Regarding the Heat Exchange Coefficient, the formula used is the following:

A = 1/(Partial Exchange Coefficient Air Side * Finned Surface Efficiency)

B = (External/Internal Surface Ratio) / (Partial Exchange Coefficient Fluid Side)

K = 1 / (A+B)

Regarding the partial exchange coefficient tubes side and fins side, the formulas are property of Unilab SRL and cannot be shared because they are part of our know how. In the next versions of Coils , it will be available a functionality that will let you choose the most appropriate formula from a list.

For a detailed reference of the calculation of the global and partial heat transfer coefficients, please take a look to the Hewitt, Shires & Bott book “Process Heat Transfer”.