The over-all heat transfer coefficient is a measure of
performance. It evaluates the ability of the tube in a given
mechanical arrangement to transmit heat from one fluid to another.

A clearer concept of heat transfer can be gained by
considering the reciprocal of heat transfer coefficient, which we may
term heat flow resistance. The advantage of this mathematical
inversion lies in the fact that over-all resistance is the simple sum
of five individual resistances. These are the resistance to the flow
of heat through the tube side fluid, tube side scale, tube metal,
shell side scale and shell side fluid.

Usually the smallest individual resistance to heat
transfer is that of the metal tube wall itself.

How the various heat flow resistances affect U, the
heat transfer coefficient, can be shown in the equation:

Where:

=
resistance to flow of heat through the tube side fluid film

=
resistance to flow of heat through scale deposits inside the tube
-Fouling resistance

=
resistance to flow of heat through metal tube wall

=
resistance to flow of heat through scale deposits outside the tube
-fouling resistance