UFC 3-240-13FN
25 May 2005
content of the remaining water. As steam is produced, additional mineral-laden
feedwater enters the boiler, resulting in additional minerals being introduced into the
boiler and increasing the amount present in the boiler. Mineral content in an operating
boiler water system can be limited only by blowdown.
3-1.5.2
Blowdown. Blowdown consists of draining some of the boiler water with
its accumulated solids, and replacing it with treated feedwater before harmful levels of
solids are reached. Dissolved solids tend to concentrate near the water surface in the
steam drum. Therefore, surface blowdown is most effective in reducing the
concentration of dissolved solids. Bottom blowdown is used to remove precipitated
sludge from the boiler mud drum. However, blowdown results in the loss of heated
water and treatment chemicals. Economical operation requires careful control of
blowdown to maintain sage solids levels, while minimizing both heat and chemical
additive losses.
3-1.5.3
Water Balance for Feedwater, Evaporation, and Blowdown. The total
volume of the water (feedwater) that is added to the boiler must equal the total volume
of the water (steam plus boiler water blowdown) that is removed from the boiler. By
convention, these water quantities are commonly expressed in kilograms per second
(pounds per hour) in this equation for water balance:
EQUATION
F=E+B
(1)
where
F = feedwater, kg/s (lb/hr)
E = steam generated, kg/s (lb/hr)
B = blowdown, kg/s (lb/hr)
3-1.5.4
Calculation of Feedwater and Blowdown Water. The feedwater or
blowdown water (or both) can be calculated in relation to the COC using these
equations:
COC = F B, or F = B x COC
EQUATION
or
(2)
B = F COC
where
COC= cycles of concentration, no units
F= feedwater, kg/s (lb/hr)
B= blowdown, kg/s (lb/hr)
It is common to express blowdown as a percentage:
EQUATION
% B = 100/COC
(3)
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