Question

Two identical double-pipe heat exchangers are constructed of a 2-in standard schedule 40 pipe placed inside a 3-in standard pipe. The length of the exchangers is 10 ft; 40 gal/min of water initially at 80◦F is to be heated by passing through the inner
pipes of the exchangers in a series arrangement, and 30 gal/min of water at 120◦F and 30 gal/min of water at 200◦F are available to accomplish the heating. The two heating streams may be mixed in any way desired before and after they enter the heat exchangers. Determine the flow arrangement for optimum performance (maximum heat transfer) and the total heat transfer under these conditions. Use both NTU and LMTD methods.

Answer 1

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The plot of the heat transfer rate to the inlet flow of hotter fluid is shown in the following figure The plot of Heat Trans fer Rate Versus the inlet mass flow rate x 10 4.5 Th-26.7 C Th-30 C 3.5 | Th 48.9 C 2.5 1.5 0.5 40 30 35 50 60 Mass fow rate, gaVmin