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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 [Fi [13 Lin 0.3 —— No PgE [13 CHAPTER 19 Direct Contact Heat Transfer ROBERT F. BOEHM University of Nevada–Las Vegas Las Vegas, Nevada 19.1 Introduction 19.2 Sensible heat exchange 19.2.1 General comments 19.2.2 External convection to spheres 19.2.3 Heat transfer inside spheres 19.3 Evaporation and condensation 19.3.1 General considerations 19.3.2 Condensation of a vapor on or in a liquid Film condenser Condensation on liquid droplets Condensation on liquid jets Condensation in a liquid 19.3.3 Evaporation of a liquid by a surrounding vapor, gas or liquid Droplet evaporation in a vapor or gas Droplet evaporation in a liquid 19.4 Columns and other contactors 19.4.1 Spray columns Global treatments Differential treatment Melting and solidification applications 19.4.2 Baffled columns 19.4.3 Packed columns 19.5 Concluding comments Nomenclature References 19.1 INTRODUCTION Direct contact heat transfer can occur whenever two substances at different temper- atures touch each other physically. The implication is that there is not an interven- ing wall between the two substances. Heat transfer where there is a surface between the two streams is sometimes called indirect , or the heat transfer device is one of 1359
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1360 DIRECT CONTACT HEAT TRANSFER 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 [136 Lin 0.0 —— Nor PgE [136 the closed types. The physical interaction of the two streams can accomplish heat transfer very efficiently. Without an intervening wall, the energy transport between the two streams can take place across small thermal resistances. In addition, the fact that a wall is not present can allow a mass transfer process to take place. In some cases, this is a desirable phenomenon (open cooling towers), but in other cases it may not be. Costs are often more favorable for direct contact heat transfer devices than for their closed counterparts. The thermal resistances present in closed heat exchangers result in less heat transfer than might be accomplished in direct contact, and this often trans- lates to lower operating costs for the latter. In addition, the equipment to accomplish the direct contact processes is generally less expensive than the counterpart closed heat exchangers. Both aspects can result in considerable life-cycle cost savings for the direct contact approach over that of the closed type of heat exchanger. Some potential limitations are inherent in direct contact processes. There is a requirement that the two streams be at the same pressure. Although this requirement does not often cause significant problems, it could be very important. Also, as noted above, the mass transfer possibility in direct contact may not be desirable.
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