5HE Heat Transfer - Oct'1997
PART A - (20 x 2 = 40 marks
PART B - (5 x 12 = 60 marks)
UNIT 1
(b) A plane brick wall, 25 cm thick, is faced with 5 cm thick concrete layer. If the temperature of the exposed brick face is 70oC and that of the concrete is 25oC, find out the heat lost per hour through a wall of 15 m x10 m. Also, determine the interface temperature. Thermal conductivity of the brick and concrete are 0.7 W/m.K and 0.95 W/m.K respectively.
Or
(b) A steel pipe (K = 50 W/m.K) of I.D. = 100 mm and O.D. = 110 mm is to be covered with two layers of insulation, each having a thickness of 50 mm. Thermal conductivity of the first insulation material is 0.06 W/m.K and that of the second is 0.12 W/m.K. Calculate the loss of heat per metre length of pipe and the interface temperature between the two layers of insulation when the temperature of the inside tube surface is 250oC and that of the outside surface of the insulation is 50oC.
UNIT II
(b) Air at 20oC is flowing along a heated flat plate at 150oC at a velocity of 3 m/sec. The plate is 2 m long and 1.5 m wide. Calculate the thickness of the hydrodynamic boundary layer and the skin friction coefficient at 30 cm from the leading edge of the plate. Kinematic viscosity of air at 20oC is 15.06 x 10-6 m2/sec
Or
(b) Determine the rate of heat loss from a 100 mm diameter steam pipe placed horizontally in ambient air at 30oC. The length of the pipe is 4 m and wall temperature, Tw = 170oC.
Use the following empirical expression:
Nu=0.53 (Gr x Pr)1/4
Properties of air at 100oC are as following
b
=1/373 K-1
g
= 23.13 x 10 -6 m2 /sec
K= 0.0325 W/m.K
Pr = 0.7
Nu = 114
UNIT III
(b) A parallel flow heat exchanger has to cool 2500 kg/hr of oil from 70oC to 30oC. Cooling water enters the exchanger at 10oC and leaves at 20oC. Specific heat of oil is 2.1 kJ/kg.K. Determine the effectiveness of the heat exchanger and heat transfer capacity.
Or
(b) A heat exchanger heats 25,000 kg/hr of water entering at 30oC while cooling 20,000 kg/hr of water from 100oC to 80oC. Determine the area necessary for
(i)Parallel flow arrangement
(ii)Counter flow arrangement.
Overall heat transfer coefficient may be assumed as 1,600 W/m2K.
UNIT IV
Or
(b) Determine the net heat transfer by radiation between the two surfaces A and B per hour per unit area if the temperatures of A and B are 800oC and 350oC respectively. Emissivities of A and B are 0.9 and 0.25 respectively . Both surfaces are gray and are infinite parallel lines, 3.5 m apart.
UNIT V
Or
(b) A triple effect evaporator concentrates, a liquid with no appreciable elevation in boiling point. If the temperature of the steam to the first effect is 395 K and vacuum in the last effect brings down the boiling point to 325 K, what are the approximate boiling points of liquid in first and second effect? Assume the overall heat transfer coefficient as 3.1, 2.3 and 1.1 kW/m2.K in first, second and third effects respectively.
Last Modified on: 04-Feb-2022
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