Hard or Soft Water for Cooling Towers

Q: I have two cooling towers using softened water. It has 0 hardness after softening, pH is 8.75, conductivity is 150 and alkalinity is 40, chlorides are 10.

Pennwell web 250 254

Q: I have two cooling towers using softened water. It has 0 hardness after softening, pH is 8.75, conductivity is 150 and alkalinity is 40, chlorides are 10. The one tower water has a pH of 8.2 in the sump as well as 140 hardness, 514 conductivity, 30 chlorides and 180 alkalinity. The other tower has a pH of 7.84, hardness of 200, 549 conductivity, 40 chlorides and 200 alkalinity. What is causing my hardness to be so high in the tower and am I wasting water. Also, do you have any ideas to improve my treatment. The towers both have rust on the inside walls, and we’re using a treatment with sodium hydroxide in it.

Pennwell web 250 254

A: The use of softened water in cooling tower systems requires very special treatment since softened water is very corrosive to ferrous metals. You didn’t specify your treatment program, but following are my comments. The iron build-up in cooling tower indicates that corrosion is going on in the system. Dissolved iron will oxidize as it falls through a cooling tower allowing precipitation of iron in fill and in basin. A corrosion inhibitor and iron dispersant will be required to stop corrosion and excess dissolved iron from the system. Note: The amount of iron that has already been deposited will require mechanical and/or chemical cleaning to remove.

You’re operating at about 5 cycles of concentration based on alkalinity readings. The hardness in cooling tower is coming from an outside source, most likely wind blown dust from the atmosphere. The amount of hardness in cooling tower cannot be predicted, so you just have to maintain calcium/alkalinity balance to prevent precipitation of calcium carbonate. Your pH range is within normal limits for the calcium/alkalinity you’re currently maintaining.

I don’t believe you’re wasting water, although you are at the lower limit of normal range for cycles of concentration (5-7).


Cooling FAQs are provided by the Houston-based Cooling Technology Institute (CTI), a non-profit organization dedicated to improvement in technology, design, performance and maintenance of cooling towers and evaporative heat transfer systems (EHTS). They’re from the “Ask the Expert” column of CTI Journal. Contact: 281-583-4087 or www.cti.org

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