The cell effectiveness E_c required for evaluating the heat exchanger effectiveness, may be expressed by (see Section 97)

\[\label{eq1} E_{c}=f(\mbox{NTU}_c, R,\,\mbox{cell flow configuration}) \tag{1}\]

where

\[\label{eq2} \mbox{NTU}_c = \dfrac{UA_c }{\dot{C}_1 }\;(\mbox{number of transfer units of a cell}) \tag{2}\]

Apart from a few tube rows in the baffle cut near the shell, the shell-side flow is more or less across the tube bundle. The numerical value of the cell effectiveness lies between those for cross flow with both fluids unmixed and for cross flow with the shell-side fluid laterally mixed. The tube-side fluid is virtually always laterally unmixed. Comparing the numerical values of cell effectiveness for the two limiting cases shows that both values in the range NTU_c ≤ 1 are practically equal (Gaddis, 1978). Since the number of transfer units of a cell is seldom higher than unity, either of the two relationships may be used. A number of the available equations are reviewed in Section 100 and Section 107.