Shell boilers—also called fire‑tube or smoke‑tube boilers—work best in low to medium pressure ranges (1–30 t/h at under 1.6 MPa). Their simple setup makes upkeep straightforward, and they’re among the most budget‑friendly choices.
Definition and Key Features
Shell boilers are defined by the fact that their heat transfer surfaces are entirely contained within a single, cylindrical pressure vessel—or “shell.” The combustion gases flow inside the tubes, while the surrounding water absorbs the heat, effectively making the shell both the pressure boundary and the water drum.
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Fire-tube: The hot flue gases pass through tubes immersed in water.
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Smoke-tube: An alternative name highlighting that “smoke” (combustion products) travels inside the tubes.
Construction and Working Principle
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Combustion Chamber
Fuel is burned in a furnace within the boiler’s shell, producing high-temperature flue gases. -
Tube Passes
These gases traverse one or more “passes” through tubes in the water space. Common designs include single-pass, two-pass, and three-pass arrangements, where the number of passes refers to how many times the gases reverse direction before exiting. -
Heat Transfer
As gases flow through the tubes, the heat transfers through the tube walls into the surrounding water, raising its temperature until steam forms. -
Steam Collection
The steam rises to the upper part of the shell and is drawn off for process applications or heating.
1-20t/h Shell Condensing Steam Boiler
Types of Shell Boilers
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Cornish Boiler
Features a single large flue tube surrounded by water. -
Lancashire Boiler
Utilizes two parallel flue tubes for higher capacity. -
Three-Pass Shell Boiler
Incorporates three sets of tubes, increasing heat transfer efficiency and steam output.
Advantages and Limitations
Advantages
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Simple Construction: Easier to manufacture and maintain compared to water-tube designs.
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Lower Initial Cost: Generally less expensive for low- to medium-pressure applications.
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Good Steam Storage: The large water volume provides a buffer for fluctuating load demands.
Limitations
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Pressure Constraints: Typically limited to pressures below 25 bar due to shell thickness restrictions.
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Size and Weight: Heavy construction and large footprints make them less suitable for space-restricted installations.
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Thermal Stress: Localized heating of tube surfaces can lead to hot spots if water circulation is insufficient.
2-30t/h Shell Condensing Steam Boiler
Applications
Shell boilers are widely employed in industries where moderate steam or hot water demands exist:
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Heating Plants for buildings and district heating
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Process Steam generation in food, paper, textile, and chemical industries
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Backup Boilers for peak-load support due to their simple start-up and shutdown characteristics.
Historical Context
Early 1800s engineers swapped the old flat‑top “haystack” boilers for a new shell design that tucked a large, curved flue inside the vessel. That simple change made the boilers stronger and safer, letting them run at pressures up to about 25 psi—a major leap back then.
Even today, shell boilers are a go‑to for moderate steam and hot‑water needs. Their no‑frills build, long track record of safe operation, and easy upkeep keep them in service across countless industrial and heating setups.
