Rate of Blowdown

Definition - What does Rate of Blowdown mean?

Rate of blowdown is a rate normally expressed as a percentage of the water feed.

In the case of boilers, rate of blowdown is the removal rate of water from a boiler. Its purpose is to control boiler water parameters within prescribed limits to minimize:

  • Scale
  • Corrosion
  • Carryover
  • Other problems

Blowdown is also used to remove suspended solids from the system.

Insufficient blowdown may lead to carryover of boiler water into the steam, or the formation of deposits. Excessive blowdown wastes energy, water and chemicals.

Corrosionpedia explains Rate of Blowdown

The rate of blowdown is the intentional periodic removal of water from the cooling tower, boiler or any other heat exchangers. The primary purpose of blowdown is to maintain the solids content of boiler water within certain limits. For boiler water, a high blowdown rate is required to eliminate the contaminants as rapidly as possible.

The blowdown rate required for a particular boiler depends on:

  • Boiler design
  • Operating conditions
  • Feedwater contaminant levels

In many systems, the blowdown rate is determined according to total dissolved solids (TDS). In other systems, alkalinity, silica or suspended solids levels determine the required blowdown rate. Variations in these factors change the amount of blowdown required, causing a need for frequent adjustments to the manually operated continuous blowdown system. Even frequent manual adjustment may be inadequate to meet the changes in operating conditions.

A blowdown of the boiler is a routine operation necessary due to the increased concentration of total dissolved solids in the boiler during steam production, which causes formation of sludge or sediments in the boiler, leading to degraded heat transfer. Therefore, water is periodically discharged or blown down from the boiler.

Minimizing blowdown not only reduces chemical treatment costs, but also energy loss in the boiler. Blowdown rates typically range from 4% to 8% of boiler feedwater flow rate, but can be as high as 10% when makeup water has a high solids content.

Blowdown rate can be calculated as:

    qBD = qS fc / (bc - fc)


    qBD = blowdown rate (kg/h)
    qS = steam consumption (kg/h)
    fc = total dissolved solids (TDS) in the feed water (ppm)
    bc = maximum allowable TDS in the boiler water (ppm)
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