Cycles of Concentration for Boilers

Two weeks ago we talked about options for calculating the cycles of concentration (CoC) in cooling water systems. I promised we would follow that up with a discussion about calculating the CoC for boiler water, so let’s get started.

As with cooling water systems, maximizing boiler cycles means reduced fuel, water, and chemical costs. Far overshadowing all the other costs is fuel, which represents the highest annual operating cost for a boiler. In a boiler, we have “invested” in fuel to heat the boiler water to make steam at the given pressure.

To make quality steam, the total dissolved solids (TDS), total alkalinity, and silica levels have maximum limits. Meanwhile, some boiler water chemicals will break down and become ineffective if left in the system too long. To control these parameters, we need to blowdown water from the boiler water.

For boilers with softened make-up water (typically lower pressure boilers), enough TDS usually exists in the waters to allow for calculating CoC. Dividing the boiler water TDS (or neutralized conductivity) by the feedwater TDS is sufficient to give us a reasonably close value. This can be verified in the field by testing for silica — dividing boiler water silica by feedwater silica.

Example: A 50 psig boiler has a feedwater conductivity of 100 umhos and a silica level of 0.50 ppm. The boiler water “neutralized” conductivity is 3000 umhos and the silica level is 15 ppm. What are the cycles of concentration for this boiler? (This is not a trick question.) Extra points: What would you expect the unneutralized conductivity to be?

However, when systems have very high quality feedwater from extremely high levels of condensate return or from using reverse osmosis makeup or demineralized water makeup, the TDS and analyte levels can be too low to use to get an accurate measurement of CoC. This is especially true in high-pressure boilers, which require a high level of quality water. Using standard calculations can lead the surveyors to an incorrect number.

Here is a challenge question I would like to leave you with: There is a 1200 psig boiler on congruent phosphate with a feedwater silica level of 0.020 ppm. The boiler water silica is 0.080 ppm. Surely the cycles of concentration cannot be 4 (0.080/0.020). To calculate the correct CoC for this system, what else do you need to know (assume no flow measurement is available of the blowdown)?