d. A heat exchanger is used to heat cold water entering at 8 degree C at a rate
ID: 2077378 • Letter: D
Question
d. A heat exchanger is used to heat cold water entering at 8 degree C at a rate of 1.2 kg/s by hot air entering at 90 degree C at rate of 2.5 kg/s. The highest rate of heat transfer in the heat exchanger is (show work for) (i) 205 kW (ii) 411 kW (iii) 311 kW (iv) 114 kW (v) 78 kW e. In a parallel-flow, liquid-to-liquid heat exchanger, the inlet and outlet temperatures of the hot fluid are 150 degree C and 90 degree C while that of the cold fluid are 30 degree C and 70 degree C, respectively. For the same overall heat transfer coefficient, the percentage decrease in the surface area of the heat exchanger if counter-flow arrangement is used is (show work for) (i) 3.9% (ii) 9.7% (iii) 14.5% (iv) 19.7% (v) 24.6% f. Consider a surface at -5 degree C in an environment at 25 degree C. The maximum rate of heat that can be emitted from this surface by radiation is (show work for) (i) 0 w/m^2 (ii) 155 W/m^2 (iii) 293 W/m^2 (iv) 354 w/m^2 (v) 567 W/m^2 g. Hot water flows through a PVC (K = 0.092 W/m degree k) pipe whose inner diameter is 2 cm and outer diameter is 2.5 cm. The temperature of the interior surface of this pipe is 50 degree C and the temperature of the exterior surface is 20 degree C. The rate of heat transfer per unit of pipe length is (show work) (a) 77.7 W/m (b) 89.5 W/m (c) 98.0 W/m (d) 112 W/m (e) 168 W/m h. An 18-cm-long, 16 cm-wide, and 12-cm-high hot iron block (rho = 7870 kg/m3, cp = 447 J/kg degree C) initially at 20 degree C is placed in an oven for heat treatment. The heat transfer coefficient on the surface of the block is 100 W/m2[degree C. If it is required that the temperature of the block rises to 750 degree C in a 25-min period, the oven must be maintained at (show work for) (a) 750 degree (b) 830 degree C (c) 875 degree C (d) 910 degree C (e) 1000 degree CExplanation / Answer
here,
d)
initial temprature , Ti = 8 degree C
final temprature , Tf = 90 degree C
m = 1.2 kg/s
so, the highest rate of heat exchange , P = m * Cw * ( Tf - Ti)
P = 1.2 * 4186 * ( 90 - 8) W
P = 411902 W
P = 411 KW
the heighest rate of heat exchange is b) 411 KW
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