Shell And Tube Heat Exchanger .docx

Single shell-1 tube pass heat exchanger

Flowrate Q1 hot (LPM) 4

Flowrate Q2 cold (LPM) 4

Hot tank T1 (°C) 44.8

Hot water IN T2 (°C) 44.2

Hot water OUT T3 (°C) 40.6

Cold water IN T4 (°C) 26.9

Flow condition: co-current Logarithmic mean temp difference

DTlm 

DTlm 

(T2  T4 )  (T3  T5 )  T T  ln  2 4   T3  T5 

(44.2  26.9)  (40.6  29.3)  44.2  26.9  ln    40.6  29.3 

DTlm 

(17.3)  (11.3)  17.3  ln    11.3 

DTlm  14.087 oC Power Emitted by hot water:

WE  QH  H CPH (TH ,in  TH ,out ) = (0.0000666)(991)(4.174)(44.2-40.6) = 0.9828 KW = 982.8 W Power Absorbed by cold water:

WA  QC C CPC (TC ,out  TC ,in ) = (0.0000666)(995.8)(4.179)(29.3-26.9) =0.6645 KW = 664.56 W Efficiency:



0.6645 100 % 0.9828

  67.61%

Cold water OUT T5 (°C) 29.3

Over all Heat Transfer Coefficient:

U

Power absorbed tm  Area

Area =  DLN Where: D = Tube outside diameter=0.01 m L = Tube length = 1m N = No. of tubes = 21 Therefore: Area = 0.659 m2

U

0.6645 14.08  0.659

U  71.61 W/m2 . oC

Single shell- 2 tube pass heat exchanger

Flowrate Q1 hot (LPM) 4

Flowrate Q2 cold (LPM) 4

Hot tank T1 (°C) 48

Hot water IN T2 (°C) 47.8

Hot water OUT T3 (°C) 42

Cold water IN T4 (°C) 25

Flow condition: co-current Logarithmic mean temp difference

DTlm 

DTlm 

(T2  T4 )  (T3  T5 )  T T  ln  2 4   T3  T5 

(47.8  25)  (42  30)  47.8  25  ln    42  30 

DTlm 

(22.8)  (12)  22.8  ln    12 

DTlm  16.8 oC Power Emitted by hot water:

WE  QH  H CPH (TH ,in  TH ,out ) = (0.0000666)(991)(4.174)(47.8-42) = 1.583 KW = 1583.4 W Power Absorbed by cold water:

WA  QC C CPC (TC ,out  TC ,in ) = (0.0000666)(995.8)(4.179)(30-25) =1.373 KW = 1373.2 W Efficiency:



1.373  100 % 1.583

  86.73%

Cold water OUT T5 (°C) 30

Over all Heat Transfer Coefficient:

U

Power absorbed tm  Area

Area =  DLN Where: D = Tube outside diameter=0.01 m L = Tube length = 1m N = No. of tubes = 21 Therefore: Area = 0.659 m2

U

1.373 16.8  0.659

U  124.01 W/m2 . oC CONCLUSION: Multi pass exchangers helps to prolong the time of fluid to remain in the exchanger for longer periods as compared to single pass, for heating or cooling to take place. In a 2 pass tube heat exchanger, same quantity of water that would have passed in one through (single pass tube) stays in the heat exchanger for longer periods and transfer more heat than in the single pass tube. That is why the temperature differences between hot water inlet and outlet is increased in case of 1 shell and 2 pass (1-2 exchanger) heat exchanger. Same is the case for cold water IN and OUT.