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U in Heat Exchangers

May 10 2012 08:19 AM | bspang in Heat Transfer ****- Share this topic:
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The surface area A of heat exchangers required for a given service is determined from

Attached Image: uexchangers1.gif

where


Q    = rate of heat transfer
U    = mean overall heat transfer coefficient
ΔTm   = mean temperature difference

For a given heat transfer service with known mass flow rates and inlet and outlet temperatures the determination of Q is straightforward and ΔTm can be easily calculated if a flow arrangement is selected (e.g. logarithmic mean temperature difference for pure countercurrent or cocurrent flow). This is different for the overall heat transfer coefficient U. The determination of U is often tedious and needs data not yet available in preliminary stages of the design. The following is a table with values for different applications and heat exchanger types. More values can be found in the sources given below.

The ranges given in the table are an indication for the order of magnitude. Lower values are for unfavorable conditions such as lower flow velocities, higher viscosities, and additional fouling resistances. Higher values are for more favorable conditions. Coefficients of actual equipment may be smaller or larger than the values listed. Note that the values should not be used as a replacement of rigorous methods for the final design of heat exchangers, although they may serve as a useful check on the results obtained by these methods.


Typical Overall Heat Transfer Coefficients in Heat Exchangers
Type Application and Conditions U
W/(m2 K)1)
U
Btu/(ft2 °F h)1)
       
Tubular, heating or cooling Gases at atmospheric pressure inside and outside tubes 5 - 35 1 - 6
  Gases at high pressure inside and outside tubes 150 - 500 25 - 90
  Liquid outside (inside) and gas at atmospheric pressure inside (outside) tubes 15 - 70 3 - 15
  Gas at high pressure inside and liquid outside tubes 200 - 400 35 - 70
  Liquids inside and outside tubes 150 - 1200 25 - 200
  Steam outside and liquid inside tubes 300 - 1200 50 - 200
       
Tubular, condensation Steam outside and cooling water inside tubes 1500 - 4000 250 - 700
  Organic vapors or ammonia outside and cooling water inside tubes 300 - 1200 50 - 200
       
Tubular, evaporation steam outside and high-viscous liquid inside tubes, natural circulation 300 - 900 50 - 150
  steam outside and low-viscous liquid inside tubes, natural circulation 600 - 1700 100 - 300
  steam outside and liquid inside tubes, forced circulation 900 - 3000 150 - 500
       
Air-cooled heat exchangers2) Cooling of water 600 - 750 100 - 130
  Cooling of liquid light hydrocarbons 400 - 550 70 - 95
  Cooling of tar 30 - 60 5 - 10
  Cooling of air or flue gas 60 - 180 10 - 30
  Cooling of hydrocarbon gas 200 - 450 35 - 80
  Condensation of low pressure steam 700 - 850 125 - 150
  Condensation of organic vapors 350 - 500 65 - 90
       
Plate heat exchanger liquid to liquid 1000 - 4000 150 - 700
       
Spiral heat exchanger liquid to liquid 700 - 2500 125 - 500
  condensing vapor to liquid 900 - 3500 150 - 700
Notes: 1) 1 Btu/(ft2 °F h) = 5.6785 W/(m2 K) 2) Coefficients are based on outside bare tube surface

Sources

Schlünder, E. U. (Ed.): VDI Heat Atlas, Woodhead Publishing, Limited, 1993, Chapter Cc.
Perry, R. H., Green, D. W. (Eds.): Perry's Chemical Engineers' Handbook, 7th edition, McGraw-Hill, 1997 , Section 11.
Kern, D. Q.: Process Heat Transfer, McGraw-Hill, 1950.
Ludwig, E. E.: Applied Process Design for Chemical and Petrochemical Plants, Vol. 3, 3rd edition, Gulf Publishing Company, 1998.
Branan, C. R.: Process Engineer's Pocket Handbook, Vol. 1, Gulf Publishing Company, 1976.

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