True-colour image processing of liquid crystal (LC) images has been established as a new powerful experimental technique to yield full-field measurements of temperature and heat transfer coefficient distributions. In this study the LC technique was developed successfully and applied to a study of six element-shapes of rotary air heat preheaters are used extensively in power stations. A complete mapping of temperature, heat transfer coefficient and pressure drop was obtained for all of them at every angle and Reynolds number required. The measuring technique comprising the use of LC flexible sheets and true-colour processing may also be used for a great variety of applications and should be of considerable use in improving the design of all types of compact heat exchanger.