Thermal energy is valuable. Therefore, the most important characteristic of heat exchangers must be high efficiency. The aim is to manufacture units with high efficiency and low pressure drop in the most economical way. How this requirement can be met is basically shown by the fundamental equation of heat transfer:
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General
 |
 heat exchanger performance |
 heat transfer coefficient |
 heat transfer surface |
 logarithmic mean temperature difference |
In the majority of heat transfer problems, the logarithmic mean temperature difference is a fixed (given) value. Therefore, the only solution is to improve the product (  ) in order to increase the efficiency of the heat exchanger. The reciprocal of ( ) is referred to as overall thermal resistance of the heat exchanger. It must be kept to a minimum in order to achieve the above mentioned aim. |
 |
 overall thermal resistance |
 outside thermal resistance |
 thermal resistance (wall) |
 inside thermal resistance |
 outside, inside heat transfer coefficient |
 tube wall thickness |
 thermal conductivity |
The following design rules can be derived:
| • Use thin-wall tubes made of materials with good thermal conductivity, for example, copper or aluminium materials |
• Reduce thermal resistance through increase of heat transfer surfaces  and/or reduction of  ; this means use tubes with outside and/or inside structures |
• Reduce thermal resistance through special surface structures, thus increasing the outer  and/or inner  heat transfer coefficient |
Depending on the type of heat exchanger, the heat transfer coefficients may vary considerably on both sides of a tube which makes different types of finned tubes necessary. Based on this finding, we have developed tubes with structures on the outside, inside and on both sides in the last few decades.
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