How can the overall heat transfer coefficient of a heat exchanger be calculated, assuming no fouling?

The overall heat transfer coefficient (U) for a heat exchanger, assuming no fouling, is accurately determined using the relationship involving the individual heat transfer coefficients of the fluids involved. The correct formulation is based on the concept that the total resistance to heat transfer is the sum of the resistances from each side of the heat exchanger.

In this case, the heat transfer from one fluid to the other occurs through the heat exchanger wall, and heat transfer coefficients are defined for both the inside and outside fluids. The overall heat transfer coefficient is defined in terms of the thermal resistances associated with these coefficients. As heat transfer can be viewed as a series of thermal resistances:

The relationship given by 1/U = (1/h_inside fluid) + (1/h_outside fluid) means that the overall heat transfer coefficient U is inversely related to the sum of the individual resistances to heat transfer from both fluids. This indicates that each fluid’s contribution to the heat transfer process is significant, and both resistances must be accounted for.

In this context, if you sum the inverses of the individual heat transfer coefficients, you effectively capture the total thermal resistance encountered by the heat being transferred, allowing for a comprehensive understanding of the heat exchanger's performance under ideal conditions