How to calculate Coils with variable fin pitch?

Sometimes, for many reasons, you need to build coils which have parts with different fin pitches. This technical solution can be adopted, for example, when you need to ensure a perfect airflow distribution trough the coil in particular applications where the coils shape or disposition can hinder it: in many projects in fact it’s not possible to design simple linear coils because of space needs. A bad airflow distribution influences the local heat exchange coefficient obtained in some coils zones and therefore it doesn’t permit the optimal design of the coil. Variable fin pitch is also used in specific cooling applications where you want to avoid or delay as well as it possible the frost formation on the fins surface.

In Unilab Coils it’s not possible to insert more than one fin pitch in the same project, but you can calculate anyway a coil with variable fin pitch, simply considering a “medium” fin pitch resulting from a weighted mean of the real fin pitches. For example, you can consider a coil with length = 1000 mm, where there are three parts: the head with length = 200 mm and fin pitch = 1,5 mm, the central part with length = 500 mm and fin pitch = 2,5 mm, and the tail with length = 300 mm and fin pitch = 1 mm.

In this case the fin pitch to input in Coils for the calculation will be: