A heat exchanger is used to heat cold water entering at 8 degree C at a rate of
ID: 2077374 • Letter: A
Question
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 (i) 205 kW (ii) 411kW (iii) 311 kW n (iv) 114 kW (v) 78 kW In a parallel-flow, liquid-to-liquid heat exchanger, the inlet and outlet temperatures of the heat 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 it counter-flow arrangement is used is (i) 3.9 % (ii) 9.7% (iii) 14.5% (iv) 19.7% (v) 24.6% Consider a surface at -5degree C in an environment at 25 degree C, The maximum rate of heat that can be emitted from this surface by radiation is (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 Hot water flows through a PVC (k =0.092W/m degree K) pipe whose inner diameter is 2 cm outer diameter is 2.5 cm. The temperatures of the interior surface of this pipe is 50 degree C and the temperatures of the exterior surface is 20 degree C. The rate of heat transfer per unit of pipe length is (a) 77.7 W/m (b) 89.5W/m (c) 98.092W/m (d) 112W/m (e) 168 W/m 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 temperatures f the block rises to 750 degree C in a 25-min period, the oven must be maintained at (a) 750 degree C (b) 830 degree C (c) 875 degree C (d) 910 degree C (e) 1000 degree CExplanation / Answer
We know that,
Q = m Cp (T2-T1)
hence for heat exchange, neglecting loss of heat
total heat lost by air = Total heat gain by water
2.5 * 1 * (90 - T) = 1.2 * 4.2 * (T - 8)
solving we get, T = 35.14 degree celcius
hence heat tranfer rate = 1.2 * 4.2 * (35.14 - 8) = 114kW