*7.1.4 Variation in cooling water flow rate*

The speed of water flow through the tubes of the exchanger is a determining factor in the growth of biofouling. In addition to influencing the contribution of nutrients that determine the growth of organic matter that is part of the biofouling film, deposits adhered to the inner surface of the tubes can be detached as a result of the greater cutting effect on the surface are exerted to higher water flow rates. The heat exchanger can be designed to work at high flow rates (from 1 to 2 ms<sup>−</sup><sup>1</sup> ) or so that the water flow rate can be varied for a short period of time (e.g., from 1 to 3 ms<sup>−</sup><sup>1</sup> ) at regular intervals. This increase in flow velocity is only possible if the pumping capacity of the system can be increased.

Pulsed injections can also be made by introducing a gas (compressed air or nitrogen) at very high speed (3 ms<sup>−</sup><sup>1</sup> ) periodically at short intervals of time (5, 10, and 60 minutes). The effect is to increase the turbulent regime, creating pressure fluctuations inside the tube and higher shear stresses on the surface. As a consequence, a greater detachment of biofouling is obtained than that obtained by simply increasing the speed of water flow [5]. However, according to Mussalli and Tsou [79], the effectiveness of this system decreases as the biofouling film becomes more compact.
