Forced draft-fired boilers

Figure 20.2 Forced draft-fired boiler simplified schematic drawing.

Although the engine was not a success, Ericsson s trip to London allowed him to meet John Braithwaite, a machine manufacturer, who had the expertise to put Ericsson s ideas into practice. In 1828 Ericsson, with Braithwaite, patented the principle of artificial draft in steam boilers. The principle of forced draft was applied to a fire engine and a locomotive entered for the Rainhill locomotive trials of 1829. 

Packaged FT boilers are usually fired by oil or gas using integrated, forced-draft burners and are available as wet- or dry-back types. 

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. 

Steam systems. An investment cost of 35 per lb/h of total steam generation capacity is recommended for preliminary estimates (costs as of 2001). This represents the total installed costs for gas- or oil-fired, forced-draft boilers operating at 250 to 300 psig, and all appurtenant items such as water treating, deaerating, feed pumps, yard piping for steam, and condensate. 

Boilers normally are forced draft (FD) versus most fired heaters that are natural draft. It is cheaper to combine FGR with an FD system because the FD fan can be used to induce the flue gas and the plenums already exist to distribute the air. 

At the Tenneco Oil Refinery in New Orleans, 1 first learned how to optimize the excess oxygen content of the flue gas from a boiler. The fired boiler was equipped with a forced-draft (FD) fan. Due to mechanical deficiencies of the FD blower, the combustion air supply was limited, but constant. The boiler produced 650-psig steam, which was in short supply in the refinery. Flence, the boiler was operated to maximize steam production. 

Combustion calculations show that an oil-fired watertube boiler requires 200,000 lb/h (25.2 kg/s) for air of combustion at maximum load. Select forced- and induced-draft fans for this boiler if the average temperature of the inlet air is 75°F (297 K) and the average temperature of the combustion gas leaving the air heater is 350°F (450 K) with an ambient barometric pressure of 29.9 inHg. Pressure losses on the air-inlet side are, in inFLO air heater, 1.5 air supply ducts, 0.75 boiler windbox, 1.75 burners, 1.25. Draft losses in the boiler and related equipment are, in inH20 furnace pressure, 0.20 boiler, 3.0 superheater, 1.0 economizer, 1.50 air heater, 2.00 uptake ducts and dampers, 1.25. Determine the fan discharge pressure and horsepower input. The boiler burns 18,000 lb/h (2.27 kg/s) of oil at full load. 

Figure A1.5 Typical scheme of coal-fired thermal power plant (AuthorAJser BillC https // commons.wikimedia.org/wiki/File PowerStation2.svg website approached January 26, 2016) (1) Cooling tower (2) cooling-water pump (3) transmission line (3-phase) (4) step-up transformer (3-phase) (5) electrical generator (3-phase) (6) low-pressure (LP) steam turbine (7) condensate pump (8) surface condenser (9) intermediate-pressure steam turbine (10) steam control valve (11) high-pressure (HP) steam turbine (12) deaerator (13) feedwater heater (14) coal conveyor (15) coal hopper (16) coal pulverizer (17) boiler steam drum (18) bottom ash hopper (19) superheater (20) forced draught (draft) fan (21) reheater (22) combustion air intake (23) economizer (24) air preheater (25) precipitator (26) induced-draught fan and (27) flue gas stack.

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