In Boiler, Water is evaporated to convert it into steam. During evaporation salts gets concentrated in the boiler drum. The salts in the boiler drum needs to be removed to operate boiler in safe and efficient condition.
Based on the pressure condition of boiler drum or pressure rating of boiler, concentration of salt is limited. To avoid excess concentration of salt in the boiler, concentrated water is blown out from the boiler which is called as boiler blow down.
During boiler blow down, Boiler water is removed at high temperature condition, ie equivalent to steam temperature. The concentrated water leaving boiler is the area of direct energy loss.
Water coming out from the boiler is good source of heat which can be recovered and utilized at boiler deaerator or feed water preheating.
Boiler water coming out from boiler blowdown may be fed to flash separator, where flash steam can be generated and used at deaerator directly.
It may be noted that during manual blowdown, heat is released for a short interval of time, therefore flash separator and heat recovery design becomes difficult and expensive. It is recommended to have water quality based on auto blow down for better and consistent recovery of energy from boiler blowdown.
Auto or TDS based blowdown also helps in monitoring and maintaining desired quality of water in the boiler drum.
Schematic arrangement of boiler blowdown and heat recovery is shown below.
Potential of Heat recovery :
Heat recovery potential differs from case to case basis because TDS limit depends on the boiler pressure, For low pressure boiler operating concentration can be as high as 3500 mg/lit,
For boiler water specification refer below link :-
Based on the feed water quality, total blowdown water requirement can be estimated by
Blowdown Water Qty (kg/hr) = F / (B-F) X S
F= Feed TDS
B = Blowdown TDS
S = Steam Qty (Kg/hr)
With the right blowdown qty heat saving can be calculated.
Below attached excel sheet can be used for calculating Boiler blow down, flash steam, condensate, and saving calculation:-
Boiler blowdown heat recovery is easy and effective method of energy conservation and must be planned where there is opportunity.
In general investment in boiler auto blowdown and heat recovery gets recovered within a year.