% Blow down = quantity of ...

% Blow down = quantity of blowdown water / quantity of feed water x 100. Blowdown rate varies widely with feed water quality. It can be as low as 1% going upto 20% of feed water. I am assuming that all good practices for pre-treatment of feed water are in place, and still you want to reduce blowdown. Poor control or judgement can further add extra blowdowns. From operating economy point, frequent short blowdowns are preferred to infrequent lengthy blows. Generally, in low pressure boilers, bowdown controls are manual and poor with wide range of conductivity. Changing from a manually adjusted continuous blowdown to automatically controlled continuous blowdown  can save about 20% blowdown.