Scale Management

The Importance of Scale Management

Scale management is critical to any cooling water application as the presence of scale can cause efficiency losses with heat transfer and water transport. These losses can then be compounded by mechanical losses due to plugging and low flow. As part of the Condair team, I perform end-of-life simulations for our media. The goal of these simulations is to provide customers with a cycle-of-concentration setpoint that considers the most common concerns when operating a system utilizing evaporative media. These simulations take into account many potential limiting factors including but not restricted to pH control, chloride levels, hygiene limits, and of course, scale formation. Looking back at over two years of requests from customers with real-life water qualities, I have seen over 70% of these limiting factors be connected to scale concerns within the system.

Chart 1: Files requested per end-of-life failure mode. There are many kinds of scale, some more prevalent than others. But the required action stays the same

While this 70% is split into multiple different types of potential scale, the required action stays the same. The question is, how do I, as a system operator, keep the common and rare salts solubilized within my recirculating water? In the previous paper discussing high purity water we can see this problem solved by removing almost all salts all together therefore leaving little to no scale to form regardless of the potential to solubilize within the water. While this solution is highly effective it can present its own issues with corrosivity as well as costs associated with more sophisticated treatment equipment. Since scale management when operating evaporative media has become a prevalent need, our team has focused on a common solution found in other cooling applications to see if it can fit this growing need.

Scaling animation shows how minerals’ solubility in water creates precipitation, which can then become scale on media.

Common Scale Formations

The most common scale formation in cooling applications is calcium carbonate. This is due to the high natural presence of both calcium and bicarbonate ions in water sources such as groundwater and wells. Here, they dissolve into contact water from limestone as well as from other rock formations. The presence of bicarbonate ions then causes shifts in the water’s alkalinity, as carbon dioxide dissolves into water and forms carbonic acid. This high prevalence in the environment is just one reason why we see calcium carbonate as the most common scale. It is also prevalent due to its low solubility within water in relation to other scales. In our end-of-life studies we see 57% of our simulations are limited by the formation of calcium carbonate scale.

Meanwhile, just roughly 15% of other simulations are limited by silica, sodium, or sulfate scales. While presence in the environment is a key factor in why this prevalence is seen, the solubility of the compounds themselves also play a key role. Calcium carbonate has a solubility constant (Ksp) of 3.8 x 10-9, which means it precipitates far more easily than other compounds like sulfate or magnesium scales. A few other scale compounds are provided in chart 2, and it is clear that as the solubility constant greatly increases, scale is more and more unlikely to form.

Chart 2: Scale compounds’ solubility constants. Calcium Carbonte’s solubility constant means it precipitates far more easily than many other compounds. As solubility increases, scale is more likely to form.

How Compounds Form Scale Differently

It is important to note that amorphous silica does not form scale in the same way as the other scale compounds. Instead of dissociating into ions, and precipitating based on a Ksp-defined equilibrium like the other compounds listed, silica involves polymerization to form solid deposits. This is why the Ksp provided is seen as a much more general estimate for comparison. In addition, the very high solubility of sodium carbonate is also seen as an estimate, since this salt is often considered as freely soluble. When discussing both of these compounds, it’s important to understand how their impact on evaporative media differentiates them from the other compounds. Sodium, since it is so highly soluble in water, typically isn’t seen as a scale when a system is operating. But when the media is allowed to dry out, it can potentially form dust particulate. This is a major concern when media is allowed to dry and run in economizer mode, where the dust can be carried into the recirculating air system. Silica scales are also a major concern due to their physical properties that lead to greater detrimental effects on heat transfer as well as higher difficulty in removing them, often requiring more hazardous treatment chemicals or physical mechanical cleaning.

An animation of common scale formations.

Scale Inhibitors in Recirculating Water

Since the scale variations present many different and unique problems for pretreatment, it’s expected that treating recirculating water to keep compounds solubilized has its own unique issues. This is where the previous mention of cooling applications ties in with our consideration for expanding water quality sources for evaporative media. It is understood that, as demand for cooling expands and water resources become more highly monitored, systems will need to operate with lower-quality water. These lower quality waters can be pre-treated to meet cooling requirements, but this often leads to large volumes of concentrate being exhausted from equipment, and the electrical and financial implications of operating the equipment must be considered as well.

This is where the use of scale inhibitors ties in with our discussion. Since their development in the early 70s, scale inhibitors have been used in almost all water industries to keep our most common ions solubilized. These chemistries are highly complex and are constantly evolving to meet both environmental and performance demands. A high-level explanation of the more common form of scale inhibitors is that a recirculating ppm dosage of chemical that, when dispersed, causes disruptions in the salts’ ability to form crystalline structures. This then blocks the salts’ ability to deposit and form scale structures on surfaces within the system. This can be accomplished with a wide range of strategies, which is why numerous water treatment vendors have invested into researching the most optimal treatment plan for your water quality.

Water Treatment, Present and Future

This investment in water treatment vendors is now playing a role in Condair’s evaporative media as we look to expand the utilization of varying water quality sources around the globe. However, when introducing a new variable into our cooling systems, we need to be certain it’s compatible with all of its parts. Typically, any metallurgy concerns are addressed and disclosed in the associated SDS, but for evaporative media we perform compatibility studies to ensure this information. These studies are essential to confirm that the chemistry not only does not impact the media’s strength or composition, but also that it does not negatively influence the performance of the media. A system cannot function without proper water transport, so this is an essential part of how our team can learn whether a provided chemical can be used to increase cycles of concentration. It is also important to understand increasingly holistic approaches to water treatment which not only require a focus on scale prevention, but also on preventing microbial growth, as well as controlling effluent parameters for blowdown of the system.

We’re Always Evolving to Optimize Our Media

It is a constant process for our team to evaluate these scale inhibitors as the water savings they can provide to our customers is substantial.

We offer free media scaling analysis, so you know your media challenges…and solutions. Arrange for yours today.

We work with water treatment vendors directly to evaluate products they see as applicable for clients, as well as working with customers who’ve already decided on a treatment plan to see if the media they use is compatible. That is why it is critical that, as a provider of treatment or as a system management team, you review not only the chemistry’s efficacy, but also, its compatibility. As demand for cooling grows and new challenges for treatment arise, we continue to optimize our media to expand its potential for utilization.

Complementary source: CRC Handbook of Chemistry and Physics, 97th Edition (Haynes, W. M., Ed., 2016), Section 8: “Aqueous Solubility of Inorganic Compounds.”