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:
| Fluid | Material in Transmission Surface | Fluid | Overall 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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