Calcmeister

Boiler blowdown calculator

Enter the steam output, the dissolved solids in the feedwater and the limit for the boiler water. The calculator gives the blowdown you need and what it costs.

kg/h
ppm

Total dissolved solids in the water entering the boiler, after mixing make-up water and condensate.

ppm

From the boiler manufacturer. Shell boilers often 2,000 to 3,500 ppm.

bar
°C
h
%
per kWh
Blowdown rate
– kg/h
Blowdown share of feedwater
– %
Heat lost
– kW
Fuel cost per year
–

    How the blowdown rate is calculated

    When water evaporates in a boiler, the dissolved solids stay behind and concentrate. To keep them below the limit, part of the boiler water must be drained continuously. In steady operation the solids coming in with the feedwater equal the solids leaving with the blowdown:

    Blowdown = Steam output · F ÷ (B − F)

    Heat loss = Blowdown · (hf − hfeed)

    F
    total dissolved solids (TDS) in the feedwater, ppm
    B
    maximum TDS allowed in the boiler water, ppm
    hf
    enthalpy of saturated water at boiler pressure
    hfeed
    enthalpy of the feedwater, about 4.19 × temperature in °C

    Worked example

    A boiler produces 5,000 kg/h of steam at 10 bar g. The feedwater carries 150 ppm TDS and the boiler water may hold 3,000 ppm. The blowdown is 5,000 × 150 ÷ 2,850 = 263 kg/h, or 5 % of the feedwater. Each kilogram leaves as saturated water at 184 °C carrying 780 kJ, against 335 kJ in the 80 °C feedwater. The heat lost is 32.6 kW, around 10,900 per year at 0.05 per kWh of fuel and 90 % boiler efficiency.

    Recovering blowdown heat

    Blowdown water at boiler pressure partly flashes into steam when it drops to low pressure. A flash vessel recovers this steam for the deaerator or feed tank, typically 10 to 15 % of the blowdown mass. The remaining hot water can preheat make-up water in a heat exchanger. Together, flash and heat recovery can save 50 to 80 % of the blowdown heat.

    Reducing blowdown

    Questions and answers

    How do I calculate boiler blowdown?

    Blowdown rate = steam output × F ÷ (B − F), where F is the feedwater TDS and B the maximum allowed boiler water TDS. With 150 ppm feedwater and a 3,000 ppm limit, a 5,000 kg/h boiler needs about 263 kg/h of blowdown.

    What is a normal blowdown percentage?

    Typically 1 to 8 % of the feedwater. High values point to poor feedwater quality or low condensate return.

    How can I reduce blowdown losses?

    Return more condensate, improve make-up water treatment (softening, reverse osmosis), use automatic TDS control instead of manual blowdown, and recover heat with a flash vessel and heat exchanger.

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    Last reviewed: 2026-10-14. Results are estimates for planning. Check them before you rely on them for design, purchase or safety decisions.