Page 161 - Boiler plant and distribution system optimization manual

146 Boiler Plant and Distribution System Optimization Manual

level of concentration is regulated by the percent- 4. AUTOMATIC BLOWDOWN CONTROL
age of blowdown. If a low level of concentration
Cost
is maintained, then the blowdown rate could be
Moderate
very high, wasting energy (Figure 10.39). High
cycles of concentration require less blowdown Potential Savings
and less energy for make up water heating. Moderate (2%-3%)

Advantages/Disadvantages Description
The higher the cycles of concentration to be When high temperature, high energy blow-
maintained, the closer the scale formation limit down water is not closely controlled, energy is
is approached so good control is required. If this wasted by dumping too much water out of the
process is not closely regulated, there is a chance boiler at one time (Figure 10.41). This water then
for scale to form in the boiler which could lead must be replaced by cold make up water. Lack
to tube failure or costly cleaning. The general of control may also cause the dissolved solids
practice has been to stay away from higher lev- concentration to rise above the scaling level. Au-
els of concentration if good control could not be tomatic control eliminates these problems and
assured, this can waste money, energy and water saves water, chemicals and energy.
(Figure 10.40) so the pros and cons should be care-
fully evaluated. Feedwater purity is another issue Advantages/Disadvantages
which must be evaluated to really understand This option requires good reliable equip-
the situation. If make-up feedwater has high im- ment which must be kept calibrated.
purities, then higher cycles of concentration will
be approached more quickly whereas very pure
5. MECHANICALLY SCALE REMOVAL
feedwater allows very high cycles of concentra-
tion, eliminating blowdown losses and make up Cost
water heating to a large degree. Low (essential maintenance)

Figure 10.39—Cycles of Concentration.
The purer the feedwater, the less blow-
down required and the higher cycles of
concentration that can be maintained.
For example, if feedwater quality is
improved to increase the concentration
from 3 to 6 times, the blowdown rate for
a 100,000 lb/hr boiler is reduced from
33.3 to 16.7 percent, or about 30,000 lb/
hr of hot water drained from the boiler.