By Eamonn Ryan, from a TechTalk hosted by SAIRAC Cape Town Centre.
In a recent technical talk, Pieter de Jager, owner of Procontrol, a leading provider of water treatment solutions, shed light on the critical importance of effective water treatment in cooling systems. This is Part 2 of a 5-part series.
De Jager highlights the concept of cycles of concentration to illustrate how water quality changes within a cooling tower. As water evaporates, dissolved minerals concentrate. Using Cape Town’s tap water as an example, with an initial conductivity of 60-80 microsiemens (μS – a unit of electrical conductivity, equal to one millionth of a siemens), he explains that after ten cycles of concentration, the conductivity could rise to 800 μS. To manage this buildup, conductivity controllers are employed to automatically open a bleed valve, releasing a small amount of concentrated water. This is then replenished with fresh make-up water, typically controlled by a ball valve.
However, De Jager cautions that manual bleeding, especially when coupled with manual biocide dosing, can lead to significant chemical waste. If biocide is added in the morning while the system is continuously bleeding, the expensive chemicals are simply diluted and lost down the drain. Achieving the right balance between bleed-off and chemical addition is crucial for both cost efficiency and environmental responsibility.
Shifting focus to closed-loop circuits, such as chilled water systems, de Jager outlines a different approach. These systems, ideally, should not lose water. A critical step for new closed-loop systems is pre-treatment. Due to welding and the potential presence of debris from pipework installation, the system is typically flushed, filled with water and a pre-treatment chemical, circulated for a day or two, and then flushed again before the maintenance chemical is added.
The primary chemical used in closed-loop systems is usually nitrite-based. Nitrites act as an oxygen scavenger, removing dissolved oxygen from the water and thereby preventing corrosion. However, even in closed systems, the addition of biocide is essential to address potential microbial contamination introduced during filling. De Jager explains that regular monitoring for nitrite levels is important, as a decrease could indicate water loss or the presence of bacteria consuming the nitrites.
Furthermore, the pH in closed-loop systems is typically adjusted to around 9 using a small amount of soda ash. A common visual indicator in these systems is a pinkish colour in the water, which is due to the presence of phenolphthalein. This chemical is colourless below a pH of 7.8 but turns pink at higher pH levels. This serves two key purposes: it provides a quick visual check for potential pH drops or water loss (if the colour disappears), and it can help identify leaks in multi-story buildings with complex piping systems. A pink droplet immediately indicates a leak in the chilled water circuit.