Scale control in heat exchangers

Whenever natural water is used in a heat-exchanger, scale deposits tend to build up on heat transfer surfaces. This results mainly from the chemistry associated with the hardness and alkalinity of natural water. To ameliorate the problems created by scale deposition, a wide range of methods and processes are available. This thesis investigates the efficiency of one of these methods, magnetic treatment. Two pilots plants were used to measure the effect of a magnetic field created by a commercial non-intrusive clamp-on device on scale formation. The first pilot plant consisted of four domestic boilers, two treated and two controls, filled and operated four times daily with mains water dosed to specific hardness and alkalinity levels. This rig operated in single-pass mode, with the water contained in the boilers being flushed out after each heating cycle. The second rig was based on two loops, treated and control, filled with deionised water to which reagents were added to control hardness and alkalinity levels. Each loop consisted of a 75 litre tank feeding a scaling cell via a centrifugal pump. The scaling cells included a copper element, exposed on one side to a regulated flow of heating water (set temperature) and on the other side to the flow of scaling water. The boiler rig results showed no significant influence of magnetic treatment on the scaling of the boilers’ heating elements. On the recirculating rig, no significant effect of the magnetic devices on the amount of scale formed was observed. However, magnetic treatment appeared to have reduced by 50 % the thickness of the hard scale layer on the copper elements. The reproducibility of the experiments was significantly improved by precoating the copper element with a scale layer to overcome discrepancies introduced by the induction process.