Any building operator located in a hard water area will be all too aware that limescale in hot water systems can be a big and expensive problem, impinging on both performance and running costs. Studies in the US have shown that a 25mm thick layer of limescale on a heat exchange surface will reduce heat transfer by 95 per cent and there is a knock-on effect in terms of energy costs. Just a 1mm layer of limescale will increase energy costs by 7.5 per cent, while a 12mm layer will raise this increase to 70 per cent.
Traditional limescale control methods include the use of aggressive chemicals to dissolve the scale, which has implications for storage under COSHH regulations and disposal of toxic chemicals after treatment. Or the system can be shut down and dismantled so the scale can be removed manually, but this is expensive and disruptive.
An alternative is to use benign chemicals that prevent scale settling in the system while offering no threat to the environment. This principle has been explored since as far back as 1821, when it was found that potato starch would reduce the rate of scale formation. More recent research has shown that one of the most effective scale inhibiting chemicals is zinc, even in tiny quantities. And the higher level of zinc ions in the water, the greater the effect on scale formation - to the extent of removing existing scale deposits.
This phenomenon, which has been exploited in electrolytic scale inhibitors using a zinc anode for many years, was recently put to the test at Cranfield University.
Tests were carried out on a new style of electrolytic inhibitor that uses a zinc anode and copper cathode to create electrolytic action, releasing minute quantities of zinc into the water. The new style of unit also incorporates electronic controls that adjust the rate of release of zinc in relation to the hardness of the water.
The tests at Cranfield assessed the amount of scale formation using a temperature-controlled heating element with a removable sleeve. It was found that the scale inhibitor reduced the amount of scale on the heater sheath by 80 per cent when compared to the control.
The zinc approach has also been tested in live field trials. One such trial was carried out at Critchill Court care home in Somerset, which uses a low- temperature hot water heater to provide domestic hot water and a high-temperature heater for the kitchen and laundry.
Severe limescale problems led to a need for descaling with acid every 16 weeks and the high-temperature heater, running at 65ºC, had to be renewed at 18-20 month intervals, at a cost of around £4,000. The low-temperature heater was also experiencing progressive 'rocking up'.
Scale inhibitors were fitted after both heaters had been replaced and six months later the heaters were drained down and opened for inspection. As Clive Sowells, Area Building Services Engineer for the Council's Property Services division, explained: "In both cases the internal surfaces were 'clean' and in the low-temperature heater small pieces of scale, shaped like the inside of a pipe, were found, which seemed to indicate that the system was cleaning the pipework as well," he recalled.
Zinc ions have this effect because they cause the mineral salts in the water to clump together and remain in suspension. They influence crystallisation, encouraging the formation of soft aragonite crystals rather than the harder calcite crystals associated with stubborn limescale. The result is that any calcium crystals flushed through the system can be wiped off with a damp cloth. Also, the change to the water is permanent so this approach is suitable for stored water.
With virtually no maintenance requirements beyond occasional visual checks, scale inhibitors using a zinc anode offer a low cost of ownership solution that meets the need to reduce energy consumption and minimise environmental impact.
Please visit: www.watermatic.co.uk
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