How Boiler Blowdown Affects Water and Energy Use

Why blowdown is necessary at all

A boiler concentrates its feedwater by design. Water leaves as steam; every dissolved and suspended solid that came in with the feedwater stays in the drum. Without removal, concentration would rise until solids deposit on heating surfaces or carry over into the steam.

Blowdown is that removal. The rate required is a direct function of feedwater quality: a boiler on high-quality make-up needs far less blowdown than one running on partially treated water, for the same steam output.

Where the cost actually is

The water itself is usually the smallest part of the cost. Blowdown discharges water that has already been through every treatment step and has been heated all the way to saturation temperature at boiler pressure.

• The thermal energy in the discharged water, which leaves the system entirely unless heat is recovered.
• The treatment chemical dissolved in it, discharged at the concentration the boiler was holding.
• The cost of producing the replacement make-up — softening, filtration, deaeration and the chemical it needs.
• The fuel required to heat that replacement from make-up temperature back to saturation.

Continuous and intermittent blowdown

Continuous blowdown, usually drawn from the steam drum at the level where dissolved solids concentrate, controls the dissolved solids concentration. This is the flow that should be controlled on conductivity.

Intermittent or bottom blowdown is a short, sharp discharge from the lowest point, and its purpose is different: it removes settled sludge and suspended solids. It cannot be replaced by continuous blowdown, and a system that has stopped doing it accumulates sludge in the mud drum.

Confusing the two is common. A plant that increases continuous blowdown because it is finding sludge is spending thermal energy on a problem that a short bottom blowdown would address directly.

Getting the rate right

Blowdown on a fixed schedule is wrong in one direction or the other almost all the time. Steam demand varies, condensate return varies, and feedwater quality varies — so the required blowdown rate varies with them.

Conductivity-controlled blowdown holds dissolved solids at the specified limit continuously. When steam demand rises, blowdown rises with it; when condensate return improves, blowdown falls automatically because the feedwater is cleaner.

Two further improvements are usually available. Heat recovery from the continuous blowdown stream recovers part of the thermal loss. And improving condensate return reduces the required blowdown at source, because returned condensate is high-quality feedwater that carries almost no dissolved solids.