concerns the different designs and construction features
of fire-tube boilers.
The basis for identifying the two types is as follows:
WATER-TUBE BOILERS are those in which the
products ofcombustion surround the tubes through
which the water flows.
FIRE-TUBE BOILERS are those in which the
products of combustion pass through the tubes and
the water surrounds them.
Water-tube boilers may be classified in a number of
ways. For our purpose, they are classified as either
straight tube or bent tube. These classes are discussed
separately in succeeding sections. To avoid confusion,
make sure you study carefully each illustration referred
to throughout the discussion.
The STRAIGHT-TUBE class of water-tube boilers
includes three types:
1. Sectional-header cross drum
2. Box-header cross drum
3. Box-header longitudinal drum
In the SECTIONAL-HEADER CROSS DRUM
boiler with vertical headers, the headers are steel boxes
into which the tubes are rolled. Feedwater enters and
passes down through the downcomers (pipes) into the
rear sectional headers from which the tubes are supplied.
The water is heated and some of it changes into steam as it
flows through the tubes to the front headers. The
steam-water mixture returns to the steam drum through
the circulating tubes and is discharged in front of the
steam-drum baffle that helps to separate the water and
Steam is removed from the top of the drum through
the dry pipe. This pipe extends along the length of the
drum and has holes or slots in the top half for steam to
Headers, the distinguishing feature of this boiler. are
usually made of forged steel and are connected to the
drums with tubes. Headers may be vertical or at right
angles to the tubes. The tubes are rolled and flared into
the header. A handhold is located opposite the ends of
each tube to facilitate inspection and cleaning. Its
purpose is to collect sediment that is removed by blowing
down the boiler.
Baffles are usually arranged so gases are directed
across the tubes three times before being discharged from
the boiler below the drum.
BOX-HEADER CROSS DRUM boilers are shallow
boxes made of two platesa tube-sheet plate that is bent
to form the sides of the box, and a plate containing the
handholds that is riveted to the tube-sheet plate. Some are
designed so that the front plate can be removed for access
to tubes. Tubes enter at right angles to the box header and
are expanded and flared in the same manner as the
sectional-header boiler. The boiler is usually built with
the drum in front. It is supported by lugs fastened to the
box headers. This boiler has either cross or longitudinal
baffling arranged to divide the boiler into three passes.
Water enters the bottom of the drum, flows through
connecting tubes to the box header, through the tubes to
the rear box header, and back to the drum.
BOX-HEADER LONGITUDINAL DRUM
boilers have either a horizontal or inclined drum. Box
headers are fastened directly to the drum when the
drum is inclined. When the drum is horizontal, the
front box header is connected to it at an angle greater
than 90 degrees. The rear box header is connected to
the drum by tubes. Longitudinal or cross baffles can be
used with either type.
Bent tube boilers usually have three drums. The
drums are usually of the same diameter and positioned at
different levels with each other. The uppermost or
highest positioned drum is referred to as the STEAM
DRUM, while the middle drum is referred to as the
WATER DRUM, and the lowest, the MUD DRUM. Tube
banks connect the drums. The tubes are bent at the ends to
enter the drums radially.
Water enters the top rear drum, passes through the
tubes to the bottom drum, and then moves up through the
tubes to the top front drum. A mixture of steam and water
is discharged into this drum. The steam returns to the top
rear drum through the upper row of tubes, while the water
travels through the tubes in the lower rear drum by tubes
extending across the drum and enters a small collecting
header above the front drum.
Many types of baffle arrangements are used with
bent-tube boilers. Usually, they are installed so that the
inclined tubes between the lower drum and the top front
drum absorb 70 to 80 percent of the heat. The water-tube
boilers discussed above offer a number of worthwhile
advantages. For one thing, they afford flexibility in
starting up. They also have a high productive capacity