Book - School of Science and Technology

324 Combustion and chimneys. The temperature of the gases should be maintained, throughout the stack, at near to theentry level by effective insulation.. The use of draught stabilisers (as distinct from draught diverters) and leakages whichadmit cold air should be avoided.. The inner surfaces of the flue construction should be smooth and changes in crosssection, bends, etc., should be designed aerodynamically.. The use of a single flue-way to serve more than one boiler should be avoided whereverpossible. In the rare case where a common flue for a multi-boiler plant is genuinelyunavoidable, means should be provided to isolate the outlets of idle boilers and tomaintain a constant efflux velocity at the chimney terminal.Chimneys for solid or liquid fuelsFor the purpose of this present text the design process may be simplified by reducing thenumber of variables which relate to the particular cases of solid fuel and oil. Since theremainder, including the calorific value of the fuel envisaged for design purposes, will almostcertainly vary during the life of the chimney, further precision will serve little purpose.. Products of combustion, quantityAssume excess air provided to be 75%Assume flue gas temperature, on average, to be 200±300 CAssume boiler efficiency to be 75%For these conditions, it may be shown that the products of combustion per MJ of boileroutput are between 1.1 and 1:3m 3 /MJ. It is convenient to take a mean of 1:2m 3 /MJ. Thetemperatures assumed cover the range mentioned previously.. TimeBringing a time scale into consideration, the volume of the products as above will derivefrom combustion associated with 1 MJ/s, i.e. 1 MW.. Velocity of flue gases in the chimneyIt may be assumed that the range of velocities appropriate to natural and to mechanicaldraught are, respectively, 4±8 m/s and 10±12 m/s.. AreaFigure 12.4 gives areas and diameters for round chimneys direct, having selected avelocity. The equivalent diameter of a square or rectangular chimney, in mm, is 1000the square root of the area as read from the figure.The cross-sectional area of the chimney may then be derived according to boiler duty,fuel and type of draught. Thus, if the boiler duty were 1 MW, oil-fired, with naturaldraught (5 m/s) and a chimney temperature average of 250 C,1:2/5 ˆ 0:24 m 2 ˆ 550 mm in diameter. Efflux velocityIn order that the plume of flue gas should rise clear of the chimney top and not flow downthe outside (down-wash), the diameter at the top should be reduced so as to maintain ashigh a velocity as practicable. For small boilers with natural draught, a velocity of 6 m/s isadvised. Larger boilers with mechanical draught should achieve 7.5±15 m/s.The velocity pressure corresponding to these rates may be read from the bottom line ofTable 12.3 as appropriate to a flue gas temperature of 250 C.