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Unformatted text preview: Chap 23 Heat Exchangers 2354 Glycerin is heated by ethylene glycol in a thinwalled doublepipe parallelflow heat exchanger. The rate of heat transfer, the outlet temperature of the glycerin, and the mass flow rate of the ethylene glycol are to be determined. Assumptions 1 Steady operating conditions exist. 2 The heat exchanger is wellinsulated so that heat loss to the surroundings is negligible and thus heat transfer from the hot fluid is equal to the heat transfer to the cold fluid. 3 Changes in the kinetic and potential energies of fluid streams are negligible. 4 There is no fouling. 5 Fluid properties are constant. 6 The thermal resistance of the inner tube is negligible since the tube is thinwalled and highly conductive. Properties The specific heats of glycerin and ethylene glycol are given to be 2.4 and 2.5 kJ/kg. ° C, respectively. Analysis ( a ) The temperature differences at the two ends are ∆ ∆ T T T T T T T T h in c in h out c out h out h out 1 2 60 20 15 = = ° ° ° = = ° ° , , , , , , ( C C = 40 C C) = 15 C and ∆ ∆ ∆ ∆ ∆ T T T T T lm = = = ° 1 2 1 2 40 15 40 15 255 ln( / ) ln( / ) . C Then the rate of heat transfer becomes kW 19.58 = = ° ° = ∆ = W 584 , 19 C) 5 . 25 )( m C)(3.2 . W/m 240 ( 2 2 lm s T UA Q ( b ) The outlet temperature of the glycerin is determined from C 47.2 ° = ° + ° = + = →  = C) kJ/kg. kg/s)(2.4 3 . ( kW 584 . 19 C 20 )] ( [ glycerin p in out in out p C m Q T T T T C m Q ( c ) Then the mass flow rate of ethylene glycol becomes kg/s 3.56 = ° ° ° = = = C] 60 C 15) + C)[(47.2 kJ/kg. (2.5 kJ/s 584 . 19 ) ( )] ( [ glycol ethylene glycol ethylene out in p out in p T T C Q m T T C m Q 2337 Glycerin 20 ° C 0.3 kg/s Hot ethylene 60 ° C 3 kg/s Chap 23 Heat Exchangers 2355 Air is preheated by hot exhaust gases in a crossflow heat exchanger. The rate of heat transfer and the outlet temperature of the air are to be determined. Assumptions 1 Steady operating conditions exist. 2 The heat exchanger is wellinsulated so that heat loss to the surroundings is negligible and thus heat transfer from the hot fluid is equal to the heat transfer to the cold fluid. 3 Changes in the kinetic and potential energies of fluid streams are negligible. 4 There is no fouling. 5 Fluid properties are constant. Properties The specific heats of air and combustion gases are given to be 1005 and 1100 J/kg. ° C, respectively. Analysis The rate of heat transfer is kW 103 = C) 95 C C)(180 kJ/kg. kg/s)(1.1 1 . 1 ( )] ( [ gas. ° ° ° = = out in p T T C m Q The mass flow rate of air is ( . . m PV RT = = × = (95 kPa)(0.8 m / s) kPa.m / kg.K) K kg / s 3 3 0 287 293 0 904 Then the outlet temperature of the air becomes C 133 ° = ° × + ° = + = = C) J/kg. kg/s)(1005 904 . ( W 10 103 C 20 ) ( 3 , , , , p in c out c in c out c p C m Q T T T T C m Q 2338 Air 95 kPa 20 ° C 0.8 m 3 /s Exhaust gases 1.1 kg/s 95 ° C Chap 23 Heat Exchangers...
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This homework help was uploaded on 04/18/2008 for the course EML 3007 taught by Professor Chung during the Spring '08 term at University of Florida.
 Spring '08
 Chung
 Heat Transfer

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