Learn & Understand

Cycles of Concentration: The Hidden Fuel Cost of Every Blowdown

The companion calculator tells you what percentage of feedwater to blow down to hold total dissolved solids in check. That number is correct, but it hides two things every boiler operator learns the hard way: blowdown is best understood through cycles of concentration, and every drop of it dumps energy you already paid to add. Understanding both turns blowdown from a chore into a cost you can manage.

The Concept Behind the Percentage: Cycles of Concentration

Steam leaves the boiler as nearly pure water vapor, but the dissolved solids in the feedwater stay behind, concentrating in the boiler water. Cycles of concentration is the ratio of the dissolved-solids level in the boiler water to that in the feedwater, in other words, how many times over the incoming water has been concentrated. Running at ten cycles means the boiler water is ten times as concentrated as the feedwater. Blowdown is simply how you cap that number: the more cycles you allow, the less you blow down, but the closer you push toward scale and carryover. The blowdown percentage the calculator gives is the flip side of the cycles you are trying to hold.

Why You Cannot Just Stop Blowing Down

Letting solids concentrate without limit invites three failures the boiler cannot tolerate.

Scale is especially costly because it conducts heat poorly, even a thin layer forces the burner to work harder to push heat through it, quietly raising fuel use while overheating the tube underneath.

The Energy You Throw Away

Here is the part the percentage does not show. Blowdown water is boiler water, hot, pressurized, and full of the energy your fuel put there. Every kilogram sent down the drain takes that heat with it, so blowdown is a direct fuel loss, not just a water loss. This is exactly why excessive blowdown is expensive and why operators fight to run as many cycles of concentration as their water chemistry safely allows, fewer cycles means more blowdown means more wasted heat. It is also why flash steam recovery and heat exchangers on the blowdown line exist: to claw back some of that energy before it escapes.

Continuous Versus Intermittent Blowdown

There are two complementary ways to blow down, and good practice uses both. Continuous blowdown skims a small, steady stream from near the water surface, where dissolved solids concentrate most, holding TDS at a stable level and making heat recovery practical. Intermittent (bottom) blowdown briefly opens a valve at the lowest point to flush out the heavier sludge and sediment that settles there. Continuous control manages dissolved solids; periodic bottom blowdown clears the settled solids the surface stream never reaches.

Automating the Balance

Because the right blowdown rate shifts with feedwater quality and load, modern systems measure boiler water conductivity (a proxy for dissolved solids) and modulate a blowdown valve automatically, holding the target without an operator guessing. This keeps the boiler as close as possible to the maximum safe cycles of concentration, minimizing the fuel thrown away while still preventing scale.

Using the Blowdown Percentage Well

Take the calculator's blowdown percentage as the rate needed to hold your target dissolved-solids limit, and read it as the inverse of your cycles of concentration. Push cycles as high as your water treatment safely permits to cut the fuel that blowdown carries away, use continuous plus periodic bottom blowdown together, and consider recovering heat from the blowdown stream, because that hot water is money leaving the system.