Page 55 - Boiler plant and distribution system optimization manual

40 Boiler Plant and Distribution System Optimization Manual
Figure 4.7—Stack losses measured using
the Heat Loss method.
BLOWDOWN LOSSES
Blowdown losses are often overlooked be-
cause they are hard to measure and water chemis-
try work is not always fully understood by plant
personnel. As shown in Figure 4.8., this boiler has
a blowdown rate of 4% of its total steam output.
Hot water is being dumped into a drain or sew-
er system and excess heat is being vented to the
atmosphere as flash steam. This amounts to 6,000
lb/hr of very hot water which is responsible for
an energy loss of 1.57%.
Figure 4.8—Blowdown losses
SURFACE LOSSES
Another significant loss is the boiler surface
loss (Figure 4.9), caused by radiation and convec-
tion from the boiler surface and valves and piping
in the boiler plant. Usually this loss is between 1
and 3 percent but this number can not be general-
ized.
The percentage of this loss grows as boiler
load decreases. When the boiler load starts to
drop off, this percentage continues to go up be-
cause the boiler is still losing the same amount
of heat but the output is less so the loss is a big-
ger percentage at low firing rates could be 8 to 10
percent. This becomes a real significant problem
of their design output capacity. Figure 4.9 shows
the percentage loss of this boiler at 100% of its full
rated output.
Figure 4.9—Radiation and convection losses from the
boilers surface
Figure 4.10 illustrates a method of survey-
ing a boiler for surface losses using infrared mea-
surements. These measurements of both surface
temperature and surface heat losses are useful for
estimating energy losses and for planning main-
tenance. Changing surface temperatures will in-
dicate casing and insulation failures. A modern
spread sheet can be used to determine total and
average heat losses and record changes over time.
Figure 4.11 illustrates how this radiation
and convection losses increase as the boiler load
goes down. The lower plot represents radiation
and convection losses from the boilers exterior
surface, the upper curve shows the effect of on/
off operation at the same loads. When a boiler
stops firing a purge of at least 4 air changes is
necessary to sweep out any remaining fuel va-
pors. This same purge is required on light off.