CPD PROGRAMME | HEAT EXCHANGERS the brazing material, which has good resistance to corrosion in the majority of likely water qualities. If the water has a low pH (more acidic), the copper can start to corrode or dissolve in the water. Copper is also very sensitive to ammonia and sulphur. Ammonia has been used in some district energy systems to regulate the pH, so if copper is used as the brazing material, it is recommended that the ammonia level be kept very low. (A normally preferred alternative chemical to regulate pH is, for example, sodium hydroxide.) BPHEs are available that employ a nickel alloy as the brazing material (instead of copper) resistant to both high sulphur and ammonia contents. Both copper and stainless steel are susceptible to corrosion in strongly acidic solutions, but copper is the more vulnerable metal in an alkaline environment. Copper may also be compromised by the presence of dissolved salts, but maintaining electrical conductivity of the water within the recommended range will minimise this source of corrosion. Conductivity measures the ability of a solution to carry an electric current because of the presence of certain ions, and is usually expressed in micro siemens per centimetre (S.cm-1). Conductivity is also affected by temperature the warmer the water, the higher the conductivity and so is normally reported as conductivity at 25C. The concentration of total dissolved solids in milligrams per litre (mg.L-1) can also be calculated by multiplying the conductivity result by a factor. Oxygen content Figure 1: Example of heat interface unit (Source: Altecnic) stainless steel nor copper corrodes easily. However, high chloride levels in an oxidising environment can initiate the corrosion of stainless steel, particularly at higher water temperatures, as the chlorides form a galvanic cell with oxygen and the metals of the BPHE. Chlorides are widely distributed in nature as salts, such as sodium chloride (NaCl), potassium chloride (KCl) and calcium chloride (CaCl2). The most common form of resulting damage is pitting corrosion, with the chloride attacking areas of the steel where the passive layer is damaged (the passive layer is the protective surface film that is formed when stainless steel is exposed to air). Pitting corrosion is hard to detect and might only be identified when a unit has started to leak. Another common and very similar type of corrosion for stainless steel is crevice corrosion, which can occur at, or immediately adjacent to, a gap or crevice between two joining surfaces. The gap or crevice can be formed between two metals or a metal and non-metallic material, and may occur in welds that fail to penetrate, in flange joints and under deposits on the steel surface where the passive layer has failed to form. Outside the gap or without the gap, both metals may be resistant to corrosion.5 Higher temperatures make chlorides more aggressive towards stainless steel. The grade of stainless steel used in the systems should account for the temperature and chloride concentrations manufacturers will be able to provide advice. The majority of BPHEs use copper as The presence of oxygen increases the risk of corrosion, so the oxygen content should be kept as low as possible. When adding make-up water, it is important to ensure that either the water has been de-oxygenated or that appropriate additives have been used. The typical recommendations are that the water circulating in the network should have an oxygen content less than 0.02mg.L-1, be a weakly alkaline environment with a pH preferably between 7.5 and 9, and should contain the lowest possible presence of ammonia and sulphide. Fouling is the term used to describe the tendency of a fluid to form a film or scale on the heat transfer surface, and is an undesirable phenomenon. Inorganic materials may crystallise as salts, resulting in scaling. Organic deposits include biofilms or microbial organisms. If inorganic or organic material starts to build up inside the BPHE, it will result in lower heat transfer and a higher pressure drop. Scaling is a type of fouling caused by inorganic salts in the water passing through the BPHE, which may precipitate and form a scale on the heat transfer surface. Most scaling is the result of either calcium carbonate (CaCO3) or calcium sulphate (CaSO4) precipitation. It increases the pressure drop and insulates the heat transfer surface, thus preventing efficient heat transfer. Scaling is more likely to occur when the fluid velocity is low (laminar flow) and the liquid is distributed unevenly through the passages on the heat transfer surface. COATING STAINLESS STEEL FOR ENDURING PERFORMANCE Ceramic SiO2-based surface coatings are being employed to improve corrosion resistance on stainless steel plates in BPHEs to enhance the mechanical and thermal stability. These organic coatings improve the surface behaviour in relation to scaling and fouling. The self-cleaning nature of the coating reduces deposit formation. The protective layer is applied to all inner surfaces of the heat exchanger that come into contact with untreated water. Examples of coated stainless steel plates (Source: Swep) 46 January 2020 www.cibsejournal.com CIBSE Jan20 pp45-48 CPD v2.indd 46 20/12/2019 15:30