Book - School of Science and Technology

326 Combustion and chimneys. Draught requiredThe following may be calculated from the data on duct sizing in Chapter 16, adjusted fortemperature:At boiler exit, consult makers' data butOil-fired boilers vary from7 to 50 PaSolid fuel fired, if burning rate is 5 kg/m 2 grate area70 PaFlue connections, boiler to chimney, depending on number of bendsand other losses, average15 to 30 PaEfflux velocity pressure, e.g. for 6 m/s12.1 PaThe new total for an oil-fired boiler may then be between 50 and 100 Pa and for a solidfuel boiler 100 to 150 Pa.. Draught produced by chimneyThe theoretical draught of a chimney at the two temperatures named varies with theexternal ambient temperature. Assuming that this is 20 C in summer and 0 C in winter,unit values are:Winterper metre height at 300 C6.7 PaSummerat 200 C5.5 Paper metre height at 300 C5.8 Paat 200 C4.5 PaFigure 12.5 is drawn on this basis and thus a chimney of 30 m height will in summerproduce, theoretically, a draught of 155 Pa with flue gases at 250 C.. Draught loss in chimneyThe pressure loss per metre height may be taken from Table 12.3. Values for othervelocities may be interpolated. Figure 12.6 gives the pressure loss for chimneys of givenheights, the loss per unit length having been determined from Table 12.3. The pressureloss arising from the flow of gases in rough brickwork or concrete chimneys will be asmuch as three or four times greater than that for relatively smooth sheet steel.. Rectangular equivalentsFor square or rectangular chimneys, the effective areas are those of the circle or ellipsewhich may be inscribed within them. The equivalent diameter of such flues is therefore thesquare root of the square or rectangular area. If the flue must be rectangular, it is ageneral rule that a ratio of sides of 3:1 should not be exceeded.In conclusion:. Using Figure 12.4 select a velocity according to whether draught is natural or mechanicaland find chimney area and diameter.. From Table 12.3 determine pressure loss per metre of height for this diameter.. Make an assumption as to chimney height and hence from Figure 12.6 note pressureloss in chimney. Add for loss through boiler and flue connection and for efflux velocity.. Using Figure 12.5, the available draught may be found for the same assumed chimneyheight. If this were equal to or in excess of the sum of the losses, the assumption as toheight may stand. If the available draught were insufficient, either the height must beincreased or the velocity reduced, or both. If the calculated draught were much in excessof requirements, a smaller chimney or less height might be the solution, or if neitherwere possible, or desirable, a damper may be used.