1. Boiler Designs
Introduction
So firstly, what is a boiler? And what is it used for? These are the two questions that pop
up as soon as we hear the word “boiler”. A boiler is a device used for converting a liquid
to vapour. A boiler consists of a furnace in which fuel is burned, surfaces to transmit heat
from the combustion products to the water (or other liquid), and a space where steam (or
vapour) can form and collect. A conventional boiler burns a fossil fuel or waste fuel; a
nuclear reactor may instead supply the heat. There are two types of conventional steam
boiler. In a fire-tube boiler, the water surrounds the steel tubes through which hot gases
from the furnace flow; easy to install and operate, fire-tube boilers are widely used to
heat buildings and to provide power for factory processes, as well as in steam
locomotives. In a water-tube boiler, the water is inside tubes, with the hot furnace gases
circulating outside the tubes; water-tube boilers, which produce more and hotter steam,
are used in ships and factories. The largest are found in the central-station power plants of
public utilities; other large units are used in steel mills, paper mills, oil refineries, and
chemical plants. Now let’s look at the main two types of boilers, ie, the water tube and
the fire tube boilers.
1. Water tube boiler- The industrial water tube boiler typically produces steam or
hot water. In the water tube design, tubes contain steam and/or water and the
products of combustion pass around the tubes. Typically, water tube designs

2. consist of multiple drums. A steam drum (upper) and mud drums (lower) are
connected by the tubes, which form both the convection section and the
furnace area. Packaged industrial water tube boilers are typically rated in
pounds of steam per hour output.
Small water-tube boilers
may be manufactured and assembled into a single unit, just like packaged shell boilers,
whereas large units are usually manufactured in sections for assembly on site.
Many water-tube boilers operate on the principle of natural water circulation.

3. In this diagram, Cooler feedwater is introduced
into the steam drum behind a baffle where, because the density of the cold water is
greater, it descends in the 'down comer' towards the lower or 'mud' drum, displacing the
warmer water up into the front tubes. And then, continued heating creates steam bubbles
in the front tubes, which are naturally separated from the hot water in the steam drum,
and are taken off. However, when the pressure in the water-tube boiler is increased, the
difference between the densities of the water and saturated steam falls, consequently less
circulation occurs. To keep the same level of steam output at higher design pressures, the
distance between the lower drum and the steam drum must be increased, or some means
of forced circulation must be introduced.
A watertube boiler can also be divided into
several sections, namely-the radiant, convection and conduction sections. These are the
three major ways in which heat energy is obtained. The radiant/furnace section is an open
area accommodating the flame from the burner. If the flames were allowed to come into

4. contact with the boiler tubes, serious erosion and finally tube failure would occur. The
walls of the furnace section are lined with finned tubes called membrane panels, which
are designed to absorb the radiant heat from the flame.
And now the convection section is designed to absorb the heat from the hot gases by
conduction and convection. Large boilers may have several tube banks (also called
pendants) in series, in order to gain maximum energy from the hot gases.
There are several main designs of watertube boilers
which allow better heat transfer and performance. Some these types are:-

5.  Longitudinal drum boiler- The longitudinal drum boiler was the original type of
water-tube boiler that operated on the thermo-siphon principle. Cooler feedwater
is fed into a drum, which is placed longitudinally above the heat source. The
cooler water falls down a rear circulation header into several inclined heated
tubes. As the water temperature increases as it passes up through the inclined
tubes, it boils and its density decreases, therefore circulating hot water and steam
up the inclined tubes into the front circulation header which feeds back to the
drum. In the drum, the steam bubbles separate from the water and the steam can
be taken off.
 Cross drum boiler- The cross drum boiler is a variant of the longitudinal drum
boiler in that the drum is placed cross ways to the heat source. The cross drum
operates on the same principle as the longitudinal drum except that it achieves a
more uniform temperature across the drum. However it does risk damage due to
faulty circulation at high steam loads; if the upper tubes become dry, they can
overheat and eventually fail. The cross drum boiler also has the added advantage
of being able to serve a larger number of inclined tubes due to its cross ways
position.

