**7.2 Calculation of** *Up*

In the phosphoric acid concentration unit, the operating conditions at the limits of the heat exchanger unstable, it is necessary to disclose the heat exchange coefficients in the proper conditions *Up* corresponding to the new operating conditions. Assuming that the cleaning between operational runs is perfect and that the heat exchangers are totally free of fouling at the beginning of a new run. The initial value of the overall heat transfer coefficient at the beginning of every cycle is considered as the value of the overall heat transfer coefficient in the clean state.

$$U\_p = U(t=\mathbf{0}) = \frac{\dot{\mathbf{m}}\_{\rm ac, cir} \* \mathbf{C}p\_{\rm ac} \* (T\_{out, ac} - T\_{in, ac})}{A \* F \* \Delta T\_{ml}} \tag{13}$$
