The Limitations of Thermal Design using Kays and London’s Colburn and Friction Factors
Since it was first published in 1955, the book Compact Heat Exchangers, by Kays and London, has been considered one of the best references for heat exchanger and cold plate design. Kays and London’s Colburn and friction factors have probably been used by thousands of thermal engineers over the past five decades. In addition, many thermal design software programs use this fin data. However, although the Kays and London Colburn and friction factors have served engineers well over the years and continue to be a valuable resource, they have their limitations.
One of the limitations of the Kays and London Colburn and friction factors is their accuracy. Research has shown that the accuracy of the Colburn and friction factors in Kays and London can be off by as much as 30%, which is particularly critical when designing vacuum-brazed cold plates and plate-fin heat exchangers. For this reason, Lytron uses its own empirical data approximately 95% of the time. Lytron has found that its data, which has been collected over the last 50 years and refined with actual product testing, is much more reliable. Using the highest accuracy mass flow, temperature, and pressure measurements, Lytron’s engineers can achieve an overall F & J test accuracy to within 3-5% for most Reynolds numbers. Lytron generally only uses the Kays and London data to fill in any gaps that may exist in its own data.
When using thermal design software based on Kays and London Colburn and friction factors to design a cold plate or heat exchanger, it’s also important to realize that software cannot account for manufacturability or cost. What may appear to be an optimal thermal design in theory may prove in reality to be difficult or even impossible to manufacture. It may also be a design that is not the most cost-effective or the most optimal, since there are so many permutations of fin geometries that no software would include all of the fin options.
When designing cold plates or heat exchangers, it’s important to understand the limitations of off-the-shelf thermal software. During the design process, thermal engineers should also consult with manufacturing engineers to elicit feedback on the manufacturability of the design. This can help eliminate future problems, particularly when a cold plate or heat exchanger prototype moves into production. A concurrent engineering process provides for designs that meet thermal as well as mechanical specifications, while also remaining within budget.
1 W. M. Kays and A. L. London, Compact Heat Exchangers, 2nd Edition, McGraw-Hill Series in Mechanical Engineering. New York, 1964.
2Testing conducted by Lytron Inc. and outside testing suppliers.