Tackling Cooling Tower Scale: Strategies and Solutions
Scale is enemy #1 that often constrains cooling towers from being able to operate safely at higher cycles of concentration. Scale commonly forms on metal surfaces in towers from minerals such as calcium carbonate, calcium phosphate, magnesium silicate and calcium sulfate. Today we’ll cover how to perform a scale risk measurement assessment, basic chemical solutions, chemical free solutions, and advanced strategies for combatting cooling tower scale.
Start By Measuring Your Risk
There are many variables that drive scale formation in cooling towers, such as the pH of the water, the calcium carbonate content, the temperature, and the level of conductivity / total dissolved solids (TDS). Together, these variables are combined into a risk measurement for scale formation called the Langelier Saturation Index (LSI). When the LSI index is positive, then you are operating the tower in a scale-forming state. Remember that the LSI level at which the tower is operating is a major factor in determining how many cycles of concentration it can safely run, meaning it is also something we are going to target for water efficiency improvements. LSI is often the most significant limiting factor for blowdown in the majority of cases. In a smaller number of cases, scale can be driven by high silica, magnesium, or phosphate content in recirculating cooling water.
Strategy #1 Chemical Solutions
Traditional scale inhibitor chemicals are a highly proven and reliable method for reducing scale-forming potential.
Facilities begin optimizing their chemistry by analyzing water quality to determine if the facility is over- or under-feeding anti-scalant. Properly employing scale inhibitors requires you to make sure you are not over or under-feeding chemicals. Underfeeding can leave you at risk of scaling, while over-feeding can waste money.
How to tell if you are overfeeding?
- A water treatment professional should be able to tell you the exact target residuals and active chemical concentrations that should be present in your system to achieve industry standards. You should seek additional opinions if you want to verify the strength of circulating chemicals in your system. A trained service technician will be able to perform a variety of onsite tests to confirm the strength of your current scale inhibitors.
Small investments in a new controller, or in add-on capabilities to your existing controller, can also help reduce scale and OpEx by both boosting chemical dosing precision and by giving you the confidence to run your cooling tower at a higher Water Efficiency Score without sacrificing safety.
Strategy #2 Water Softening
If you’ve already dialed-in your traditional chemical treatment program, then there are additional measures you should be looking at to allow your system to run at a higher WES without pushing the system into an “unsafe” scaling condition.
Water softeners are a valuable asset for improving water efficiency and protecting cooling tower equipment. When run properly, a softener removes scaling minerals like calcium and magnesium from your makeup water. The effectiveness of a water softener depends on factors like:
- Incoming water quality
- Softener resin type
- Regeneration cycle
If you do already have a softener:
Just like going to the doctor’s office for your annual check-up, softeners need to be periodically checked, especially if your internal team is not highly experienced in softener operations and maintenance. Here is a list of things that should be checked regularly:
- Controller settings are regularly re-calibrated to changes in incoming water quality.
- Regeneration Process Inspection: Verify injection and backwash flow rates.
- Elution Study: Compare theoretical vs. actual salt brine concentrations.
- Resin bead condition: check for damage and pore condition.
- Hardware condition inspection: Check for leaks and corrosion.
Water softeners require regular check-ins and control calibrations. This process includes monitoring incoming feedwater quality and ensuring the controls are appropriately set for your water source’s hardness and flow rate. It’s also crucial that the softener resin is adequately cleaned and regenerated regularly. If addressed improperly, the softener operates less efficiently, resulting in higher water consumption and sometimes equipment damage.
If your team needs training on industrial water softeners to incorporate them into your process, search for water softener professionals like EAI Water for training and assessments.
