TB 45: Boiler feed water degassing lowers operations costs and chemical usage

ShenLan Environment Inc. located in Shanghai China uses Liqui-CelĀ® Membrane Contactors in their boiler
feed water treatment systems. These systems realize lower operating costs with the added benefit of reducing the chemicals added to the boiler feed water.

Proper treatment of boiler feed water is an important component of a boiler system. As steam is produced, dissolved solids become concentrated and deposit inside the boiler. This leads to poor heat transfer and efficiency reduction of the boiler. Dissolved gasses such as oxygen and carbon dioxide will react with the metal surfaces inside the boiler promoting corrosion. Degassing is an important step for protecting the boiler.

Liqui-Cel Membrane Contactors offer many advantages over forced draft deaerators, vacuum towers, and chemical treatment programs for feed water treatment. Membrane Contactors utilize microporous membranes to create 10X the surface area compared to mechanical technologies. Contactors are highly efficient, compact and can be used inline under pressure.

Chemical Treatment
Chemical treatment is widely used to control dissolved oxygen in a boiler. The cost of operating a chemical treatment program consists of chemical costs and blow down costs. Periodically the water in the boiler must be flushed out to remove non-volatile compounds. They are flushed out of the boiler in a process called blow down. Chemical addition to the water can increase the frequency of blow down, which increases the operating cost of the boiler.

There are two components of blow down costs. Water and steam that is purged from the boiler during blow down is sent to drain. This water must be replenished by fresh makeup water and there is a cost associated with it. The second cost is heat or energy cost. The water blow down from the boiler is hot. It is replaced with cold water that must be reheated in order to produce steam.

Example Using Membrane Contactors
Membrane Contactors can be used to remove the dissolved oxygen from water. By removing the dissolved oxygen the volume of chemicals added to the boiler will be reduced. By reducing the chemicals added to the boiler the frequency of blow down can be potentially reduced. The example in figure 1 compares operating costs of two systems. One system is a chemical only treatment system with a blow down rate of 10%. The other system assumes that the oxygen content of the feed water is reduced to 0.5 ppm and that the blow down rate can be reduced to 5% due to the reduction of chemicals in the boiler.

The boiler specifications used in this example are for reference. These calculations can be modified in order to apply them to boilers with different operating conditions.

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