Heat Pumps Today

18 By Marcus Boskey, Head of Communications – Hydratech Heat pump thermal fluid: engineering, safety, environmental and cost considerations As well as performing its primary role of temporarily absorbing captured heat and transferring it to points of usage, the heat transfer fluid (heat pump antifreeze, glycol, working fluid, brine etc) used in a heat pump system must also be a reliable antifreeze and o er e ective corrosion protection. The type of fluid selected and installed in GSHP, ASHP and geothermal heat recovery systems can have a direct and long-lasting impact on performance, e ciency, output, energy use, and down-time. In addition to the thermal and pumping characteristics of any heat transfer fluid (HTF), when making the appropriate selection for any heat recovery system it is also important to consider: ° Corrosivity and Corrosion ° Toxicity ° Biodegradability ° Biological susceptibility and durability (aka lifespan) ° Flammability ° Energy e ciency and cost Corrosivity and Corrosion Unsuitable and uninhibited HTF can cause corrosion of metal components, as mainly found in the manifolds, pump rooms and energy centres of geothermal and GSHP installations. The corroding part of a component is called the anode, which will tend to dissolve into the HTF and/or coat other metals of higher nobility. E.g., carbon steel will ‘sacrifice’ itself to copper. Oxidation Corrosion, as caused by air and/or dissolved oxygen, will tend to dissolve metals evenly across their surface, and most often a ects metals which do not form passive protective layers or are naturally resistant to oxidation corrosion. The most susceptible metals to oxidation corrosion include magnesium, zinc (galvanised pipe), cast iron, ductile iron and carbon steel. Whilst iron April | May 2023 T R A I N I N G Marcus Boskey, Head of Communications, Hydratech and steel can be protected using the appropriate corrosion inhibitors, it is virtually impossible to prevent magnesium and zinc from corroding in a submerged aqueous environment, such as a flooded pipework system. Galvanic Corrosion can take place when two di erent metals of varying nobility are ‘connected’ by a fluid, which acts as an electrolyte. E.g., HTF inside a geothermal or GSHP system. An electric potential di erence is generated between the metals and the less noble (less precious) metal acts as an anode and dissolves, while the more precious metal acts as the cathode. Pitting and Crevice Corrosion are typical in installations where metals protected by a passive coating, such as galvanised or some stainless-steel pipes. An increased local corrosion rate can occur where there is a flaw in the passive coating, when the potential di erence is concentrated. Erosion Corrosion is where metal and synthetic components are worn down over time, due to one or more of the following causes: ° High levels of entrained sediment in the HTF, acting as an abrasive inside pumps and bends. ° Cavitation is most often observed inside centrifugal pumps. E.g., worn impellers and volutes, but can also be found where there are severe changes in HTF direction, or poorly designed pipework etc.