practice3

Practice3 - PROBLEM 11.10 KNOWN Heat exchanger with two shell passes and eight tube passes having an area 925m 45,500 kg/h water is heated from 80C

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PROBLEM 11.10 KNOWN: Heat exchanger with two shell passes and eight tube passes having an area 925m 2 ; 45,500 kg/h water is heated from 80 ° C to 150 ° C; hot exhaust gases enter at 350 ° C and exit at 175 ° C. FIND: Overall heat transfer coefficient. SCHEMATIC: ASSUMPTIONS: (1) Negligible losses to surroundings, (2) Negligible kinetic and potential energy changes, (3) Constant properties, (4) Exhaust gas properties are approximated as those of atmospheric air. PROPERTIES: Table A-6 , Water ( 29 ( 29 c T 8 0 15 0 C/ 2 388K: =+°= c = c p,f = 4236 J/kg K. ANALYSIS: The overall heat transfer coefficient follows from Eqs. 11.9 and 11.18 written in the form m,CF U q/AFT =∆ l where F is the correction factor for the HXer configuration, Fig. 11.11, and m,CF T l is the log mean temperature difference (CF), Eqs. 11.15 and 11.16. From Fig. 11.11, find ( 29 ( 29 ( 29 ( 29 h, i o c, o c,i c, o c, i i c,i T T TT 35 0 17 5 C 15 0 8 0C R 2. 5 P 0.26 T T 15 0 8 0 C T T 35 0 8 -- - ° = = = = == - - ° - find F 0.97. The log-mean temperature difference, Eqs. 11.15 and 11.17, is ( 29 ( 29 ( 29 ( 29 ( 29 12 35 0 15 17 5 8 T 141.1 C. n T /T n 35 0 150/ 17 5 80 - ° - -° -∆ = = ∆∆   l l l From an overall energy balance on the cold fluid (water), the heat rate is ( 29 c c c, o c,i q m c =- & ( 29 6 q 45,500kg/ h 1h/3600 s 4236J/kgK 15 0 8 3.74 81 0 W. = ×× - ° Substituting values with A = 925 m 2 , find 6 22 U 3.74 8 1 0 W/925 m 0.9 7 141.1 K 29.6W/ m K. = × × × =⋅ < COMMENTS: Compare the above result with representative values for air-water exchangers, as given in Table 11.2. Note that in this exchanger, two shells with eight tube passes, the correction factor effect is very small, since F = 0.97.
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PROBLEM 11.56 KNOWN: Inlet temperature and flow rates for a concentric tube heat exchanger. Hot fluid outlet temperature.
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This note was uploaded on 08/08/2008 for the course ME 364 taught by Professor Rothamer during the Spring '08 term at Wisconsin.

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Practice3 - PROBLEM 11.10 KNOWN Heat exchanger with two shell passes and eight tube passes having an area 925m 45,500 kg/h water is heated from 80C

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