FTFS Chap23 P054 - Chap 23 Heat Exchangers 23-54 Glycerin...

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Unformatted text preview: Chap 23 Heat Exchangers 23-54 Glycerin is heated by ethylene glycol in a thin-walled double-pipe parallel-flow 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 well-insulated 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 thin-walled 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 23-37 Glycerin 20 ° C 0.3 kg/s Hot ethylene 60 ° C 3 kg/s Chap 23 Heat Exchangers 23-55 Air is preheated by hot exhaust gases in a cross-flow 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 well-insulated 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 23-38 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.

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FTFS Chap23 P054 - Chap 23 Heat Exchangers 23-54 Glycerin...

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