Physical changes in electrolytic scale control

By Charles Nicolson
This is the third and final article looking at how the relatively new application of electrolysis is being increasingly used for water treatment, particularly for scale control in evaporative cooling water circuits as well as other circuits in which evaporation takes place, such as adiabatic humidification.

Previous articles have described anti-scaling in evaporative water circuits in terms of chemical reactions and processes. However, as anti-scaling treatment moves away from traditional chemical dosing, it becomes easier to understand when presented in more physical terms, although chemists still continue to refer to this as physical chemistry.

Effective prevention of the build-up of scale deposits in water circuits has always been a process carried out over two or sometimes three stages. The first stage in one traditional method, sometimes referred to as ‘primary scaling prevention’, is to stop potential scale-forming substances (mostly dissolved calcium and magnesium carbonates) from coming out of solution when circulating water comes into contact with hotter surfaces such as condenser tubes. This is accomplished by dosing chemicals designed to raise solubility thresholds so that dissolved carbonates can be maintained in solution at higher than normal maximum solubility levels. Another widely adopted method is by softening supply water, which has always been a reliable way of ensuring that the nature of circulating water, controlled at predetermined cycles of concentration, is neither potentially scale-forming nor corrosive from a practical point of view. Commonly used ion-exchange water softening is a mature process that alters the nature of supply feedwater but does not materially alter amounts of total dissolved solids and therefore has no significant effect on the electrical conductivity of the water.

Without water softening, maintaining carbonates in solution at supersaturated levels by using threshold chemicals is not a thermodynamically stable state.

Until about five years ago, the costs of installing automatic softener plants compared to far less costly chemical dosing systems could be justified by avoiding the need to use speciality scale control chemicals. The cost difference was generally amortised over periods between three and five years. However, softener costs have increased more rapidly than prices of chemicals and will probably continue to do so until at least 2025. The resulting increase in amortisation periods has therefore made softening supply water a less attractive option. In addition, although bleed-off from circuits supplied with softened water is minimal and can usually be disregarded as an environmental contaminant, drainage during frequent regular regeneration periods of softener ion-exchange resins contain high levels of sodium chloride or common salt, now regarded as contributing far more to pollution.
Without water softening, maintaining carbonates in solution at supersaturated levels by using threshold chemicals is not a thermodynamically stable state. The total amount of supersaturated carbonates must be kept relatively low, which means that as amounts of dissolved carbonates rise according to increasing circuit cycles of concentration, proportionately increased precipitation forming carbonate crystals suspended in circulating water will occur. Controlling the total number of crystals depends on accurately controlling amounts and frequencies of bleed-off in relation to varying loadings on evaporative cooling water circuits. Also, the ‘dwell time’ of supply water entering circulation until leaving the circuit by evaporation or bleed-off should be kept relatively short to limit the size to which crystals grow by combination and agglomeration. Achieving short dwell times, however, may not be possible in circuits having high total volume/evaporation rate ratios.

The second stage of scale control when using this method is to further limit the total number of carbonate crystals by physically removing some of them from the water circuit. Theoretically, this can be done by filtration as is used in some small volume specialised evaporative water circuits mostly found in modern processes involved in electronic and medico-pharmaceutical manufacturing and assembly. However, regular or continuous filtration of sub-micron-sized crystal particles is far too costly and maintenance-intensive for use on evaporative circuits found in HVAC and the normal sizes of commercial and industrial evaporative cooling installations. Removing enough crystals to maintain a set maximum number in circulation therefore depends on bleed-off which, in turn, directly increases supply water usage, thereby counteracting one of the basic benefits of evaporative cooling.

Chemical scale control in cooling water circuits is almost always a trade-off involving several technical parameters. Other factors such as corrosion and bio-fouling also have to be taken into consideration, which can impact on minimum scale control chemical dosing needed through dosing of inhibitors and biocides with opposite electrical charges. This complication can be reduced by optimum selection of dosing points and methods but never completely eliminated. Reduced efficiency of anti-scaling chemicals is often the reason why scale deposits begin, slowly at first but speeding up as temperatures and refrigerant pressures rise in response to reductions in heat transfer across heat exchange surfaces. When scaling starts to affect cooling,
particularly when condensing temperatures increase and sub-cooling decreases, total plant efficiency reduces and costs rise through longer periods of maximum loading and higher discharge pressures requiring increased power demand by refrigerant compressor drive motors. Increasing bleed-off substantially will often slow down the overall rate of efficiency reduction, but only temporarily, because the amount of supply water used increases sharply, which also results in a proportional increase in automatically dosed chemicals.

Previous articles have explained in chemical terms how electrolysis can replace chemical scale control in cooling water circuits. Looking at scale control stages, electrolysis accomplishes both stage one and stage two of scale control almost simultaneously by elevating the pH of the water close to and in contact with the surface of the cathode up to a pH of 11.0 or even higher. Dissolved carbonates migrating in ionic form under electrolysis to the cathode area cannot remain in solution at this high pH; therefore, they precipitate out of solution and immediately deposit onto the cathode surface. In physical terms, electrolysis takes dissolved scale-forming salts out of circulating water and deposits them onto cathode surfaces. This has the effect of reducing dissolved concentrations of scale-forming carbonates throughout the whole volume of the water circuit and provided that the amounts taken out by electrolysis are at least equal to those coming in with supply water, the tendency will be that the carbonates will remain in solution in circulating water rather than precipitate and deposit onto hotter surfaces, particularly heat exchange plates or tubes.

