Theoretical techniques alone are usually inadequate in designing new heat exchangers. An accurate evaluation of each new heat exchanger core configuration requires that experimental data be obtained on heat transfer coefficient and flow friction pressure drop vs flow rate for both sides of the exchaiJ.ger. For the air or finned side this has best been measured in laboratory heat exchanger test stands. As a rule, convenience and economy dictate that test stands handle only small heat exchanger samples; thus it is quite common for the air side sample to have a face area of approximately 8 x 10 in. and an air flow length of -1 to 4 in. While the existing, machine-built production of heat excchangers allows that the required core sample for performance testing be obtained by cutting one down, a new heat exchanger design requires that speciai samples be handbuilt. Unfortunately, the latter is a very expensive, time consuming and complicated task for which tube and plate fin heat exchanger samples with heat transfer characteristics representative of those of machine-built production heat exchangers must be hand-built in a model shop. As a resUlt, budgetary and time constraints usually restrict the number of .heat exchanger configurations that can be built and tested in typical hardware developmeht projects, and the fewer the number of different heat exchanger samples used, the less likely the development of an optimum design.