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. This chemical treatment may either occur in the softening plants or by chemical dozing.
While chemical addition is necessary, it leads to the presence of suspended solids in the boiler. These will inevitably collect in the bottom of the boiler and 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 and we now have to give more blowdowns, increasing the quantity of associated heat loss.
Condensate being as good as distilled water, it can be returned directly to the feed water tank. Doing so maintains water quantity without addition of chemicals. This reduced the TDS levels in the boiler there by reducing the frequency of blowdowns. Lesser blowdowns translate to reduced loss of heat energy. Besides reducing the costs of chemical treatment and cost of addition of fresh water into the system, this leads to incremental energy savings.
Consider a steam load of 6 Tons/hr and of which 50% (3TPH) is indirect consumption.
TDS of feedwater (soft water): 100ppm
TDS of Condensate: 10ppm
TDS maintained in boiler: 3500ppm.