Furthermore, the electrolysis process theoretically eliminates the need for carbonates, either in solution or crystalline forms, to be removed from water circuits by bleed-off. However, in practice, some bleed-off will be necessary to limit concentrations of other substances dissolved in feedwater supply, such as chlorides, but far less than minimum bleed-off amounts needed when chemical scale control is used. This bleed-off reduction can lead to substantial water savings as well as minimising environmental impact if supply water contains any other potential polluting substances. From the comparison between traditional chemical and the more recently adopted electrolysis method for controlling scaling, the question arises as to why electrolysis did not come into general use earlier than about nine years ago?


A typical single tube and rod electrolysis unit designed for side-stream installation on a water circuit.

Getting Technical has previously commented that during the 30-year period from 1980 until 2010, many laboratory scale experiments were conducted to determine and measure what happened when various strengths of hard water solutions were subjected to electrolysis. Published results universally agreed that dissolved hardness salts, particularly by far the most common one, calcium carbonate, migrated in ionic form to cathode electrode surfaces where they precipitated out of solution and accumulated as scale deposits. It seems somehow at variance with technical progress that none of these tests went on to field-scale trials, which would certainly have promoted more awareness that electrolysis could be used as a practical method not only to reduce the scale-forming potential of evaporative circulating water, but also to remove the deliberately precipitated calcium carbonate (and other precipitated scale-forming salts) by simply cleaning or replacing the cathode. Some development work was done in Israel primarily for agricultural applications, but commercial side-stream electrolysis installations on cooling water systems began during 2004 in cities and countries remote from the US and Europe, namely Singapore, Malaysia, and Japan.

One technical factor that has emerged is that many of the earlier pilot-scale trials did not use electrodes with large enough surface areas. Minimum electrode surface areas are sized on water flow rate through the electrodes, electrical conductivity of the water, and voltage applied across the electrodes. Insufficient electrode area, particularly cathode area, does not remove carbonate scaling salts and crystals fast enough to avoid scaling. With sufficient and even some extra electrode area, voltages and amperages across the electrodes can be reduced and are easier to maintain within an optimum range. Further benefits arising from additional electrode areas are that larger deposits of scaling can accumulate before cleaning or replacement of cathodes is needed. This results in increased operating periods, and during these periods, more carbonate salts that circulating water may have picked up by re-dissolving existing scaling, can also be included.

Another factor that has undoubtedly discouraged adoption of electrolysis, has been a perception that the costs of additional electric power would be unacceptable. Here again, sufficient electrode area in side-stream electrolysis units treating low water flow rates has proven capable of reducing the power needed to levels that, vendors claim, are sufficient to ensure that heat exchange surfaces are automatically cleaned and maintained in clean condition, resulting in reduced overall average power usage by cooling plants in comparison to plants that tend to use more power due to less effective control of scaling by traditional chemical methods.

One other water circuit mentioned earlier on, which electrolysis has proven to be effective in preventing scaling, is adiabatic humidification, as illustrated in Figure 1.

Charles2 redrawn
Figure 1: Sketch diagram of adiabatic evaporative humidifier.

These older types of evaporative humidifiers have been largely replaced by units that are supplied with water at higher pressures so that all supply water is directly converted through suitable spray nozzles to vapour and fine mist droplets small enough to remain suspended in the airflow drawn through by the fan. However, many units having evaporative spray water circuits are still in operation and new installations crop up occasionally. Scaling resulting from untreated supply water results in blocked spray nozzles, which have to be cleaned manually. Supply water flow rates to even large humidifiers are, however, low enough to be treated by relatively small-diameter rod and tube type electrodes so that all supply water is subjected to treatment by electrolysis. Four of these types of humidifiers fed by unsoftened municipal water in Perth, Western Australia, have reportedly been operating on electrolysis-treated supply water for over a year without experiencing any spray nozzle blocking by scaling compared to previous years during which nozzles had to be manually cleaned on a regular monthly schedule.

In summary, side-stream electrolysis installations on evaporative water circuits control scaling by precipitating scale-forming substances, which are then removed from the water circuit. In the process, circulating water becomes partially softened, allowing gradual re-dissolving of existing scale deposits as well as higher cycles of concentration to be used with commensurate reduction in supply water demand. In addition, polluting chemical content of the lower amount of bleed-off water is minimised or even possibly reduced to zero. Some of the suppliers of these electrolysis units claim additional benefits relating to reduction of corrosion and growth of bacteria and other microorganisms.

Water treatment companies have been anxious for a real technical step forward for many years. On the evidence to date of effective scale control, water savings, and compliance with anti-pollution regulations provided by electrolysis installations, this could well be what they have been waiting for.

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