The lack of confidence isn’t a result of these products being faulty – far from it. When they work, they work very well. The trouble is, those results have tended to be unpredictable because water conditions vary so much from one building to another. It’s not surprising, therefore, that PWCs have developed something of a patchy reputation with many engineers. Some think they are the best thing since sliced bread while others wouldn’t touch them with a barge pole.

In fact, all of this uncertainty is totally unnecessary if you understand how PWCs interact with the water they are treating and take the trouble to analyse the water before proceeding with the project. Unfortunately, many suppliers of PWCs don’t fully understand how their units work and analysis of the water before installation of PWCs is surprisingly rare.

So, in an ideal situation, the supplier of the PWC will analyse the water and predict whether their unit will deliver the required results – basing that decision on solid science rather than hoping for the best. To that end, it makes sense to understand something about the chemical processes that contribute to limescale formation.

A scaly problem

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.

Disruption of that equilibrium occurs when there are changes in the environment, such as an increase in temperature or pressure in a heating or cooling system. Disturbing the equilibrium in this way can lead to the rate of crystal formation exceeding the rate of dissolution, resulting in precipitation of calcium carbonate crystals as limescale.

A change in the pH of the water can have a similar effect, with an increase in alkalinity increasing the likelihood of scale formation in areas where there is no temperature change such as spray nozzles.

At this point it’s important to note 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 that PWCs work is to encourage the formation of the softer aragonite crystals so they pass through the system and do not leave hard limescale behind. In this respect they do not soften the water because they do not remove calcium carbonate; they simply change its structure.

However, as indicated here, the 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.

Getting it right

PWCs can be broadly divided into magnetic, electronic and catalytic conditioners and it is widely accepted that catalytic conditioners offer the most consistent results. However, in order to be able to predict whether a catalytic conditioner will work before it’s installed it is vital to understand the interactions between the conditioner and the water.

To that end, a research project was commissioned at the UKAEA Harwell laboratory to increase the understanding of these interactions and to use that understanding to develop a model that would enable accurate predictions of performance.

Experience had already shown that placing a ‘spinner’ made from a special alloy in the system would prevent limescale formation under certain conditions. However, this experience was just the type of anecdotal evidence that has created the smoke and mirrors image of PWCs. The research, 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) developed by our company for use in catalytic water conditioners.

The results showed that the special alloy was more successful in promoting the formation of soft aragonite crystals than either the control (copper) or the rapidly corroding metal (zinc).

Furthermore, it was shown that this was achieved by the special alloy producing an increase in pH in the water close to its surface, so that the aragonite crystals remained in suspension, aided by the turbulence created by the special shape of the spinner. As a result, the calcium carbonate in the water is stabilised and flushed through the system. The fact that it is a soft aragonite crystal means that any deposits can be removed with a damp cloth.

However, understanding what was going on was just the first step in developing a model that would enable us to accurately predict performance in relation to the chemistry of the water. The next step was to use this scientific knowledge of the water chemistry, combine it with the results of some 5,000 installations from the 250,000 units we have installed worldwide, and develop a software program that removes the guesswork from such installations.

The program analyses variables such as temperature, pressure, pH and bicarbonate concentrations to predict whether a project will be successful. If it predicts success we can have the confidence not just to supply a unit but to back it up with a guarantee. If it 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.

Having confidence in such a specification can make all the difference to arriving at the best solution for the end client. It’s important in any installation but especially so when the PWC is going to represent a significant investment.

This was the case for a chemical company in Argentina that was experiencing severe limescale build up in a chilled water circuit with a flow rate of 1700m3/h through 16 inch pipework. The total cooling water volume of the circuit is 400m3/h and requires a daily make up of 25m3. As a result, the circuit is continually topped up with mains water, so the calcium salts in the circuit are being constantly replenished.

A PWC offered significant savings compared to the heavy chemical dosing regime and frequent downtime for manual descaling – with a payback of less than two years. However, it also involved significant investment in manufacturing what is believed to be the world’s largest PWC and neither party would have been prepared to proceed without a high level of confidence.

Analysis of the water predicted that the PWC would indeed resolve the limescale issues and, two years on, the company no longer uses descaling chemicals in this cooling circuit and there is no downtime for manual descaling.

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