Chemical treatment of make up water increases the TDS levels in the boiler, leading to increased blowdown. Returning condensate
reduces the TDS levels and thus blowdown, leading to fuel savings.
Fresh water added to the boiler needs to undergo chemical treatment. Among
many other factors, this is necessary to prevent scaling in boiler tubes because
of hardness in water and to maintain the pH level. This chemical treatment
may either occur in the softening plants or by chemical dosing.
While chemical addition is necessary, it leads to the presence of suspended
solids in the boiler. These solids will eventually collect at the bottom of the
boiler and will have to be removed by bottom blowdown.
The blowdown water is at the same pressure and temperature as water in the
boiler drum and this heat is now drained. As fresh water addition increases, the
quantity of chemical in the boiler drum also increases. This raises the boiler
TDS, necessitating more blowdowns and increasing the quantity of associated
heat loss.
Condensate, being as good as distilled water, can be returned directly to the
feed water tank. Doing so maintains water quantity without addition of
chemicals. This reduces the TDS levels in the boiler, thereby reducing the
frequency of blowdowns. Lesser blowdowns translate to reduced loss of heat
energy. Besides reducing the cost of chemical treatment and cost of addition
of fresh water into the system, this leads to incremental energy savings.
Illustration: Consider a steam load of 6 TPH, of which 50% (3TPH) is indirect
consumption.
TDS of feedwater (soft water): 100ppm
TDS of Condensate: 10ppm
TDS maintained in boiler: 3500ppm.
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Case 1
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Case 2
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Condensate recovered
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Nil
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3TPH
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Avg TDS addition to boiler
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100 ppm
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(100+10) / 2 = 55 ppm
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Blowdown per day
|
4.2 TPH
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2.3 TPH
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