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3.9) A heat exchanger consists of two E-shells connected in series and contains a total of 488 tubes arranged for four passes (two per shell). The tubes are 1 -in. OD, 16BWG with a length of 10ft. The heat exchanger was designed to cool a hot fluid from 250F to 150F using a cold fluid entering at 100F. When the exchanger was first placed in service, the hot fluid outlet temperature was 130F, but it gradually increased to 160F over several months and then remained essentially constant. Fluid data are as follows: (a) To the nearest square foot, how much heat-transfer surface does the heat exchanger contain? (b) Use the initial operating data to calculate the clean overall heat-transfer coefficient, UC - (c) Use the final operating data to calculate the fouled overall heat-transfer coefficient, UD - (d) Use the results of parts (b) and (c) to determine the experimental value of the total fouling factor, RD RDi(Do/Di)+RDo Ans. (a) 1,278ft2. (b) 153Btu/hft2F. (c) 66Btu/hft2F. (d) 0.0086hft2F/Btu