By Charles Nicolson
Water treatment programmes, particularly for cooling water circuits, have now reached a stage of development where it is unlikely that further significant advances will occur from a purely technical point of view unless new technology is introduced and applied.
Refinements introduced into current cooling water treatment are almost entirely focused on minimising costs while ensuring that the qualities of both circulating and bleed-off water remain compliant with the ever-increasing numbers and complexity of toxicity and environmental regulations.
Going back fifty years to 1970, cooling water treatment was simple. Supply water to cooling circuits was softened to eliminate any scaling potential and small concentrations of soluble chromate chemicals were dosed which not only prevented corrosion by ‘chrome plating’ wetted surfaces but were also highly effective broad-spectrum biocides. This straightforward and easy treatment covered not only closed water circuits but also the whole diverse range of applications of closed and evaporative cooling water circuits found in HVAC installations as well as other water-based machinery such as humidifiers and misting/fogging systems.
If there were restrictions on availability of municipal or potable quality water from a local authority which was sometimes the case, then water sources were usually boreholes or other types of underground aquifers. Regardless of the sources, however, little thought was generally given to ensuring that supply water pressure at points of entry into evaporative cooling units and other equipment such as backwash facilities for filters was enough. Cooling towers and other equipment needing supply water were often located on top of office and other commercial buildings along with concrete or ‘Braithwaite type’ galvanised steel water holding tanks fed by municipal or alternative supply water. These relatively large holding tanks ensured that enough water was always available in the event of disruptions in normal supply lasting for several hours or even for a few days at a time.
The tanks also distributed fresh water for drinking, domestic toiletry, sewage and cleaning purposes. More often than not, water for these purposes distributed by the tanks flowed purely by gravity which resulted in sufficient water pressures at lower levels in the building but reduced pressures at higher levels causing low flow rates. Even worse were water flows from the holding tanks to cooling towers and other machinery mounted on the roof at the same level. Provided that the water level in the tank did not drop more than about halfway (50% of tank capacity), cooling towers and humidifiers could run under normal loading conditions. However, unexpectedly high ambient conditions or even relatively small leaks in water circulating systems could raise demand from the tank to greater than the low-pressure supply capability which often ended up in system failures through trip-outs.
Discomfort caused by air conditioning plant trip-outs tended to draw attention away from higher expenses and energy losses inherent in additional plant restarts. In general, electrical power for refrigerant compressors, water pumps and axial or centrifugal air moving fans was a minor initial design consideration with little or no consideration of unexpected energy costs during operation from commissioning through whatever periods it was expected (or hoped) that the plants involved would continue to run reliably.
Another of the most common potential problem points in water circuits, including circuits in HVAC installations, up until around the end of the 1980s was ‘dead ends’ where the flanged ends of water pipes were blanked off for future plant expansions. These dead ends were only small volumes but the water in them became rapidly stagnant and de-oxygenated resulting in growth of acid-producing anaerobic bacteria which spread throughout the water circuits causing corrosion. Some of the blanked off pipes were fairly large, between 250 and 350mm in diameter. However, it was normally relatively easy to weld small valved fittings onto the flange blanks, drill through the blanks and connect 6mm tubing from the fitting to the inlet sides of nearby circulating water pumps – or other lower pressure points of re-entry into the circuit. The small amount of water flow through the 6mm tube was sufficient to eliminate the “dead end” situation and had no measurable impact on design circuit water flow rates. ‘Dead ends’ are seldom seen today but new potential problem points arise for which mechanical modifications which may seem expensive initially are often the optimum solution choices.
Water circuits supplied with softened water usually require little or no bleed-off. Drift and spray losses from evaporative cooling towers supplemented by low flow rate continuous trickle drains provided enough bleed-off to prevent cycles of concentration rising too rapidly between regular draining of cooling tower sumps which tended to be done as frequently as needed without any regard to the accompanying increases in fresh water and treatment chemical usages.
Only very few people seemed to perceive the situation as being too good to last! It was.
During the 1970s chromium, in its most active and toxic hexavalent form [Cr6] was progressively banned. Suddenly, cooling water treatment became much more expensive. Corrosion inhibitors based on zinc could handle corrosive water reasonably well, but no product could replace chromium as both a wide-spectrum bactericide and an algaecide. Then zinc was banned, leaving the water treatment industry with only far more costly non-toxic phosphate corrosion inhibitors which also had the disadvantage of being nutrients for bacteria requiring much higher biocide dosing levels. To avoid these highly escalated costs of treatment, the use of softened supply water to cooling water circuits was virtually universally discontinued. However, using unsoftened supply water immediately introduced scaling problems which rapidly became the number one cooling water problem until the early 1980s.
Prior to the Internet it often took a while for new technical changes to be implemented in South Africa. By September in 1984 the large companies which had invested huge amounts of money and effort into researching and developing anti-scaling polymer chemicals were able to supply some of their new products locally.
These new anti-scaling chemicals for use in unsoftened supply water were the biggest single technical change which had occurred so far in chemical cooling water treatment programmes worldwide and their costs were also far higher which not surprisingly generated widespread customer resistance.
Technical developments and changes in air conditioning plants since 1984 have been far-reaching involving in particular, computerised applications, pollution and electrical grid energy demand reduction. The Montreal Protocol was signed in 1987 and continues to influence air conditioning, for example, no more new R22 installations. A fairly recent phenomenon is proliferation of data centres requiring substantial air chilling air conditioning plants, not for people but for the computers themselves. In 1984, energy savings were low priority and sustainable energy use was virtually unheard of. The term ‘load-shedding’ only came into use about 25 years later and although it is still used almost exclusively in regard to electrical grid power supply it is also gradually being applied to other non-grid electrical generating installations.
There were no heat recovery systems designed into the plants like there are today including desiccant type energy wheels. Variable speed drives for water circulating pumps were available but expensive. Capacity controls other than basic on/off systems comprised thermostatically operated airflow restrictors on the suction side of centrifugal fans. Current air conditioning plants are indeed vastly more sophisticated in respect of programmable automated controls. Generally, there are also additional water circuits installed for heat recovery, heat pumps and solar energy collection and transmission.
Water treatment in HEVAC installations has also become far more advanced in automated control and monitoring systems. Water treatment providers have had to become more service focused but still generate most of their revenue from chemical sales. This will only change when traditional steel pipework and other potentially corrosive wetted steel surfaces are replaced with non-corrosive substitutes which will undoubtedly take many years to eventuate. Water treatment of heating and cooling water circuits is a large worldwide industry. It seems, therefore, inconsistent compared to so much other technical progress that no new chemicals for this purpose have been developed since 1984 which is curiously similar to the fact that no completely new antibiotics have appeared from the much larger pharmaceutical industry for the same period. (At the time of writing – the coronavirus outbreak is still making headline news. Thankfully there are no reports of any cases reaching Southern Africa as yet – mid-February).
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, were precipitated out of solution and migrated in ionic form to cathode electrode surfaces where they accumulated as scale deposits. Some development work was done in Israel primarily for agricultural applications, but commercial electrolysis installations on cooling water systems began only during 2004 in Singapore, Malaysia and Japan.
In summary, side-stream electrolysis installations on evaporative water circuits control scaling by precipitating scale-forming substances which automatically remove them 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 due to 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 suppression of growth of bacteria and other micro-organisms.
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.