Page 16 - Boiler plant and distribution system optimization manual

Chapter 1

Boiler Plant Orientation

[Fast Track]

MODERN BOILERS dom detected.
When water is heated to form steam we are
Modern boiler designs go back well over 100 restricted to certain temperatures for the steam.
years. It has been a century of changes in design, This pressure-temperature relationship is called
construction and operation of boiler plants. Pres- the saturation temperature. Simply stated, all of
sure vessel design has advanced from riveted to the heat transferred to the water just generates
welded construction. Natural draft firing using more steam. If we want to raise the temperature
tall brick stacks has given way to forced draft and of the steam above the saturation temperature we
balanced draft operations. Controls have taken take the water out of the process and apply heat
over most of the tedious watchstander functions to steam only. The steam now becomes superheat-
and now computerized systems can almost think ed as shown on the right-hand side of Figure 1.1.
for plant personnel. When steam is generated in a pressure vessel,
the same process occurs but conditions change.
Figure 1.2 illustrates when steam is generated at
GENERATING STEAM 3,000 psi (200 Bar). The saturation [boiling] tem-
perature is now 695°F. It takes a lot more Btus to
Boilers must deliver dry clean steam at a heat the water to the boiling point, but it takes
specified pressure and temperature. Figure 1.1 fewer Btus to form a pound of steam. At atmo-
shows the basic steam generation cycle. The Brit- spheric pressure it takes 970 Btus and at 3,000 psi
ish thermal unit (Btu) is a term used to identify it takes only 213 Btus. The difference is the 802
the heat value of fuel and steam. The scale begins Btus needed to bring the water up to the boiling
at the freezing point for water or 32°F. Adding point at 695°F for this high pressure.
one Btu to a pound of water will raise its temperature Table 1.1 illustrates the relationship of pres-
1°F. It takes 180 Btus to heat water from the freez- sure and energy values. It shows the information
ing point to the boiling point, but we don’t have available from standard steam tables, a source of
steam yet. At atmospheric pressure it takes an essential information about the properties of
additional 970 Btus to form one pound of steam. steam. Engineers use steam table values for the
This steam is the energy that goes to work for us. design and operation of steam plants.
When the pound of steam, with its 970 Btus, gives
up its heat in the steam distribution system and
condenses back into water, its volume shrinks TYPES OF BOILERS
tremendously to 1/700 its steam volume. High
pressure replacement steam rushes in to fill the Package boilers are used for most commer-
void caused by the shrinkage in volume caused cial and smaller industrial operations. They are
by the steam to water phase change. Because this built at a factory and then shipped to the plant
process is continuous and the replacement steam via truck, barge and railroad. They are usually
moves in very quickly, pressure changes are sel- complete units which need only be connected up
1