Blowdown water is at the same pressure and temperature as water in the boiler drum. Thus, by recovering this heat we can save on the blowdown losses.

The boiler feedwater contains suspended particles in the form of impurities and chemicals used for water treatment. These will inevitably collect at the bottom of the boiler in the form of sludge, which is removed by a process known as bottom blowdown.

Additionally, the water in the boiler drum contains dissolved solids. Their concentration increases as steam is produced and the boiler needs to be purged regularly. This purging is achieved during blowdown at which time some water is drained and fresh water gets added.

This blowdown water is at the same pressure and temperature as water in the boiler drum. Thus substantial potential for recovering heat exists.

For small process boilers (up to 20TPH), it is possible to recover the heat in the blowdown water by separating the flash steam from the blowdown before draining it. For large boilers (20TPH and above) we can additionally look at recovering the blowdown heat via a heat exchanger. These methods minimise the heat loss through blowdown, saving fuel.

Illustration:

Consider the following case:

The boiler has 2.9TPD of blowdown. This blowdown is at 10.5 bar (190°C) and if we flash it at 1.5 bar (120°C), we would have about 338 Kg/day of flash steam.

Returning this flash steam will save 7.7 tons of fuel per year!

Related Links: Boiler Blowdown Control System (BBCS)