Heat Transfer
A metal ball of radius 0.1 m at a uniform temperature of 90°C is left in air at 30°C. The density and the specific heat of the metal are 3000 kg/m³ and 0.4 kJ/kg.K respectively. The heat transfer co-efficient is 50 W/m².K Neglecting the temperature gradients inside the ball, the time taken (in hours) for the ball to cool to 60°C is

0.15
55.5
0.55
555

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Heat Transfer
In a gas-liquid shell and tube heat exchanger, the

Gases to be heated/cooled is normally routed through the shell side, because the corrosion caused by the cooling water or steam condensate remain localised to the tubes
Presence of a non-condensible gas decreases the condensing film co-efficient
Gases under high pressure are routed through the tube side, because high pressure gases are corrosive in nature
All of these

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Heat Transfer
Nusselt number is the ratio of the

Temperature gradient of the wall to that across the entire pipe
Heat flux at the wall to that across the entire pipe
None of these
Temperature difference to the temperature gradient at the wall

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Heat Transfer
1000 Kg of liquid at 30°C in a well stirred vessel has to be heated to 120°C, using immersed coils carrying condensing steam at 150°C. The area of the steam coils is 1.2 m² and the overall heat transfer co-efficient to the liquid is 1500 W/m².°C. Assuming negligible heat loss to the surrounding and specific heat capacity of the liquid to be 4 kJ/kg.°C, the time taken for the liquid to reach desired temperature will be

44 min
51 min
22 min
15 min

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