Cooling Towers

An open recirculating cooling tower carries heat away from chillers, condensers and process equipment by evaporating a small part of the circulating water. The water that evaporates leaves its dissolved salts behind, so the circulating water becomes more concentrated than the make-up water. Part of it is drained as blowdown to keep that concentration in a workable range, and fresh make-up replaces both the evaporated and the drained water.

How does a cooling tower system work?

Warm water from the heat source is sprayed over fill inside the tower. Air moves through the fill, a little water evaporates, and the remaining water cools and returns to the heat exchangers. A basin collects the cooled water. Because the water is open to air, it picks up dust, oxygen and airborne microorganisms, and it is warm and well lit, which suits biological growth.

What are the water-chemistry risks?

• Scale: as water concentrates, sparingly soluble salts can come out of solution on the hottest surfaces, which insulates the exchanger and cuts heat transfer.
• Corrosion: oxygen-rich, conductive water attacks carbon steel and other metals in the circuit, and corrosion products settle as deposits.
• Biological growth: bacteria, algae and slime form biofilm that fouls surfaces, shelters corrosion and calls for routine microbiological control.
• Fouling: dust, debris and corrosion products accumulate in the basin and on fill and surfaces.

How is a cooling tower monitored?

Monitoring tracks conductivity to follow concentration and control blowdown, pH, the residual of the corrosion and scale inhibitor in use (for a zinc-based program, the zinc residual), and corrosion indicators such as coupons. Microbiological monitoring checks that biocide control is effective. Visual inspection of fill, basin and exchangers completes the picture. A recent water analysis of the make-up and circulating water is the starting point for deciding what to monitor and how often.

Which chemical categories are relevant?

Our datasheets describe a zinc-based scale and corrosion inhibitor for cooling towers, and cooling-system biocides of two kinds, isothiazolinone (CMIT/MIT) and DBNPA. Treatment rates are set by our technical team from the water analysis and the operating conditions, and are fed with a chemical metering pump, not estimated by rule of thumb.

Points to take into account when treating an open cooling tower. These are qualitative guidance, not a treatment recommendation for a specific system.

• Make-up water quality and how it changes with season and source, since it sets the scaling and corrosion tendency.
• The balance between concentration and blowdown, which affects water use as well as scale risk.
• Compatibility between the inhibitor and the biocide, and between both and the metals in the circuit.
• Rotating or alternating biocides so that microbial control does not weaken over time.
• Where chemicals are fed, so that they mix into circulating water and not into the drain.
• Handling and storage of chemicals in the plant area, and who is trained to do it.
• The condition of the fill, basin and exchangers, which treatment cannot repair if deposits are already hard.

Our technical team reviews the water analysis and the operating description before any program is proposed.