Figure 4-3.—Downcomer tubes.
Figure 4-2.—Steam drum, water drum, and header.
and sidewall header. These tubes are the
is the sidewall header (fig. 4-2). Notice the header
is smaller than the water drum. Most boilers have
more than one header. They are identified by their
location. For example, a header at the back of
the boiler is called a rear wall header. A header
on a screen wall is called a screen wall header.
WATER DRUM
The water drum is larger than the header, but
both are smaller than the steam drum. The water
drum equalizes the distribution of water to the
generating tubes. Both the water drum and the
header collect the deposits of loose scale and other
solid matter present in the boiler water. Both the
drum and the header have bottom blowdown
valves. When these valves are opened, some of
the water is forced out of the drum or header and
carries any loose particles with it. DO NOT OPEN
THE BOTTOM BLOWDOWN VALVES ON A
STEAMING BOILER. Opening these valves will
interrupt the circulation of the steam cycle.
DOWNCOMER TUBES
At each end of the steam drum are a number
of large tubes (fig. 4-3) that lead to the water drum
downcomers through which water flows down-
ward from the steam drum to the water drum and
the header. The downcomers range in diameter
from 3 to 8 inches.
GENERATING TUBES
Many tubes link the steam drum to the
water drum and to the header. The tubes
that lead from the water drum to the steam
drum are the generating tubes (fig. 4-4).
They are arranged in the furnace so the
gases and the heat of combustion can flow
around them. The large arrows in figure 4-4
show the direction of flow of the combustion
gases.
The generating tubes are made of steel that
is strong enough to withstand the high pressures
and temperatures within the boiler. In most
boilers these tubes are usually 1 to 2 inches
in diameter, but some may be 3 inches. These
small tubes present a large surface area to
absorb furnace heat. A 2-inch tube has twice
the surface area of a 1-inch tube but four
times the volume. A 3-inch tube has three
4-6
