Combating limescale

Published:  18 October, 2007

Physical water conditioners for controlling limescale have quite a patchy reputation. PWE asked Paul Spencer of Fluid Dynamics how his company is able to predict performance with confidence.

Limescale is a potentially serious problem for any business operating in a hard water area. A build up of scale on heat exchangers will reduce efficiency while encrustations in pipes will reduce flow rates. Limescale will also block nozzles in spray humidifiers and other process involving spraying water - all eventually leading to downtime while scale is removed.

Physical water conditioners (PWCs), claiming to control limescale formation without chemicals, have been on the market for many years but tend to meet with a mixed response from engineers. "Engineers tend to love them or hate them, depending on previous experience”, explains Paul Spencer, general manager of Fluid Dynamics. He adds: “It's not surprising that PWCs have developed something of a patchy reputation because with most of these products you don"t find out if they’re going to work until they’ve been installed.”

The problem, it seems, is that water can vary considerably in its chemistry from one area to another and even from one building to another. “It’s not a matter of these products being faulty because when they work, they work very well”, says Spencer. “It’s really down to the variation in the water chemistry making this a bit of a hit and miss affair with the majority of these units. The obvious thing is to test the water first, but then you have to be able to interpret the results in the context of whether the PWC will work or not.

“So it’s essential for all PWC suppliers to understand the science of how their units work and take away the 'smoke and mirrors’ element. That’s the only way to convince building operators and engineers to move away from their environmentally-damaging corrosive chemicals and water softeners and implement chemical-free solutions.”

To that end, the company commissioned a research programme at the UKAEA Harwell laboratory to determine just what was happening and how performance might vary with different water chemistries.

In hard water containing ions of calcium and carbonate there is an ongoing dynamic process. Free ions are continually combining to form calcium carbonate crystals while, at the same time, the crystals are dissolving to release free ions. Under natural conditions there is an equilibrium where the crystals are forming and dissolving at the same rate.

Spencer explains: “You only get limescale when the natural equilibrium is disturbed through changes in the water environment.” He adds: “Changes in temperature or pressure in a heating or cooling system, for example, can disturb the equilibrium so that the rate of crystal formation exceeds the rate of dissolution, resulting in precipitation of calcium carbonate crystals as limescale.

“An increase in the pH of the water will also increase the likelihood of scale formation in processes where there is no temperature change, such as spray nozzles.”

Spencer explained that there are two forms of calcium carbonate crystal – the hard calcite crystals that we know as limescale and softer, stable aragonite crystals that remain suspended in the water and are flushed through the system.

The way all PWCs work, he says, is to encourage the formation of the softer aragonite crystals that will not settle as hard limescale. Spencer points out: “As the aragonite crystals pass through the system they do sometimes settle on surfaces but because they are soft they can be wiped off with a damp cloth – they don’t require the hammer and chisel approach needed for calcite crystals.”

The fact that PWCs do not remove calcium carbonate means they do not soften the water; this is simply a change in the structure of the calcium carbonate.

H explains: “The key thing here is that these dynamic interactions between ion concentrations, temperature, pressure and pH are extremely complex and it is this complexity that has tended to make the use of most PWCs a rather hit and miss affair.

“Having manufactured non-chemical scale prevention products for more than 34 years, and having sold over 250,000 units worldwide, we already knew that placing a spiral insert made from a special alloy in the system would prevent limescale formation under certain conditions. We also knew that the insert needed to be shaped to create turbulence to achieve maximum effects.

“However, we were also very aware that anecdotal evidence wasn’t good enough if we wanted to guarantee performance to clients – and also decline to install units if we felt they wouldn’t work.”

The research at Harwell, therefore, focused on clarifying the interactions between the spinner and the water. Different types of water were tested, comparing spinners of copper (as an inert substance), zinc (as a material that would corrode fairly rapidly) and the special alloy (the exact nature of which remains confidential) used by Fluid Dynamics in its Colloid-A-Tron and Scaletron catalytic water conditioners.

Spencer highlighted: “The results showed that our special alloy was more successful in promoting the formation of soft aragonite crystals than either the control (copper) or the rapidly corroding metal (zinc). Crucially, it also showed that this was achieved by the special alloy producing an increase in pH in the water close to its surface. This is why the aragonite crystals remained in suspension, aided by the turbulence created by the special shape of the alloy insert.

“Armed with this scientific knowledge and understanding we were able to take the next logical step and develop a model that would enable us to accurately predict performance in relation to the chemistry of the water. To that end we applied the science to the water analysis and operating conditions of around 5000 of our installations and this enabled us to develop a software program that removes the guesswork from our installations.”

The Expert 5 program analyses variables such as temperature, pressure, pH and bicarbonate concentrations to predict whether a project will be successful. It also provides detailed information about cooling tower performance and is able to predict optimum cycles of concentration.

If the program predicts success the company will agree to supply the project and back it up with a guarantee. If the program predicts failure because the water conditions aren’t right, alternative solutions can be pursued and the end-user hasn’t wasted any time or money.

“It’s been great to have this level of confidence at the beginning of a project and neither we nor our agents will supply any industrial or commercial projects without that peace of mind”, Spencer enthuses. “Apart from the potential waste for the end user, we could also find ourselves out of pocket if we were to manufacture a special unit and then find it didn’t work.”

He recalls: “For example, a couple of years ago we manufactured what we believe is the world’s largest PWC – a 16 inch diameter unit for a chemical company in Argentina treating 1000m3 of cooling circuit water per hour. This was a very costly unit to manufacture so it was just as important for us to have confidence it would work as it was for the client – and the good news is it delivered the two year payback for the client that we predicted.”

Spencer concludes: “With several US states already banning water softeners, and many other states and countries actively considering a ban, we are seeing organisations throughout the world looking for alternatives with minimum environmental impact. Apart from the manufacturing process, PWCs have zero environmental impact. It seems clear to me that PWCs with predictable performance are the logical way forward.”

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