HEAT NETWORKS | WATER TREATMENT The latest Code of Practice on Heat Networks includes an alternative to chemical dosing to keep systems free of corrosion. Elysator UKs David Whitfield explains how demineralised water and electrochemical water treatment can be used CHEMICAL-FREE CORROSION CONTROL REMOVING GAS BUBBLES To remove micro gas bubbles, a passive de-aerator may be incorporated in the system. Gases in a closed-loop system dissolve in colder water, but are later released as tiny bubbles when the water heats up, or its pressure decreases. These micro-bubbles can coalesce to form a bubble large enough to rise to high points in the system. In some locations where the gas collects, it can be removed by an automatic air vent. However, the microbubbles can be transported to other locations where they collect, such as at the tops of radiators, where they can hinder heat transfer. By locating a de-aerator unit immediately after the boiler, where the micro-bubbles will be present in the heated water, the unit is able to passively de-gas the water to help prevent the formation of air pockets. When the system cools, the de-gassed water will act like a sponge to soak up air in contact with the de-gas the system. On a cooling system, the unit is best installed in the return leg, where the temperature is highest, so that the degassing is most effective. The units are typically installed in a sidethrough the device on a continuous basis. But the units are sized so the total system volume should pass through the unit in a 24-hour period. Several appliances can be combined for larger systems. O ne of the most significant changes in CIBSEs revised Code of Practice on Heat Networks is the inclusion of an alternative method of ensuring closed-loop heating systems are free from corrosion. The Code of Practice CP1 (2020) references a German standard that sets out how corrosion can be prevented without the use of chemical inhibitors or biocides (where the flow temperature is not intended to exceed 100C). VDI 2035 Part 1: Prevention of damage in water-heating installations is published by the Association of German Engineers, and a large number of HVAC manufacturers state that their systems have been filled to the standard in their manuals and/or warranty documents. The standard is based on the use of demineralised water and, in part, electrochemical water treatment. These include the use of ion-exchange resins and magnesium anode technology to maintain the water in a closed-loop system in a noncorrosive state. Corrosion occurs in heating and cooling systems when they are filled with untreated tap water. This contains minerals, salts and gases that can react with a systems metal components. The reaction can lead to the formation of sludge deposits, limescale and rust. The more water a system contains, the greater the quantity of minerals and salts, and the greater the potential for sludge. Corrosion reactions in hydronic systems are primarily determined by the presence of oxygen, which is why it is important that the oxygen concentration should be as low as possible. Oxygen enters the system during filling; one litre of mains supplied water contains around 8-11mg of dissolved oxygen. Generally, in a sealed system, the dissolved oxygen from the initial fill will dissipate through limited corrosion with the metal pipework and components over a short period of time, usually without significant damage. Systems with closed diaphragm expansion vessels are recommended because they help limit entry of air. If a system is not sealed effectively, or requires frequent top-ups or if oxygen can diffuse through permeable seals and pipes then corrosion will continue to be a problem for mild steel elements, such as pipework, where it will form iron oxide (FE3O4), which can manifest as sludge. To help limit corrosion, sludge and scale, the conventional approach in the UK has been to dose systems with chemicals. Some corrosion inhibitors raise the conductivity of system water and can, in the event of over-dosing or under-dosing, lead to additional corrosion. Corrosion inhibitors can also result in the formation of biofilms, which can harbour bacterial corrosion. There are about 12 bacteria known to cause corrosion of carbon steels, stainless steels, aluminium alloys and copper alloys in waters and soils with pH4~9 and temperature 10C~50C. It is well documented that some chemicals provide a source of nourishment for bacteria; ironically, the solution to this is often to add yet more 70 October 2021 www.cibsejournal.com CIBSE Oct21 pp70-71 Water treatment.indd 70 24/09/2021 15:52