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Q7. An exhaust gas economiser has a total of 60 tubes in a single pass, counter flow arrangement. Each tube has an inner diameter of 20 mm, wall thickness 2.4 mm and a length of 3 m.

The feed water enters the tubes at a temperature of 90℃ and leaves at 106℃. The exhaust gas enters the shell at a rate of 47 tonne/hour and temperature of 350℃, it leaves at a temperature of 280℃.

The heat lost to the surroundings is negligible.

Calculate EACH of the following:

(a) The rate of heat transfer; (2)

(b) The mean velocity of the feed water in the tubes; (5)

(c) The log means temperature difference; (4)

(d) The overall heat transfer coefficient based on the tube outer surface area. (5)

Note : for feed water c = 4.2 kJ⁄(kg K) and density 1000  kg⁄m³

for exhaust gas c_p = 1.1 kJ⁄(kg K).