Fire Tube Shell Boilers

Fire tube boilers (shell boilers or shell and tube boilers) convert heat from burning fuel within a furnace (combustion chamber, firebox, or furnace tube) to generate either HW or steam. Fire tube boilers are designed to direct the combustion gases through tubes (held within tube sheets) that are surrounded by BW, thus providing for a greater heat-transfer surface area and improved efficiency. 

Stay bolts are also used in many FT boiler designs. Typically, these bolts are provided with a drilled weep hole that leaks if deterioration of the stay bolt occurs due to waterside corrosion, thus providing an early warning to the operator. 

Apart from general industrial applications, FT boilers also are sometimes used for special process purposes such as waste heat recovery. 

Early twentieth century designs of FT boilers were often horizontal return tubular (HRT) designs. These boilers were classified as externally fired FT boilers and had an external, refractory brick furnace located under a near-horizontal shell and tube heat exchanger. The exchanger was supported on brick piers and tilted 1 to 2 inches down toward the blowdown pipe at the rear of the boiler to reduce the risk of burning the bottom shell plates because of sludge buildup. 

The hot combustion gases were directed through the heat exchanger tubes, and to this day FT boiler combustion gas tubes continue to be variously called fire tubes, smoke tubes, flue gas tubes (or simply tubes). 

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There are four fundamental types of boiler available today—electric boilers, fire tube (shell or FT) boilers, water tube (WT) boilers, and nuclear reactor boilers. Electric boilers apart, all other types are essentially developments from shell and tube heat-exchanger designs. 

In calculating the smoke tube inlet gas temperature of a shell boiler, A includes the effective water-cooled surface in the reversal chamber. In coal-fired boilers, any water-cooled surface below the grate is excluded from A. 

Volumetric heat release rates The rates of volumetric heat release from shell boiler furnaces fired by oil and gas are typically 175,000 to 235,000 Btu/ft3/hr. (Heat releases from the various tube passes are significantly lower than from the furnace, thus reducing the overall heat-flux rating.)... 

Early vertical boilers were constructed in several different designs, including FT and tubeless, dry-top, and wet-top versions. Typically, however, they were single-pass FT units containing an inner, combined BW and steam shell—through which a number of small fire tubes passed—and an outer combustion chamber. 

Scotch marine boilers (SM boilers) derive their name from the Scottish shipyards that built marine vessels for the British Navy. They were the first design of FT boiler to incorporate both furnace tubes and fire tubes inside the shell and replaced the brick-set boilers that used to burn through the bottoms of ships. The SM boiler was a particularly versatile design and quickly became the boiler of choice for many stationary (land) applications as well as for marine duty. Land-based SM boilers (now commonly called Scotch boilers) were not simply marine boilers adapted for stationary duty but incorporated specific design modifications to meet the requirements of land-based industry. 

AGIPIGCC Sannazzaro, Italy Shell 2005 Cracked residue (1200 mt/d) Fire-tube boiler 3.34 Power, H2... 

Leuna Methanol Anlage Leuna, Germany Shell 1985 Visbreaker residue (2400 mt/d) Fire-tube boiler 7.2 H2 (42.4 MMscf/d)... 

Most Gasification Most, Czech Republic Shell January 1971 Cracked residue (1250 mt/d) Fire-tube boiler 3.6 H2, methanol, power, steam... 

In case of fire-tube boiler, they may cost more for the same steam generation capacity since the tubes and the shell both are subjected to steam pressure. Water-tube boilers are generally used for power generation. They can start steam generation earlier than fire-tube boilers due to less volume of water in side. Only the tubes operate under pressure. The gases at low-pressure pass through the outer shell. The shell is therefore not subjected to steam pressure. Hence, cost could be less. 

The fire-tube boiler (Figure 15.1) features a large cylindrical shell, which imposes mechanical limitations on steam pressure and quantity. The economic limit on steam pressure is around 20 bar (290psia). The fire-tube boiler is the most economical and dominant in small and medium steam production and at lower pressmes. 

Fire-tube boilers where the hot gases pass through the tubes and water is on the shell side) take a longer time to generate steam due to the higher hold-up of water the cost is greater for the same capacity unit since the tubes and the shell are both subjected to steam pressure. 

Fire-tube heaters contain the combustion gases in tubes that occupy a small percentage of the overall volume of the heater. The basic components of a fire-tube boiler include a large shell that surrounds a horizontal series of tubes. A large, lower combustion tube is attached to a burner that admits heat into the tubes. The upper tubes transfer hot combustion gases through the system and out the stack. Airflow is closely controlled with the inlet air louvers and the stack damper. Water level in the shell is maintained slightly above the tubes. 

If the CO is not completely combusted to CO2 in the regenerator, a CO boiler is used to complete the combustion. The resulting heat of combustion and the sensible heat of the flue gas along with any heat from auxiUary fired fuel are recovered in the form of high pressure steam. When the regenerator is operated in total CO bum, the CO boiler is replaced with either a shell and tube exchanger or a box-type waste heat boiler (see Heat... 

A forced-draft, gas-fired burner is seated at the top of the inner tube and produces a spinning cyclonic flame that reaches down to the bottom of the furnace tube. The hot combustible gases return over the boiler shell, which is provided with heat convection fins to extract more heat before the upward flowing gases exit the boiler. The furnace tube is fitted with a top and bottom, cast-steel flame retainer. These design features act to increase flue gas residency time and provide improved structural integrity to the pressure vessel. 

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