Study of the performance of physical water treatment with a solenoid coil to prevent mineral fouling. Part 2: Effect of air bubbles

The purpose of the present study was to investigate the effect of air bubbles on mineral fouling in a heat exchanger with a cooling-tower water. Two different tests were conducted in a concentric-tube heat exchanger at 12 cycles of concentration: no-treatment, and a physical water treatment (PWT). A small flow velocity of 0.83 m/s and a high heat flux of 22.5x104 W/m2 were chosen to accelerate the fouling in the heat exchanger. Air bubbles from the cooling water adhered to the heat-transfer surface made of a smooth copper tube, significantly affecting the overall fouling process. The fouling started from the heat-transfer surface covered by the air bubbles and then spread around the area not covered by the bubbles. The fouling resistance in the case with PWT was approximately 27% less than that in the no-treatment case, indicating that the PWT could be useful in mitigating mineral fouling even in the case of a low flow velocity where air bubbles adversely affect the fouling process.