Thin Ceramic Coatings and Their Suitability Towards Scale Reduction in Heat Exchangers

O. Santos, M. Nilsson, A.H. Jensen, A.B. Christiensen,
Alfa Laval AB,
Sweden

Keywords: coatings, scaling, heat exchanger, stainless steel

Summary:

Scaling of heat exchangers in the chemical industry, district heating, power plants, etc results in a loss of performance which leads to production stops for cleaning and therefore increased operation costs. One approach to mitigate fouling is to alter the properties of the metal surface. A coating to be successful in any heat exchanger application needs to have a good adhesion to the metal but also keep the heat transfer trough the metal plates as unaltered as possible. This requires a thin coating with a relatively good thermal conductivity. SiOx has developed a ceramic coating that meets these requirements. Initial tests have been performed where the coating was tested in hard water. The ability of the coating to minimize calcium carbonate nucleation / deposition and the easiness of cleaning was assessed. The test set up consisted of two M3 plate heat exchangers (PHE) with 8 plates in stainless steel 316L (16 * 42 cm). One of these PHE was installed with coated plates and the other with uncoated plates. The units were run for 6 days with hard tap water. The tap water entered the heat exchanger at a flow rate of around 2 L / min and was heated up to 87°C by circulating water at 95°C on the adjacent channel. The PHE is running intermittently for 15 min and then 15 min without flow. Since copper brazed units are the norm in district heating applications, the coating was also applied and tested in these types of heat exchangers. The results showed that the coating led to a reduction in the amount of scaling and a lower adhesion of the crystals to the coated plate.