Problem 15-1 - Example 15-1 A cross-flow heat exchanger is...

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Example 15-1 A cross-flow heat exchanger is used for waste heat recovery. Exhaust gases, which flow at 3 kg/s, enter at 200°C and exit at 80°C. Cool air flowing at 12 kg/s enters at 20°C. The exhaust gases are unmixed and the air is mixed. If 2 55 W/m K i U = , find A i . Assume the exhaust gases have the same properties as air. Solution: Exhaust Gas 200 °C (hot side) 80 °C Air 20 °C (cold side) , kg kJ 31 . 0 1 4 sk g K HH p H Cm c ⎛⎞ == ⎜⎟ ⎝⎠ ± kW 3.04 K H C = , kg kJ 12 1.008 g K CC p C c ± kW 12.1 K C C = ,, C C o C i i H o QT T T T ⎡⎤ =− ⎣⎦ ±
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,, + H Co Ci Hi Ho C C TT C ⎡⎤ =− ⎣⎦ [] , 3.04 20+ 200 80 12.1 T , 50.15 T = In counterflow case o 1, , 200 50.15 149.8 C T ∆= = = o 2, , 80 20 60 C T = = 21 , 2 1 60 149.8 98.2 60 ln ln 149.8 LM cf T T T ∆− = = ⎛⎞ ⎜⎟ ⎝⎠ T tube, i T tube, o T shell, i Air T shell, o Exhaust Gas
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Problem 15-1 - Example 15-1 A cross-flow heat exchanger is...

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