Fluid Heat Transfer Coefficients in Heat Exchanger Surface Combinations
Average overall heat transmission coefficients for fluid and surface combinations like Water to Air, Water to Water, Air to Air, Steam to Water and more.
The overall heat transfer coefficient is used to calculate total heat transfer through a wall or heat exchanger construction. The overall heat transfer coefficient depends on the fluids and their properties on both sides of the wall, the properties of the wall and the transmission surface.
For practically still fluids  average values for the overall heat transmission coefficient through different combinations of fluids on both sides of the wall and type of wall  are indicated in the table below:
Fluid  Material in Transmission Surface  Fluid  Overall Heat Transmission Coefficient  U   

(Btu/(ft^{2} hr ^{o}F))  (W/(m^{2} K))  
Water  Cast Iron  Air or Gas  1.4  7.9 
Water  Mild Steel  Air or Gas  2.0  11.3 
Water  Copper  Air or Gas  2.3  13.1 
Water  Cast Iron  Water  40  50  230  280 
Water  Mild Steel  Water  60  70  340  400 
Water  Copper  Water  60  80  340  455 
Air  Cast Iron  Air  1.0  5.7 
Air  Mild Steel  Air  1.4  7.9 
Steam  Cast Iron  Air  2.0  11.3 
Steam  Mild Steel  Air  2.5  14.2 
Steam  Copper  Air  3.0  17 
Steam  Cast Iron  Water  160  910 
Steam  Mild Steel  Water  185  1050 
Steam  Copper  Water  205  1160 
Steam  Stainless Steel  Water  120  680 
 1 Btu/ft^{2} hr ^{o}F = 5.678 W/m^{2} K = 4.882 kcal/h m^{2} ^{o}C  Unit Converter
Note that these coefficients are very rough. They depends on fluid velocities, viscosities, conditions of the heating surfaces, size of the temperature differences and so on. For exact calculations  always check manufacturing data.
Example  Water to Air Heat Exchanger made in Copper
A roughly estimate of the specific heat transmission in a copper heat exchanger with water (mean temperature 80 ^{o}C) on one side and air (mean temperature 20 ^{o}C) on the other side  where the overall heat transfer coefficient U is 13.1 W/(m^{2} K)  can be calculated as
q = (13.1 W/(m^{2} K)) ((80 ^{o}C)  (20 ^{o}C))
= 786 W/m^{2}
≈ 750  800 W/m^{2}
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