How large a heat exchanger must be to handle a given load depends on the following:
•Over-all heat transfer coefficient
•Temperature difference between the two fluids or vapor

The surface of a heat exchanger is its total heat transfer area. Load capacity will be proportioned to this area, but the other two factors can vary so widely that it is most helpful to consider the combination of all factors together.

q = U MTD A

Where:

q = load, BTU/hr.

U = design over-all heat transfer coefficient, BTU/hr./sq. ft./ºF

MTD = mean temperature difference between hot and cold fluids, degrees F

A = effective outside area of tubes, square feet

This relationship can be expressed also in words, and the equation is then stated:
The rate that heat is transferred in a shell and tube heat exchanger is the product of three factors: (1) over-all heat transfer coefficient, (2) corrected mean temperature difference between the hot and cold fluids, (3) effective outside area of tubes and other heating surfaces.

To represent a practical application situation, the equation should be transposed so that A stands alone: