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Cranfield assessed the amount of scale formation using a temperature controlled heating element with a removable sleeve. |
The ability of zinc to prevent limescale formation in water systems has been known for many years and confirmed in both laboratory tests and field trials. Sue Clews, Sales Director of WaterMatic, describes how recent control enhancements have fine-tuned this process to provide predictable control in a wide range of applications.
For as long as people have been heating hard water, limescale has been a problem. In some cases, such as forming a coating of scale on cooking implements, it is a nuisance. But in appliances, heating systems and industrial processes that use hot water or steam, limescale can be a very costly burden indeed. In fact, the cost of limescale to UK industry has been estimated at over £1bn per annum.
It’s hardly surprising, then, that human ingenuity has been applied to this problem for many years – in fact the use of chemical inhibitors dates back to as early as 1821 when it was found that potato starch would reduce scale formation. Since then many other chemicals have been found to inhibit scale and one of the most effective 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.
The most effective way of dosing the water with zinc is through electrolysis, using a zinc anode and copper cathode in the water. Because of their respective positions in the Electrochemical Series (zinc = -850mV, copper = +80mV) there is a high electrolytic potential between these two metals, which causes zinc ions to be released into the water in minute quantities.
A recent breakthrough in the electronic control of electrolysis, investigated and proven at Cranfield University, now allows the release of zinc to be fine-tuned to the level of hardness of the water for more precise treatment. The testing was carried out on Watermatic’s new Kal-Guard+ unit, the design of which is the subject of a patent application.
Cranfield assessed the amount of scale formation using a temperature controlled heating element with a removable sleeve and looked at the scale on the sleeve, in the bulk of the water and on the surfaces of the tank. “In the tests undertaken here it was found that the Kal-Guard+ unit reduced the amount of scale on the heater sheath by 80 percent when compared to the control,” says the University’s report. “The Kal-Guard+ also reduced the level of scale formed in the bulk and when comparing the total amount of scale formed (combined heater + bulk) the Kal-Guard+ reduced scale formation by 74 percent,” it continues.
The results of these laboratory trials have been confirmed by field trials in a range of commercial and industrial applications, helping to reduce energy, maintenance and capital expenditure while also providing a fast payback on the investment.
At Imperial Tobacco’s Nottingham plant, for example, installation of a 35mm Kal-Guard+ unit enabled the company to maintain effective quality control, cut downtime and reduce maintenance costs, with a payback of less than four months. The company was experiencing problems with its re-hydrating unit, used to raise the humidity of imported, dried tobacco from five percent to 17-18 percent. This is achieved by spraying the tobacco with a mist of water to ensure even absorption of moisture across the leaves.
Before installing the scale inhibitor on the water supply, the company needed to spend around three hours a week to remove scale from the spray nozzles manually, at a cost of around £30 per hour. This also resulted in downtime of the plant, leading to further costs.
The gradual scaling up of the nozzles also changed the spray pattern of the nozzles, so regular de-scaling was essential to maintain quality control.
“Since installing the Kal-Guard+ unit there is now only an occasional slight residue of soft limescale that is easily wiped away during the routine cleaning of the machine,” explained Imperial Tobacco’s Alan Whitelock. “We are delighted with the results, which have resulted in significant cost savings,” he added.
Similar benefits were achieved by controlling scale problems in water heaters at a care home in Frome, Somerset. Critchill Court is part of Somerset Care, a group of individual care homes in the county. It provides sheltered accommodation for the elderly as well as safe care for more vulnerable residents and offers a wide range of facilities. The building uses a low temperature hot water heater to provide domestic hot water and a high temperature heater for the kitchen and laundry. Planned Maintenance Engineering, which is responsible for maintaining the systems, reported severe limescale problems, despite conventional descaling every 16 weeks. As a result, the high temperature heater, running at 65°C, had to be renewed at 18-20 month intervals because of scale build up, at a cost of around £4000. The low temperature heater was also experiencing progressive ‘rocking up’.
Local water treatment specialist Environment Plus recommended the installation of a Kal-Guard+ scale inhibitor to provide ongoing treatment of the water supply to both heaters. Following extensive discussions with all parties the two heaters were renewed and a Kal-Guard+ unit was installed. Six months later, both 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.
The end result is that the water heaters now have a much longer projected life and the cost of descaling every 16 weeks has been eliminated, providing a payback of less than a year on the investment in scale control. In addition, there is no need to dispose of the used acids from the descaling process, so environmental impact is greatly reduced.
Zinc and scale
The tests carried out at the Cranfield Institute also confirmed that it was the zinc from the electrolysis that was preventing limescale formation. Zinc ions cause the mineral salts in the water to clump together and remain in suspension, rather than settling onto the surfaces of the system. The zinc also influences the way the calcium carbonate in the water crystallises, encouraging 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 with the water can be removed from surfaces with a damp cloth.
Another benefit is that this is a permanent change to the water chemistry, providing long term protection, even for stored water, and is effective at all flow rates. Even at maximum dosage, zinc levels will not influence water taste and can be considered to be beneficial to health as the human body needs this mineral to function.
Zinc and corrosion
Interestingly, introduction of zinc ions to water through electrolysis also inhibits corrosion, because zinc reacts with oxygen in the water to form zinc oxide. This prevents oxidation of the metal surfaces in the system, and the fine film of zinc oxide that forms on them provides further protection of these surfaces. This requires the use of dedicated electrolytic corrosion inhibitors, such as Korrosion-Guard, as the rate of release is different to that for limescale inhibition.
Once all the oxygen has been removed from the system, the inhibitor unit goes into ‘standby mode’. If there is any further ingress of oxygen into the system, however, this automatically reactivates the zinc anode so the oxygen in quickly removed. Addition of zinc also acts to prevent electrolytic corrosion occurring between dissimilar metals in the heating system such as copper, brass, aluminium alloy and steel. As noted earlier, zinc is lower in the electro-chemical series than any of these other metals, so any electrolytic corrosion currents are concentrated on the zinc anode, rather than the metal elements of the heating system. In engineering terms, this is known as ‘cathodic protection’.
At a time when there is increasing pressure to reduce environmental impact and minimise total life costs, these solutions offer clear benefits. Hot water systems last longer and remain efficient, energy consumption and maintenance requirements are minimal and there is no need for toxic or corrosive chemicals.
