C2 Overall Heat Transfer

1 Introduction

In a heat exchanger hot and cold fluids are separated by a wall of one or more layers. The process of steady state heat transport from the hot fluid to the cold fluid through the separating wall is denoted with overall heat transfer and characterized by the overall heat transfer coefficient. This overall coefficient varies together with the two local heat transfer coefficients, and suitable mean values have to be introduced for the thermal design and rating of heat exchangers.

2 Local and Mean Heat Transfer Coefficient

If heat transfer is convective, the local heat transfer coefficient at a surface is directly related to the length of the flow path x, the local temperature of the fluid ϑ, and the temperature of the wall surface ϑ _w over which the fluid flows. In other words, it is given by

$${\alpha _{\rm loc}} = {\alpha _{\rm loc}}\left( {x,\vartheta ,{\vartheta _{\rm w}}} \right).$$

The direct dependence of heat transfer on the length of the flow path is caused by the...