In a boiler, the forced draught fan develops a total static pressure of 300 mmWC.
Determine the shaft power (in kW) required to drive the fan if 10,000 kg of coal is burnt
per hour with 13 kg of air per kg of coal burnt. The boiler house temperature is 20 0C and
static efficiency of the fan is 80 %.
The operating air density may be calculated from the following:
R = 847.84 mmWC m3/kg mole K and Molecular weight of air, M = 28.92 kg/kg mole.
ANSWER:
Total Pressure = 300 mm of WC
Mass of air handled, m = 10000 × 13/ 3600 = 36.11 kg/s
Atmospheric pressure, P = 1 kg/ cm2 = 10 mtr of WC = 10,000 mm of WC.
Temperature T = 20 + 273 = 293 K
Gas Constant for air, R = 847.84 mm WC m3/kg mole K
Molecular weight of air, M = 28.92 kg/kg mole
Density, kg/m3 = (P x M) / (R X T) = (10000 x 28.92) / (847.84 x 293)
= 1.164 kg/m3
Volume in m3/s = mass (kg/s) / density (kg/m3)
= 36.11 / 1.164
= 31.02 m3/s
Power to fan shaft, kW
= [Volume (m3/s) x Total pressure (mm of WC)] / [102 x fan efficiency]
= [31.02 x 300] / [102 x 0.8]
= 114 kW
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