I am currently working on a project where I am putting together an excel spreadsheet to assist technicians/engineers in designing heat exchangers for generator sets. I have come across an issue in calculating the thermal resistance in which the formula I am supposed to be using appears as follows:
Could someone please confirm the sign between fo and fi? Also my understanding is the dividing factor of fo and fi is the length of pipe, could someone confirm this also? I have looked everywhere and I am unable to find the complete equation.