Maintaining the right level of water in the feedwater tank reduced the overall boiler feedwater TDS. This leads to reduced blowdown and the associated blowdown loss.
Often the size of the feedwater tank is in multiples of the boiler capacity. For example a 10 TPH boiler has a 20 TPH feedwater tank. Such engineering though offers larger water storage, reduces the associated benefits of condensate recovery i.e. gain in temperature and lower feedwater TDS.
Expanding on the second point, with a larger feedtank the mass of makeup water present during start up is large. Condensate, being nearly distilled water, has a low value of TDS. However, because of the large quantity of the make up water already present in the feedtank, the average feedwater TDS remains high.
The higher the feedwater TDS, greater the quantity of blowdown that would be needed to be given to maintain the TDS level in the boiler drum. Though continuous condensate recovery will lower the feed water TDS, increased blowdown will lead to higher make up water addition and works in the opposite direction.
As a rule of thumb, the feedwater tank should be sized to 1.5 times the peak steam demand. Also appropriate level should be maintained in the feedtank to achieve the maximum benefit of condensate recovery.
Consider a 10TPH boiler running at a steam load of 5 TPH. The direct steam requirement in the plant is 1.5TPH and indirect is 3.5TPH, 100% of which is recovered. Therefore condensate recovery is 3.5TPH. The TDS levels of make up water are 250ppm and condensate at10ppm.