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# Heat Transfer Coefficients in Heat Exchanger Surface Combinations

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:

Heat Transfer Coefficients in Heat Exchangers
FluidMaterial in Transmission SurfaceFluidOverall Heat Transmission Coefficient
- U -
(Btu/(ft2 hr oF))(W/(m2 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/ft2 hr oF = 5.678 W/m2 K = 4.882 kcal/h m2 oC - 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 oC) on one side and air (mean temperature 20 oC) on the other side - where the overall heat transfer coefficient U is 13.1 W/(m2 K) - can be calculated as

q = (13.1 W/(m2 K)) ((80 oC) - (20 oC))

= 786 W/m2

≈ 750 - 800 W/m2

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