Blowdown in Boiler | Boiler Blowdown Calculation Formula & Calculator

Blowdown in Boiler Definition

Blowdown in Boiler, Usually, water which is fed to the boiler contains high Total Dissolved Solids (TDS) content as well as other dissolved and undissolved solids in it. On heating water or converting water to steam, these dissolved solids do not evaporate and gets separated from water or steam and settled at the bottom of the shell.

These undissolved solids are also responsible for the scaling, corrosion and carryover of solids with the steam and other specific problems. This will further prevent the transfer of heat between the gases and the water and will eventually overheat the boiler tubes or shell.

Blowdown in Boiler & Calculations

Source: Campbell

So some part of the water from the shell bottom is drained either intermittently or continuously to keep the TDS level of water under allowable operating limits. Therefore removing of TDS or impure water from the boiler shell is termed as blowdown.

Blowdown in Boilers

There are two ways of blowdown depending upon the type of design, a capacity of a boiler as well as the characteristics of a boiler feed water:-

  • Intermittent or Manual blowdown – When blowdown is done manually by the boiler operator without keeping at regular intervals is termed as Manual blowdown. This type of blowdown is useful to remove sludge formation or suspended solids from the boiler. In this blowdown valve is opened manually at regular intervals according to the set operating schedule. Although to reduce the heat losses in terms of hot blowdown water, the valve is opened frequently and slightly to allow an only small quantity of blowdown to occur but this kind of blowdown is responsible for high heat and pressure losses.

 

  • Continuous blowdown – When blowdown is done continuously and automatically to keep the dissolved and suspended solids under boiler operating limits are known as continuous blowdown. An automatic blowdown continuously monitors the quality of feed water as well as the quality of water inside a boiler shell and automatically opens as soon as the boiler water TDS exceeds the permissible operating limit.This type of blowdown helps remove a maximum amount of dissolved solids with only minimal loss of water and heat from the boiler. Also in continuous blowdown heat in terms of hot water from the boiler can be utilized to preheat the feed water by installing a heat exchanger or heat recovery equipment in the path.

Depending on the type of operation either manual or continuous and automatic, blowdown valve is provided along with suitable accessories.

Pros:-

  • Blowdown of water keeps the water TDS level under permissible limits.
  • It prevents the corrosion and scaling of boiler tubes.
  • It prevents the carryover of contaminants from the steam thereby providing the pure steam.

Cons:-

  • Blowdown of water increase the heat and pressure losses if not been taken care or proper scheduling is not done.
  • Eventually, these heat and pressure losses reduce the boiler efficiency.

Also Read: High Pressure Boiler

Boiler Blow Down Calculation Formula

The blowdown rate can be calculated with Following Formula

qBD = qS fc / (bc – fc) 

Where:

qBD is blowdown rate in kg/h

qS  is steam consumption in kg/h

FC is Total Dissolved Solids – TDS – in the feed water  is ppm

bc is maximum allowable Total Dissolved Solids – TDS – in the boiler water in ppm

Boiler Blowdown Calculation: Online Calculator

To Calculate Blowdown in boiler you need to put data in following Boiler Blowdown calculator online calculator:

Check out other Boiler Calculators created by Thermodyne technical team

 

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