Page 32 - Boiler plant and distribution system optimization manual

Boiler Plant Orientation 17

Figure 1.18—Conversion factors for boiler horsepower. In smaller boilers the capac-
ity is measured in steam produced at 212 F. Larger boiler capacity is usually given
in (000)lb of steam evaporated per hour at specified conditions of pressure and tem-
perature. A recent trend for rating large boilers is kW or mw of the turbine generator.

Figure 1.19—The three Ts of
combustion: Time, Temperature
and Turbulence. As the carbon
content of fuel increases the 3 Ts
become more important in the
design and operation of burners
and firing equipment.

Turndown is the measure of the maximum firing rate work it does is replace the heat lost to the envi-
compared to the minimum firing rate. [See Figure ronment during the shutdown.
1.20.] With larger boilers taking a boiler off line When working with other boilers and in re-
can be a complex and time consuming affair. Be- sponding to variable plant steam demands a large
ing able to reduce its firing rate provides an econ- turndown ratio has an advantage. Commercial
omy in fuel, man-hours and maintenance. Each boilers are not required to respond to large load
time a boiler is taken out of service and restarted, ranges and because of economic reasons, first cost
it must be purged with at least four changes of air competitiveness, the turndown is low about, 2-3 to
cooling it down, wasting stored heat and causing 1. Firetube boilers, because of their large water ca-
some thermal stresses from the cold air. The fuel pacity, can store a lot of energy allowing them to
used to bring it back to the point where steam is respond to large short term demands very quickly.
being exported again is wasted because the only This and the fact they are not designed for heavy