The surface area A of heat exchangers required for a given service is determined
from
where
Q
= rate of heat transfer
U
= mean overall heat transfer coefficient
= 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 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. Therefore, typical
values of U are useful for quickly estimating the required surface area. The
literature has many tabulations of such typical coefficients for commercial heat transfer
services. 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.
By: Dr. Bernhard Spang, Associate Content Writer (read the author's Profile) b.spang@gmx.net
