CPD PROGRAMME | HOT WATER valves. Such scale can significantly derate the performance of hot-water generators, and preventative methods such as water softening may be required. Acidic water can promote corrosion by developing pitted surfaces. In closed circuit heating systems, corrosion inhibitors are routinely used to limit corrosion, especially if some oxygenation of the system water occurs. Some inhibitors contain anti-scaling components, but in hard water areas, the use of water softening may be considered. A balance of factors, which will be specific to the application and the local water quality, will determine the material that provides the most effective solution taking into account the system requirements during the operational lifetime of the product. The evolution of hotwater generators and revolution in hot-water stores has seen a variety of metals including copper, cast iron, aluminium, steel and stainless steel that continue to be used today. Copper, with high thermal conductivity, high ductility and malleability, was an early choice for hot-water heaters and in recent years, predominantly for indirect hot water coils and storage vessels such as hot-water storage cylinders (calorifiers). Copper will typically develop copper oxide films that will coat its surface and, although protective to corrosion from normal water use, are not true passive layers (like the much thinner layers that form on stainless steel and aluminium). General corrosion of copper only occurs in low pH water (acidic) that should never flow from mains water in the UK.2 Increasingly, applications of pressurised, unvented hotwater systems and larger hot-water stores have reduced the application of malleable copper for cylinders. Cast iron was traditionally the material used in boiler heat transfer surfaces owing to its robust nature and resistance to the impacts of highly sulphurous flue gases produced from burning coal and oil. Cast iron heat exchangers are thick and weighty; however, cast iron is not suitable for condensing applications because of the risk of corrosion from acidic flue gases that means it struggles to meet the modern environmental, spatial and operational requirements of many applications. Aluminium has been commonly used over the past 50 years for heat exchangers in hot-water generators. It is alloyed with other metals that can be cast to produce complex, high thermal conductivity waterways in a heat exchanger block that is lightweight with excellent heat transfer properties. Aluminium is often employed because of its lower cost, low mass and high efficiency, and the low density of aluminium will typically mean that heat exchangers have a Figure 1: A pipework schematic of an example system utilising a condensing hot-water heater and a condensing boiler, both with stainless steel cylinders and heat exchangers configured to independently supply heating and hot water (Based on schematic provided by ACV) AUSTENITIC STAINLESS STEEL Steel an alloy of iron and carbon when heated to a liquid molten state, and then cooled and solidified, forms crystals. The gradual formation of crystals can mean that any one type of steel is actually made up of several crystal types as the metal passes through multiple temperature stages,4 and it is not uncommon for steels to contain a mix of structures. Austenite is created in steel at 912C, at which point it transitions to a face-centred cubic crystal structure that can absorb up to 2% carbon. The nonmagnetic austenitic crystal structure is maintained at ambient temperatures through the inclusion of additives when melting steel alloy. Stainless steel is a generic name for more than 150 carbon steel alloys that have a minimum of 10.5% chromium, each alloy having its own particular properties and applications. Chromium makes stainless steel corrosion resistant by oxidising quickly, forming a thin oxide layer on the metal surface that protects the underlying metal from corrosion. With this thin passive film completely covering the metal surface, the metal is passivised and corrosion is greatly reduced. Common austenitic alloys contain 1520% chromium together with 611% nickel that gives it strength and ductility. 300 series grade stainless steels contain approximately 18% chromium and 10% nickel. Most commonly used grades of stainless steel employed in hot-water generators are 304, 316 and 316L, which have a high level of corrosion resistance, and the ability to shape and form the metal into tanks and detailed heat exchangers. L designates a lower proportion of carbon (not exceeding 0.03%) in its composition, which reduces the risk of carbon precipitation, making it a more appropriate material choice for welding. By adding 2% of molybdenum to type 304L creates type 316L, which has increased resistance to pitting and crevice corrosion in chloride-rich environments, and is typically used in heat exchangers for condensing boilers. The higher alloygrade stainless steels will be more costly. Stainless steels perform best under fully aerated or oxidising conditions to maintain their protective film, and so are ideally suited for DHW generators and thermal stores. Any conditions that cause the protective films to break down can result in corrosion rates comparable to those of mild steel. Austenitic stainless steels, at high pressure and temperature, exposed to chloride ions can suffer rapid pitting and initiate intergranular attack and stress corrosion cracking, as discussed on the web resource Corrosionpedia.5 An example source of such chlorides is inappropriately selected encasing thermal insulation that has become wet. Some of the earlier applications of stainless steel heat exchangers suffered from corrosion failures at welded points. With the appropriate selection of materials and good welding procedures this can be minimised, as discussed in a useful web article6 by Kasay Mwiks. 78 May 2022 www.cibsejournal.com CIBSE May 22 pp77-80 CPD 195.indd 78 22/04/2022 16:15