Page 18 - Boiler plant and distribution system optimization manual

Boiler Plant Orientation 3

Table 1.1—This is an example of energy values [Btu/lb] of steam in various
phases as it is heated to superheated steam at 1,000°F from the freeze point.
Note that as pressure goes up the Btu content of the water rises significantly
and the Btus necessary to form a pound of steam decrease significantly.

to electrical power, fuel piping, steam piping and A long-standing practical limit on their size has
exhaust systems. been about 250 psig and 1,000 HP.
Field erected boilers, because of their large Figure 1.4 illustrates a very important fact
size, have to be built on-site using components of boiler pressure vessel design. The force the
either shipped by the manufacturer or fabricated pressure parts must be designed to withstand is
locally. determined by the product of pressure and area.
The large diameter drum of the fire tube boiler
produces large forces requiring a thick steel shell
FIRETUBE BOILERS to retain the pressure limiting their size.
Higher pressure construction requires the
An old and very useful boiler type is the fire- use of the water tube design where smaller diam-
tube boiler as shown in Figure 1.3. It consists of a eter tubes and drums are used. Because they have
large diameter cylinder filled with water. Tubes smaller cross sectional areas which can withstand
extending from end to end serve as the combus- much higher pressures.
tion chamber and path for hot gasses inside the
pressure vessel. It gets its name, fire tube, from
this fact. The firetube boiler design is very useful WATER TUBE BOILERS
because of its large amount of stored energy in the
large cylindrical design. When pressure drops, By using smaller pressure parts, the water
the stored energy converts to steam, meeting sys- tube design can withstand much higher pressures
tem demands. This characteristic makes burner than the firetube boiler. The water tube boiler de-
and combustion control system design simpler sign contains much less water than the firetube
and less sophisticated. Unfortunately, because of design and need much more responsive control
the large drum diameter typical of fire tube boil- systems to safely control water level and firing
ers, it has pressure and horsepower limitations. conditions.