Case Study: Keeping Chillers Scale-Free in a Hospital in Central OH

What Does Scale Do To a Chiller System?

A hospital in central OH uses a cooling tower system for their comfort cooling throughout the building, using 5 chillers, each tied into a cooling tower.  There had been a noticeable decrease in chiller plant efficiency over time, and during high demand days in the heat of the summer, the chiller plant could not meet the load properly.

Upon inspecting the chiller, Chardon examined the condenser tubes using a borescope, we found that they indeed had formed scale, with two different types: calcium carbonate (CaCO₃) scale and calcium phosphate (Ca₃(PO₄)₂ scale.  The buildup of this scale reduced the ability for heat transfer to take place and was the culprit for why the plant system was unable to keep up with demands, and why energy usage was high.

What Are The Differences in Carbonate & Phosphate Scale in Cooling Towers?

Calcium carbonate (CaCO₃) scale typically forms when the water becomes over-saturated with minerals and evaporate out onto the surfaces on the chiller system.  Calculating the saturation index using the Puckorius Scaling Index or PSI is crucial to maintain proper mineral levels in the water to maximize usage while preventing scale from forming.

Calcium phosphate (Ca₃(PO₄)₂ scale, on the other hand, forms through the excess calcium (Ca⁺) ions in the water bond with orthophosphate molecules, which can be found in some water treatment chemical products.

After chiller tubes were found to have scale, Chardon inspected the system to find the issues causing the scale growth.  Three main red flags were found:

• Incorrect scale inhibitor was causing the calcium phosphate (Ca₃(PO₄)₂ scale growth
• Approach temperatures were inconsistent
• Saturation Index (PSI) had been exceeded for an extended time causing the growth of the calcium carbonate (CaCO₃) scale

These were 3 main signs that pointed to the problems with the system scaling.  Chardon then altered the water treatment plan to correct these issues.  First, by changing the scale inhibitor used to stop the calcium phosphate scale and then changing the cycles of concentration in order to more accurately maintain a proper PSI, the approach temperatures were reduced.

How Do You Eliminate Scale on Condenser Tubes?

The water treatment program was updated to prevent scale from growing moving forward, but the scale remained from the previous program.  Chardon started the process of eliminating scale from the chiller tubes.  The process to remove scale from condenser tubes varies depending on the metallurgy and system design.

Chardon used an acid cleaning procedure on the condensers that were the most severely scaled to remove as much scale as possible.  Then over the next few months, ran increased dosages of polymer descalers and PBTC (C₇H₁₁O₉P) scale inhibitor to continue the cleaning process.  Alongside these chemical additions, monitoring the pH of the cooling tower system helped to ensure the chemicals and the system were working efficiently.

How Does Removing Scale Improve Your Cooling Tower System?

After about 3 months of the cleaning process, the results of these efforts to remove scale proved the efforts worthwhile, as the cooling tower system is operating at a much higher efficiency.  With the original 5 chillers unable to withstand the high demand of heat in the cooling system prior to these efforts, now, the system can handle any load on just 3 of those chillers.  Proper scale prevention in water treatment can make a large difference in the life of the equipment, as well as the operating efficiency and energy usage of them while running.

Have questions about scale in your cooling tower system? Contact us today for a free site survey!