Closed-Loop Chilled Water

A closed-loop chilled-water system circulates cooled water between a chiller and the air handlers, fan-coil units or process equipment that use the cooling. The water stays in a sealed circuit, so there is no evaporation and no concentration of salts as in an open tower. That makes the loop stable in principle, but it also means the same water, and whatever it carries, stays in the system for years.

How does a closed chilled-water loop work?

Pumps move chilled water from the chiller evaporator through supply pipework to the cooling coils and back. An expansion vessel absorbs volume changes, and a small make-up connection replaces losses. The metals involved are typically carbon steel pipework, copper in coils and mixed alloys in valves and pumps, so the loop contains several metals in contact with the same water.

What are the water-chemistry risks?

• Corrosion: dissolved oxygen that enters through make-up, open vents or leaks attacks carbon steel, and different metals in contact can set up galvanic attack.
• Deposits: corrosion products and any construction debris settle in low-flow areas and clog strainers, coils and valves.
• Biological growth: a loop that is poorly cleaned or filled with untreated water can support bacteria that form slime and cause under-deposit corrosion.
• Make-up variability: frequent top-up with untreated water dilutes the inhibitor and brings in oxygen and hardness.

How is a closed chilled-water loop monitored?

Monitoring follows the inhibitor residual that the datasheet for the chosen program specifies, with a nitrite residual for a nitrite-based program, together with pH, conductivity and corrosion indicators. Clear, colourless water is a good sign and discolouration or suspended iron is an early warning. Records of make-up volume reveal leaks, and microbiological checks show whether growth is establishing itself.

Which chemical categories are relevant?

Our datasheets describe a nitrite-based closed-loop chiller inhibitor, and cooling-system biocides where microbiological control is needed. Treatment rates are set by our technical team from the water analysis and the operating conditions, and the loop is monitored so that the residual stays in the intended range.

Points to take into account when treating a closed chilled-water loop. These are qualitative guidance and not a recommendation for a specific system.

• Cleanliness at the start: new pipework should be flushed and cleaned before the loop is filled and inhibited.
• Make-up water quality and a make-up meter, so that unexpected top-up is visible.
• The metals in the loop, because inhibitor choice must suit all of them.
• Air removal and venting, since oxygen is the main driver of corrosion.
• How the loop is isolated and drained for repairs, which can lose inhibitor and let in air.
• Sampling points that give representative water, not stagnant water from a dead leg.
• Handover records that state what the loop was filled with and how it should be monitored.

Our technical team can review the loop description and a water analysis before a program is discussed.