The rate equation for heat transfer is:
Q = U × A × MTD
Q = Heat duty, Btu/hr
U = Overall heat transfer coefficient, Btu/(hr × °F × ft2)
A = Heat transfer area, ft2
MTD = Mean temperature difference, °F
The area, A, and the reference area in the overall coefficient, U, must be the same. Outside bare tube surface area is the usual reference area for tubular equipment. Estimation methods for mean temperature differences and overall heat transfer coefficients are discussed in the following sections. Flow in tubes and flow across bare tube bundles for single-phase and some multiphase cases are covered.