Sample calculations Solid or liquid fuels

Sample calculations 383 Total heat loss in flue gases / % of gross CV 50 Gross CV ⫽ 46.5 MJ/kg Relative density ⫽ 0.79 40 CO 6% 2 8% 10% 12% 30 14% 20 15% 13% 10 0 50 100 200 300 400 500 600 Difference between flue gas and room temperatures/K Figure 17.2 Chimney loss for oil firing 5% 3. 5 4% % 2.5 % 3% % 2% 4. 40 1.5 2 1% CO Total heat loss in flue gases / % of gross CV 50 5% 6% 7% 8% 10% 30 12% 20 10 Gross CV ⫽ 38.7 MJ /m3 0 0 100 300 200 500 400 Difference between flue gas and room temperatures/K 600 Figure 17.3 Chimney loss for gas firing Sample calculations Solid or liquid fuels A single example common to both these fuels may be taken to introduce the use of Table 17.1 by assuming that a sample has, by mass, the following ultimate analysis: Carbon 80 per cent, hydrogen 4 per cent, sulphur 2 per cent, ash and moisture 12.5 per cent. Chimneys 387 0.35 650 0.3 Area of chimney (m2) 0.25 /s 5 /s 6 0.2 550 m m 500 /s 7 m /s 8m 0.15 450 m/s 10 m/s 12 0.1 400 350 Diameter of round chimney (mm) 4 m /s 600 300 0.05 250 200 0 0 0.2 0.4 0.6 0.8 1.0 1.2 Boiler rating (MW) Figure 17.4 Chimney areas for stated velocities Velocity of flue gases in the chimney It may be assumed that the range of velocities appropriate to natural and to mechanical draught are, respectively, 4–8 and 10–12 m/s. Area Figure 17.4 gives areas and diameters for round chimneys direct, having selected a velocity. The equivalent diameter of a square or rectangular chimney, in mm, is 1000 ⫻ the 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 natural draught (5 m/s) and a chimney temperature average of 250°C, 1. 2/5 ⫽ 0. 24 m 2 ⫽ 550 mm in diameter Efflux velocity In order that the plume of flue gas should rise clear of the chimney top and not flow down the outside (downwash), the diameter at the top should be reduced so as to maintain as high a velocity as practicable. For small boilers with natural draught, a velocity of 6 m/s is advised. 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 of Table 17.3 as appropriate to a flue gas temperature of 250°C. Draught required The following may be calculated from the data on duct sizing in Chapter 21, adjusted for temperature. 388 Combustion, emissions and chimneys Table 17.3 Pressure loss per metre height of smooth chimney (multiply values by 4 for brick or rough cement rendering) Effective Chimney diameter (mm) 4 5 6 7 8 9 10 11 12 200 250 300 350 400 450 500 550 600 650 700 750 Velocity pressure 0.45 0.34 0.27 0.22 0.18 0.16 0.14 0.13 0.11 0.10 – – 5.4 0.71 0.52 0.41 0.35 0.29 0.25 0.22 0.20 0.18 0.16 0.14 0.13 8.4 1.02 0.76 0.60 0.49 0.41 0.36 0.32 0.28 0.25 0.23 0.21 0.19 12.1 1.39 1.02 0.81 0.68 0.56 0.49 0.44 0.38 0.35 0.32 0.28 0.26 16.5 1.81 1.34 1.06 0.88 0.73 0.64 0.57 0.50 0.45 0.41 0.37 0.34 21.5 2.29 1.70 1.35 1.12 0.93 0.81 0.72 0.64 0.57 0.52 0.47 0.43 27.3 2.82 2.10 1.66 1.38 1.14 1.00 0.89 0.78 0.71 0.64 0.57 0.53 33.6 3.41 2.53 2.00 1.67 1.38 1.21 1.07 0.95 0.85 0.78 0.69 0.64 40.6 4.07 3.04 2.39 1.99 1.65 1.44 1.28 1.12 1.02 0.92 0.83 0.76 48.4 Pressure loss (Pa per m height or run) for gas flow at the following velocities (m/s) At boiler exit, consult makers’ data but: Oil-fired boilers vary from Solid fuel fired, if burning rate is 5 kg/m2 grate area Flue connections, boiler to chimney, depending on number of bends and other losses, average Efflux velocity pressure, e.g. for 6 m/s 7 to 50 Pa 70 Pa 15 to 30 Pa 12.1 Pa The new total for an oil-fired boiler may then be between 50 and 100 Pa and for a solid fuel boiler 100 to 150 Pa. Draught produced by chimney The theoretical draught of a chimney at the two temperatures named varies with the external ambient temperature. Assuming that this is 20°C in summer and 0°C in winter, unit values are: Winter per metre height at 300°C at 200°C Summer per metre height at 300°C at 200°C 6.7 Pa 5.5 Pa 5.8 Pa 4.5 Pa Figure 17.5 is drawn on this basis and thus a chimney of 30 m height will in summer produce, theoretically, a draught of 155 Pa with flue gases at 250°C. Draught loss in chimney The pressure loss per metre height may be taken from Table 17.3. Values for other velocities may be interpolated. Figure 17.6 gives the pressure loss for chimneys of given heights, the loss per unit length having been determined from Table 17.3. The pressure loss arising from the flow of gases in rough brickwork or concrete chimneys will be as much as 3 or 4 times greater than that for relatively smooth sheet steel. Chimneys 0.3 C Upper lines ⫺ 0° C ambient Lower lines ⫺ 20° C ambient 20 0° C 30 0° Theoretical chimney draught (kPa) 0.25 0.2 0.15 0.1 0.05 0 0 10 20 30 40 50 60 Chimney height (m) Figure 17.5 Theoretical draught for a chimney of given height 0.3 m 50 ⫺ ne y m 40 m ig ht o fc hi 0.2 He Loss of pressure in chimney (kPa) 60 m 0.25 1.5 30 m 20 0.1 m 15 m 10 m 0.05 0 0 1 4 2 3 Pressure loss per m height (Pa) 5 Figure 17.6 Draught loss for a chimney of given height (smooth surface) 6 389 390 Combustion, emissions and chimneys Rectangular equivalents For square or rectangular chimneys, the effective areas are those of the circle or ellipse which may be inscribed within them. The equivalent diameter of such flues is therefore the square root of the square or rectangular area. If the flue must be rectangular, it is a general rule that a ratio of sides of 3:1 should not be exceeded. In conclusion: ● Using Fig. 17.4 select a velocity according to whether draught is natural or mechanical and find chimney area and diameter. ● From Table 17.3 determine pressure loss per metre of height for this diameter. ● Make an assumption as to chimney height and hence from Fig. 17.6 note pressure loss in chimney. Add for loss through boiler and flue connection and for efflux velocity. ● Using Fig. 17.5, the available draught may be found for the same assumed chimney height. If this were equal to or in excess of the sum of the losses, the assumption as to height may stand. If the available draught were insufficient, either the height must be increased or the velocity reduced, or both. If the calculated draught were much in excess of requirements, a smaller chimney or less height might be the solution, or if neither were possible, or desirable, a damper may be used. Clean Air Act The chimney height determined by following the routine just described is that which is required for combustion of sulphur bearing fuels. It is however also necessary to consider the height in relation to the mandatory requirements of the Clean Air Act (1993). For this purpose, reference should be made to the third (1981) edition of the document Memorandum on Chimney Heights and also to Appendix A2 of the CIBSE Guide B1 published in July 2002. Memorandum on chimney heights The purpose of the Memorandum is to provide a basis for limiting the pollution due to sulphur dioxide (SO2) near to the chimney by increasing the height as the rate of emission increases. In addition, the relationship between chimney height and building height is taken into account as well as the type of locality, here classified in précis as follows: A B C D E An undeveloped area where background pollution is low with no other emission nearby. A partially developed area with low background pollution and no other emission nearby. A built-up residential area with only moderate background pollution and no other emission nearby. An urban area of mixed industrial and residential development, with considerable background pollution and with other emission nearby. A large city, or an urban area of dense residential and heavy industrial development with severe background pollution. The concentration of SO2 is determined from equations quoted in the Memorandum which require knowledge of the maximum rate of fuel consumption, the sulphur content of the fuel and the boiler efficiency. For calculation from first principles, the required information is given in Chapter 16 (Tables 16.2 and 16.4) but, as an approximation for the coals most in use and the heavier oils, emission of SO2 in g/s may be approximated by multiplying the boiler rating in MW by either 1.7 for coal firing or 1.0 for oil firing. If the calculated SO2 content is less than 0.38 g/s then the chimney height need be only 3 m higher than that which has been calculated as necessary for combustion. For SO2 contents greater than this, reference must be made either to the Memorandum, which contains a set of nomograms, or to Fig. 17.7, from which an ‘uncorrected’ chimney height may be read, taking account of the location, from the left-hand scale. Where the fuel has a sulphur content of more than 2 per cent, as used to be the case for most oils, a primary correction must then be made by adding 10 per cent to the height read from the diagram. If, after this increase, the dimension is more than 2V2 times the height of the building or any other building in the immediate vicinity, then no further correction is necessary.