Page 158 - Wastewater Solids Incineration Systems

Heat Recovery and Reuse 125

are in the flue gas. To prevent corrosion, the metal surface of the casing and tube
walls must be above the dewpoint. This is accomplished by keeping the fluid tem-
perature above the dewpoint. The range of boiler sizes in this service ranges from
approximately 2300 to 23000 kg/h (5000 to 50000 lb/hr) of steam, with pressures
from 410 to 4100 kPa (60 to 600 psig).

4.5.1 Firetube Boilers
In a firetube boiler, hot waste gases pass through straight tubes surrounded by a
cylindrical shell holding water that is heated to the boiling point. The construction is
simple, and the design offers several advantages. Because the shell is inherently tight,
the unit is well suited for gases under pressure. No internal refractory is required for
insulation because the shell is completely filled with water. Units are supplied as
packages that require minimal space.
The water circulation is self-induced. The less-dense mixture of steam bubbles in
water rises up through “risers” to the steam drum where it is separated from the
water. The separated water is denser than the steam-water mixture and sinks back
down through “downcomers” to the heated steam generating surfaces. If a boiler
design cannot generate enough natural circulation, then a pump may be used to
increase and control the flow to protect the heating surfaces. In some horizontal,
packaged firetube boilers, the shell acts as the steam drum, and risers and down-
comers are just flow paths inside the shell. Feedwater is added to the water in the
steam drum to maintain a normal water level in the drum.
Firetube boilers can be designed for steam pressures up to approximately 1720
kPa (250 psig), but cannot provide superheated steam or steam above the satura-
tion temperature. Consequently, their applications are limited to small capacities.
Horizontal tubes also will allow some particulate to settle out in the tubes and
reduce performance if velocities are too low. Use of FGTT designs with vertical
tubes avoids this problem.
4.5.2 Watertube Boilers
The design of the watertube boiler is the reverse of the firetube boiler. Hot gases are
in contact with the outside surface of the tubes and boiler water and steam are in con-
tact with the inside surface of the tubes. Figure 6.7 is a schematic diagram of a typical
watertube boiler.
Because the outside surfaces of the tubes are accessible to soot blowers for
cleaning, this type of boiler is better suited for flue gases with high solids loading.