Its chimney size is relatively simple.It has a chimney larger in size and complex in design.