Hard water has been causing issues for humanity for ages, whether it is making it difficult to wash our clothes and cars, or depositing on our hot water heaters and kettles. Hardness scale is typically the combination of calcium and magnesium compounds that have precipitated out of water (e.g., calcium carbonate, magnesium silicate). This tough deposit forms in HVAC cooling systems and process water systems and can wreak havoc by decreasing system life and increasing energy usage, maintenance, and operational costs.
Against common expectations, calcium-carbonate hardness has a retrograde solubility. This means, the higher the temperature of the water, the less soluble calcium carbonate becomes. Due to this characteristic, scale has a tendency to deposit on the hottest surfaces in the cooling water system, typically being the heat transfer sites. This highly insulating deposit reduces the system’s capacity to transfer heat. Scale also takes up space inside the pipes and may decrease the flowrate and efficiency of the system. Not only does the system now have to work harder to transfer energy through this scale, but the added corrosion inhibitors can no longer adsorb onto the system metallurgy, possibly leading to under-deposit corrosion and system damage. Table 1 demonstrates the impact scale can have on the cooling water system’s energy costs.
Table 1: Effects of Scale on Cooling System Energy Costs
Certain water quality parameters will affect the quantity and likelihood of scale deposition. High calcium, magnesium, alkalinity, and pH levels are all components that increase the potential for scale formation.
Two common approaches to mitigating scale formation include: pretreatment and chemical additions.
Softeners are commonly used in pretreatment to remove the hardness from the water before it can enter the system. The softeners contain resin beads covered in harmless sodium ions. These beads have charged sites that attract and allow the calcium and magnesium ions to stick to the beads while kicking off the sodium ions. Once all the sodium has been removed and the beads are covered in hardness ions, the softener goes into a regeneration mode, purging the captured calcium and magnesium and replenishing the beads with new sodium ions.
The aim of chemical scale control takes another approach. The overall goal of chemical scale control is to keep hardness from depositing on the cooling water system surfaces. This is accomplished through a combination of:
- Threshold Inhibitors: which chemically increase the amount of the scale ions that can be maintained in solution
- Crystal Growth Modification Polymers: which change the shape of the formation of hardness scale making it less stable and more likely to re-dissolve
- Dispersants: which change the attractive forces between the scale particles
Low levels of threshold inhibitors, like phosphonates, greatly increase the amount of hardness the system water can hold. Threshold inhibitors are most effective in systems that concentrate hardness (i.e., cycled up cooling towers). The presence of a threshold inhibitor allows these systems to achieve higher cycles of concentration with less scale deposition occurring. In hockey, players get exhausted the longer they are in play. It is important to change players often, keeping fresh strong players on the ice. Like in hockey, these inhibitors can get tired if they are holding the hardness in solution for too long. Blowdown helps remove the exhausted threshold inhibitors along with the concentrated hardness ions they are holding, making room for fresh inhibitors to be fed to protect the system.
Crystal Growth Modification polymers can also be added into cooling system water to help keep scale from depositing onto surfaces. This is accomplished by adjusting the shape of the scale’s formation, making it more difficult to form deposits that attach to the system surfaces. Imagine that scale is a wall, and the scale molecules are its bricks. The crystal growth modifier may change the shape of the molecules from rectangular (like bricks) to circular (like soccer balls). It isn’t so easy to build a wall out of soccer balls. These polymers find common use in cooling towers and boiler systems to help keep the hardness from depositing on heat transfer surfaces.
Dispersants work in a similar fashion to crystal growth modification polymers in that they prevent the scale molecules from sticking together and forming deposits on the system surfaces, but they use a different mechanism to accomplish this. Have you ever heard of opposites attract? Well, like repels like, that is, molecules with the same charge will repel each other. Dispersants attach to the scale molecules forming a unified charge around them. This causes the molecules to repel each other instead of attaching to each other and depositing on heat transfer surfaces.
Scale deposition control is an important component in any effective cooling water treatment program. Ion exchange softeners can remove hardness from the water, while chemical addition can increase the capacity for hardness and decrease its adherent properties, allowing higher cycles of concentration to be achieved with less deposition. Scale deposition control methods must be employed in order to keep these systems running longer and using less energy while minimizing operating and maintenance costs.
The end result of scale deposits can be steadily rising fuel and maintenance costs. Chem-Aqua’s experts can help you evaluate opportunities to maximize the life, efficiency, and reliability of your water systems. To learn more, visit Chem-Aqua online today!
Written by: Chenoa Hill