Strategy #3 Custom-Engineered Ion Exchange Resins
While softening can dramatically improve water efficiency performance, advanced ion exchange resins will bring your pretreatment to the next level. These IX resins selectively remove additional impurities and minerals that water softeners cannot, leading to higher water efficiency and a longer lifespan for your cooling tower equipment. Unlike with softeners, the conductivity bleed set points on cooling towers using IX pretreatment can be much lower, mitigating the risk of a scaling event in the case of equipment failure. However, these IX resins often need to be regenerated with acids or bases, which can be more operationally challenging than brine regeneration for a traditional softener. Be sure this upgrade is a good fit for your team’s SOPs before investing. Selecting the right resins for your particular site requires custom engineering beyond simple softening, but can be a major payoff for boosting WES.
Strategy #4 pH Adjustment
Depending on the starting content of scale-forming minerals (e.g calcium carbonate) in your makeup water, you may benefit from adjusting the pH of that water to keep those minerals dissolved. Recalling the Langelier Saturation Index from earlier, pH is an excellent lever available to control the scale-forming potential of dissolved minerals. Depending on the other starting conditions of your tower, mostly the existing mineral composition, concentration, and pH, you can further adjust pH to allow higher concentrations of minerals to be present, but in a dissolved state.
One of the most common ways to accomplish this pH shift is with an additional dosage of sulfuric acid. The exact pH target depends on many factors, so we cannot generalize a specific target. For some facilities, this may be a cost-effective way to achieve a boost in WES.
Downsides: Handling sulfuric acid in the facility does add operational complexity and safety considerations. With an additional dangerous chemical to manage, this approach is not for everyone. Furthermore, many facilities with sustainability goals want to reduce reliance on outside hazardous chemicals, not increase it. It is essential to consider the proper equipment and precautions when handling sulfuric acid to ensure the safety of your team. Furthermore, it is critical that any sulfuric acid program uses automated pH sensors and chemical feed pump controllers to safeguard against overfeeding, which can cause accelerated corrosion in some systems depending on the metal composition.
Strategy #5 Chemical-Free Systems
There is strong appeal for systems advertised as “chemical free”. Below, you will find an overview of some them. Please be aware that removing chemicals altogether does carry a certain amount of operational risk, especially in areas with high hardness content, like the U.S. Southwest.
- Electrodeionization (EDI) – uses positive and negative electrodes in conjunction with ion exchange resins and membranes to remove salts from your makeup water. This allows you to control scaling in your tower without chemicals. The electric field continuously regenerates the ion exchange resin, as opposed to ion exchange resins by themselves that require chemical additives to regenerate. Closely related, Capacitive Deionization (CDI) uses a similar process, often with carbon-based electrodes. Both are mainly used with feedwater but have been tried over the years for recirculating water as well.
- Electrochemical Deposition – If your system suffers from scaling, Electrochemical Deposition flows makeup water through a charged reactor rod before entering your cooling tower. The machine encourages minerals to precipitate and scale to a reactor rod before entering your cooling tower.
- Pulsed Power – uses an electric pulse both to precipitate hardness (scale) out of the water and to disrupt bacteria reproduction. The result is powdered minerals that mitigate scale formation and limit bacteria growth.
- Plant-Based Water Treatment: ProMoss™ is a product based on naturally-growing sphagnum moss that has inherent scale and corrosion inhibiting properties. In many cooling programs, it can replace a significant portion of the traditional water chemicals needed and may be able to raise the Water Efficiency Score. Aside from the water efficiency implications of this solution, ProMoss’s™ natural production process in bogs and extreme ease of transport (low density and volume compared to liquid water treatment chemicals) make it extremely attractive for boosting all aspects of facility sustainability. ProMoss™ can be deployed in either side stream contact chambers or in cages that sit in the tower sump. The product generally needs to be changed monthly. While ProMoss™ is highly unconventional in the industrial water treatment space, it does now have many years of deployment at sites across the world. Please speak with an approved ProMoss™ dealer to determine if your site is good candidate for this solution. Ultimately, it’s almost impossible to go fully chemical-free. Your cooling tower will continue to need pretreatment, especially in places with poor water quality.
The experts at EAI can assist your facility in identifying the most effective solutions for scale control and incorporating them into your process. Contact us today to learn more.