Chiller and Closed-Loop Water Treatment: Protecting HVAC Systems from the Inside

Closed water loops circulate chilled water or hot water through chillers, air handling units, fan coil units, process heat exchangers and heating systems. Because the water is recirculated and theoretically never leaves the system, it is easy to assume it needs little attention. In reality, closed loops suffer from corrosion, sludge build-up and microbial growth that reduce efficiency and cause costly failures. A modest treatment programme prevents most of these problems.

How Closed Loops Differ from Open Systems

In an open cooling tower system, water evaporates and is continuously replaced, concentrating minerals. In a closed loop, water is sealed and makes contact with air only through expansion tanks or leaks. Make-up water should be minimal. The main threats are therefore not scaling from evaporation but corrosion, deposits from corrosion products and microbial activity.

Common Problems in Closed Loops

Corrosion

Oxygen and dissolved salts attack steel pipes, copper tubes and other metals. Corrosion products form iron oxide sludge, often called black or red sludge, which circulates and settles in low-flow areas.

Fouling of Heat Exchangers

Sludge and debris settle in chiller tubes and coils, reducing heat transfer. A thin layer of deposit can noticeably reduce chiller efficiency, increasing energy use.

Microbial Growth

Bacteria can thrive in loops, especially warm heating circuits or systems with organic contamination. Some bacteria cause microbiologically influenced corrosion.

Excessive Make-Up

Leaks mean fresh water, with fresh oxygen and minerals, continuously enters the loop, accelerating corrosion and diluting treatment chemicals.

Elements of a Good Treatment Programme

Quality Fill Water

Filling the loop with softened or RO water reduces scale and dissolved salts from the start.

Corrosion Inhibitors

Specially formulated inhibitors protect metals. Different chemistries suit different metal combinations, so selection should consider all materials in the system.

pH Control

Keeping pH within a recommended range helps protect the metals in the system.

Biocides

Periodic biocide treatment controls microbial growth where testing indicates it is necessary.

Side-Stream Filtration

A small portion of loop flow passes continuously through a filter, such as a cartridge, bag or media filter, removing suspended particles and corrosion products. Side-stream filtration is one of the most effective ways to keep closed loops clean.

Dirt and Air Separators

These remove circulating debris and entrained air, reducing corrosion and improving system performance.

Monitoring the Loop

Regular testing typically includes:

• pH and conductivity
• Inhibitor concentration
• Iron and copper levels as indicators of corrosion
• Microbial counts where relevant
• Make-up water volume

Installing a water meter on the make-up line is a simple and powerful step. A sudden increase in make-up indicates a leak.

Flushing and Cleaning New Systems

New systems contain construction debris, oil and mill scale. Proper pre-commissioning cleaning and flushing, followed by passivation, sets the system up for long-term health. Skipping this step often leads to problems from the first year.

Benefits of Good Closed-Loop Treatment

• Longer life for chillers, pipes and coils
• Better heat transfer and lower energy use
• Fewer blockages and leaks
• Reduced maintenance and emergency repairs

Conclusion

Closed loops are out of sight but should never be out of mind. A combination of good fill water, appropriate chemical treatment, side-stream filtration and routine monitoring protects valuable HVAC and process equipment and keeps energy consumption under control.

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