Question

An oil engine develops 300 kW and the specific fuel consumption is 0.21 kg/kWh. The exhaust from the engine is used in a tubular water heater, flowing through 25 mm diameter tubes, entering with a velocity of 12 m/s, at 340 °C and leaving at 90 °C. The water enters the heater at 10 °C and leaves at 90 °C, flowing in counter-flow to the hot gases. The air-fuel ratio of the engine is 20, and the exhaust pressure is 1.01 bar. The overall heat transfer coefficient of the heat exchanger when designed is found to be 56 W/mK, but after running for some time a fouling factor of 0.5 m K/W must be assumed. Taking the specific heat capacity and the gas constant for the gases as 1.11 kJ/kg K and 0.29 kJ/kg K, and the specific heat capacity for the water as 4.19 kJ/kg K, calculate: The mass flow rate of water; [1096 kg/h] The number of tubes required; [110] iii. The required tube length. [1.457 m)

Answer 1

engine) (HX) exhaust gas 16 19 เ Counter flow Hx; Tg = 340°c T2 = 90°C. a Ta= 10°c -am m specific fuel consumption (86C) = (maw of fuel) work engine) RC = 021 ing= 63 Kalho e o dos (4/2) = 20 = Wair max zone 0.0195 Kgls ma A Merhaunt a nating = 2link or gas = 0.3675 kg/8 ptura i energy balance across Mxi to (as (Merhanat Dam (Ts - Tz) = (MC) water (T6-Ta) (0.3675) (111) (340-90) = (in water (449) (90-10) innte = 101.98125 = 0.30 42 kg/8 74.191180) inwater = 1095.26 Kg Ihone 7 1 82.0

(b) nexhaust gas 0.3 675 Kgls considering exhaust gas as ideal gas; Pro mRT - = f (given pz 1.0 for a 101 kla] R=0.29 kJ/kg T = 3400 Co (0.291863) = 0,568 1=0.568 kg/m Merhaut (0.3675) = (0.568) (7) (8025€ (12) W where Nez No. of tubes Nf= 109.81 e 110 takes

(0) Q = (MC) exhaust (340-90) = (01.98.125 2108 W 101.98125x10² = CUA Om) Oma LMTD = (340-90) - (98-10) - 149. 196°C In(340-90) 90-107 U = overall heat transfer coeff. including fouling factor Y te do + 0.5*10% = 0.018357 Q = 101.98125x10? (54.4749) (A) (149.196 A- 12.547 m² area of HX = 12.547 m² A = 12.547= Ex di L) (NO) = A 12.542 I adiNG (A) (0,025) (110) [[= 1, 4523 m) length of HX = 1.452m