This section is in general only intended to give an overview of the heat transfer processes; more detailed information may be found in Section 31 for single phase convective heat transfer, Section 32 for condensation and Section 33 for evaporation and boiling heat transfer.

The overall heat transfer coefficient (U) is the reciprocal of the sum of the five thermal resistances through which the heat must pass; (1) the boiling liquid, (2) the fouling deposited by the boiling liquid, (3) the wall, (4) the fouling deposited by the heating fluid, and (5) the heating fluid. Each resistance may be expressed as the reciprocal of the individual coefficient (α), corrected where necessary to refer to the same area, usually the outside.

\[\label{eq1} \frac{1}{U_{o}}=\frac{1}{\alpha_{c}}\frac{A_{o}}{A_{c}}+r_{f,c} \frac{A_{o}}{A_{c}}+r_{w}\frac{A_{o}}{A_{w}}+r_{f,h}\frac{A_{o} }{A_{h}}+\frac{1}{\alpha_{h}}\frac{A_{o}}{A_{h}} \tag{1}\]

Here, A is the surface area, A_f is the fouling resistance and α is the film heat transfer coefficient. The subscripts c, h, o and w refer to cold fluid side, hot fluid side, outside surface and mean wall respectively. Note that if boiling occurs on the outside surface, A_o = A_c whereas if boiling is inside the tubes, A_o = A_h. The estimation of these individual coefficients or resistances is dealt with below. A_w is given by:

\[\label{eq2} A_{w} =\frac{A_{o}-A_{I}}{\ln(A_{o} /A_{I})} \tag{2}\]