How much Blow-down is Needed?
This depends on how many concetrations of the various feed-water impurities a given boiler can tolerate;the more concentra-tions possible the less blow-down needed. For example, with 10 feedwater concentrations in a boiler, blow-down equal to 10 per cent of the feed-water flow rate is needed;with 20 concentrations only 5 per cent blow-down is needed. To illustrate how blow-down requirements are calculated let us assume that the maximum amount of suspended solids (sludge) in the boiler water that a particu-lar boiler can tolerate is 500 p.p.m. If the fee-water contains 50 p.p.m. of hardness it can be concentrated only about 10 times (since feed water hardness is precipitated as suspended solids in the boiler water). This means that for every 50 kg of water fed to the boiler about 5 kh of boiler water must be blown down to keep the suspended solids from exceeding 500 p.p.m. Suspended solids, however, may not be the limiting factor in all cases; other factors which may limit feed-water co
