Air at 25 oC flows over a cross flow industrial heat exchanger and cools water f

Question

Air at 25 oC flows over a cross flow industrial heat exchanger and cools water from 98 oC to 56 oC (similar to Fig. 11.5b). The water flows at a rate of 45 kg/min through 50 separate passages within the heat exchanger. Each passage has an oval cross-sectional area with a perimeter of 20 mm and an area of 30 mm2. The mass flow rate of the air is 160 kg/min. The overall heat transfer coefficient based on the cold (air) side, Uc, is 80 W/m2K.

a) Calculate the air exit temperaure and the heat exchanger effectiveness.

b) Calculate the Number of Transfer Units, NTU. Take the air side to be mixed and water side unmixed.

c) Calculate the air side heat transfer area.

d) Determine the heat transfer coefficient inside the water passages.

e) Neglecting the wall thermal resistence of the water passages (high thermal conductivity), calculate the length of each water passage if the convectine heat transfer coefficient on the cold side, hc, is 300 W/m2K.

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