6.  Bent tube boiler- Again this operates on the principle of the temperature and
density of water, but utilises four drums.
Cooler feedwater
enters the left upper drum, where it falls due to greater density, towards the lower,
or water drum. The water within the water drum, and the connecting pipes to the
other two upper drums, are heated, and the steam bubbles produced rise into the
upper drums where the steam is then taken off. The bent tube boiler allows for a
large surface heat transfer area, as well as promoting natural water circulation.
There are many advantages and disadvantages to watertube boilers. The main
advantages are that they have small water content, and therefore respond rapidly
to load change and heat input and the small diameter tubes and steam drum mean
that much higher steam pressures can be tolerated. Above all, the design may
include many burners in any of the walls, giving horizontal, or vertical firing
options, and the facility of control of temperature in various parts of the boiler.
This is particularly important if the boiler has an integral superheater, and the

7. temperature of the superheated steam needs to be controlled. Now that the
advantages are discussed, in come the several disadvantages of a watertube boiler.
Primarily, they are not as simple to make in the packaged form as shell boilers,
which mean that more work is required on site and when multiple burners are
involved, complex and expensive control systems are required to be installed.
Now as we’ve discussed briefly about the watertube boiler and some of its designs, let’s
take a look at the firetube boiler and its designs.
2. Firetube boiler- The firetube boiler is a cylindrical vessel, with the flame in
the furnace and the combustion gases inside the tubes. The furnace and tubes
are within a larger vessel, which contains the water and steam. Firetube
boilers are available for low- or high-pressure steam, or for hot-water
applications. Because of its vessel size, the firetube contains a large amount of
water, allowing it to respond to load changes with minimum variation in
steam pressure. Firetube boilers are usually built similar to a shell and tube
heat exchanger. A large quantity of tubes results in more heating surface
which greatly improves heat transfer and efficiency. The furnace and the
banks of tubes are used to transfer heat to the water. Combustion occurs
within the furnace and the flue gases are routed through the tubes to the stack
outlet. Firetube boilers are available in two, three and four pass designs. A
single “pass” is defined as the area where combustion gases travel the length
of the boiler. Generally, boiler efficiencies increase with the number of passes.
There are two main designs of firetube boilers. They are:-

8.  Dryback firetube boiler- In the dryback boiler, a refractory-lined
chamber outside of the vessel is used to direct the combustion gases
from the furnace to the tube banks. The “two pass-dry back” design
has a very high thermal efficiency and basically this design has a
cylindrical outer shell containing two large-bore corrugated furnace
flues acting as the main combustion chambers. The hot flue gases
passes out of the two furnace flues at the back of the boiler into a
brickwork setting /dry back and is deflected through a number of
small-bore tubes arranged above the large-bore furnace flues. These
small bore tubes present a large heating surface to the water. The flue
gases pass out of the boiler at the front and into an induced draft fan,
which passes them into the chimney.
 Wetback firetube boiler- In the wetback boiler design has a water-
cooled turn around chamber used to direct the flue gases from the

9. furnace to the tube banks. The wetback design requires less refractory
maintenance. The wetback design is more prone to water side sludge
build-up, because of the restricted flow areas near the turn around
chamber. A wetback boiler configuration allows for a more efficient
reversing of the hot gases than a “dryback” design.
A three pass wetback design
Now that we have briefly discussed the main two types of firetube boilers, let’s look at
the advantages and disadvantages:-
The advantages of a firetube boiler are:-
 The package arrangement of the firetube/shell design also means that it is simple
to relocate.

10.  A substantial amount of water is stored at saturation temperature, so this allows
rapid pick up of load.
 Less maintenance is required
 Control systems are usually fairly simple unlike in some watertube boiler
designs.
The disadvantages of firetube boilers are:-
 The capacity of a firetube boiler is limited due to its shell (configuration), so
several boilers should be connected together
 A watertube boiler is more suited for high pressure applications than a firetube
boiler.
In conclusion, it can be said that there are several type of boilers with varying and
interesting designs. But the selection of the type of boiler and the preferred design
entirely depends on the process its going to be used for. For example, for high pressure
applications a “watertube” boiler may be used instead of a “firetube” boiler as its
construction is sturdier and rugged which is suited to higher pressures.

11. Name-Anand Sasikumar
Assignment- # 2
Instructor-Keith Munroe
Sub-Boiler Design
POE-334