# solution

Feedwater from high pressure heater enters the inlet header of the economizer at a rate of 600 kg/sec, 140 bar, and 170Â°C. it is heated by flue gas till it becomes saturated liquid and leaves the outlet header to flow into the drum. Flue gases flow over the economizer coils at the rate of 1250 kg/sec and leaves at 450Â°C. to restrict the erosion rate by fly ash, the flue gas velocity should not exceed 12 m/sec, while the optimum water velocity leaving the coils is 1.2 m/sec. the tubes are of 70 mm OD and 60 ID. The overall heat transfer coefficient is 70 W/mÂ²K. Determine: a) The number of coils needed in the economizer. Deduce the length of one coil. Assume Cpaflue gas= 1.12 kJ/kg.K. b) Suppose the pitch of the coil is 80 mm and the clearance on the two sides of the duct of width 4.8 mis 5 mm. Find the vertical height of the economizer. Outlet header te, -815Â°C Flue gas To drum ‘sat = 336.75 Â°C 182 450Â°C Inlet header Iwi – 170 Â°C ’82-450 “C – D -280 Â°C D1 – 478.25 Â°C From h.p. heater, w, 170Â°C L (b)

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