A. INDIVIDUAL AND OVERALL HEAT TRANSFER COEFFICIENTS

Equation 89.3 is repeated here, by way of a reminder,

\[\label{eq122_1} \dot {q} = U\left( {T_1 - T_2 } \right) \tag{1}\]

where q̇ is the heat flux (rate) from stream 1 to stream 2, having cross-sectional average temperatures T₁ and T₂, per unit area of interface, and U is the overall heat transfer coefficient.

It is now time to raise and answer the questions of what precisely is meant by the interface area, and what is the relation of U to the individual resistances to heat transfer represented by the solid wall (if it exists) separating the two fluids, the deposits of dirt that may cling to the surfaces of the wall, and the low-velocity fluid “films” that adjoin those deposits or, if they are absent, the wall itself.