Firetube Boilers & Examples Watertube Boilers & Examples Boiler Fittings and Accessories Economisers & Superheaters


Boiler is a metal container in which a liquid is heated and changed into a vapor. Most boilers change water into the vapor steam. Steam is used to heat buildings and processes. It changes from vapor to liquid form as it delivers heat into a room or building, giving off even more heat as a result.

Fuel Uesd

The source of heat for a boiler is combustion of any of several fuels, such as wood, coal, oil, or natural gas. Electric steam boilers use resistance- or immersion-type heating elements. Nuclear fission is also used as a heat source for generating steam. Heat recovery steam generators (HRSGs) use the heat rejected from other processes such as gas turbines.

Steam Flow in Boilers


Firetube Boilers & Examples

The name firetube is very descriptive. The fire, or hot flue gases from the burner, is channeled through tubes that are surrounded by the fluid to be heated. The body of the boiler is the pressure vessel and contains the fluid. In most cases this fluid is water that will be circulated for heating purposes or converted to steam for process use.

Every set of tubes that the flue gas travels through, before it makes a turn, is considered a "pass". So a three-pass boiler will have three sets of tubes with the stack outlet located on the rear of the boiler. A 4-pass will have four sets and the stack outlet at the front.

Firetube Boilers are:

o Relatively inexpensive
o Easy to clean o Compact in size
o Available in sizes from 600,000 btu/hr to 50,000,000 btu/hr
o Easy to replace tubes
o Well suited for space heating and industrial process applications

Disadvantages of Firetube Boilers include:

o Not suitable for high pressure applications 250 psig and above
o Limitation for high capacity steam generation

Examples of Fire tube boilers

Cornish Boiler: This boiler has a long horizontal cylinder with a single large flue consisting the fire.

Lancashire boiler: Same as Cornish boiler, this has two large flues comprising the fires.

Scotch marine boiler: This form of boiler uses large number of small diameter tubes. It provides greater heating surface arena for both weight and volume.

Locomotive boiler: It has three major constituents, double walled firebox, horizontal and cylindrical boiler barrel consists with small flue tubes and smoke box provided with chimney for exhaust gases. They are vastly used in traction engines, steam rollers, portable engines and some other steam road vehicles.


Watertube Boilers & Examples

A Watertube design is the exact opposite of a fire tube. Here the water flows through the tubes and are incased in a furnace in which the burner fires into. These tubes are connected to a steam drum and a mud drum. The water is heated and steam is produced in the upper drum. Large steam users are better suited for the Water tube design. The industrial watertube boiler typically produces steam or hot water primarily for industrial process applications, and is used less frequently for heating applications.

Watertube Boilers are:

o Available in sizes that are far greater than the firetube design. Up to several million pounds per hour of steam.

o Able to handle higher pressures up to 5,000 psig

o Recover faster than their firetube cousin

o Have the ability to reach very high temperatures

Disadvantages of the Watertube design include:

o High initial capital cost

o Cleaning is more difficult due to the design

o No commonality between tubes

o Physical size may be an issue

Cross Section of Watertube Boilers

Examples 0f Watertube Boilers

Babcock and Wilcox Water-tube Boiler

A water tube boiler consisting in its simplest form of a horizontal drum from which is suspended a pair of headers carrying between them an inclined bank of straight tubes.

The tubes can be cleaned or replaced through hand-holes that are place ahead of each tube. The collecting boxes, on the boilers front side, are in the down-end connected to a mud disposal box with a blow down valve. The upper end of the collecting boxes by means of elbowed pipes connected to the lying steam drum. The collecting boxes at the water tubes rear side are closed at both ends. From the upper ends are tubes connected to the steam drum above the water level. At normal conditions the water level will be little below the mid-point of the steam drum.


Boiler Fittings and Accessories

Safety valve: It is used to relieve pressure and prevent possible explosion of a boiler.

Water level indicators: They show the operator the level of fluid in the boiler, also known as a sight glass, water gauge or water column is provided.

Bottom blowdown valves: They provide a means for removing solid particulates that condense and lay on the bottom of a boiler. As the name implies, this valve is usually located directly on the bottom of the boiler, and is occasionally opened to use the pressure in the boiler to push these particulates out.

Continuous blowdown valve: This allows a small quantity of water to escape continuously. Its purpose is to prevent the water in the boiler becoming saturated with dissolved salts. Saturation would lead to foaming and cause water droplets to be carried over with the steam - a condition known as priming.

Flash Tank: High pressure blowdown enters this vessel where the steam can 'flash' safely and be used in a low-pressure system or be vented to atmosphere while the ambient pressure blowdown flows to drain.

Automatic Blowdown/Continuous Heat Recovery System: This system allows the boiler to blowdown only when makeup water is flowing to the boiler, thereby transferring the maximum amount of heat possible from the blowdown to the makeup water. No flash tank is generally needed as the blowdown discharged is close to the temperature of the makeup water.

Hand holes: They are steel plates installed in openings in "header" to allow for inspections & installation of tubes and inspection of internal surfaces.

Steam drum internals, A series of screen, scrubber & cans (cyclone separators).

Low- water cutoff: It is a mechanical means (usually a float switch) that is used to turn off the burner or shut off fuel to the boiler to prevent it from running once the water goes below a certain point. If a boiler is "dry-fired" (burned without water in it) it can cause rupture or catastrophic failure.

Surface blowdown line: It provides a means for removing foam or other lightweight non-condensible substances that tend to float on top of the water inside the boiler.

Circulating pump: It is designed to circulate water back to the boiler after it has expelled some of its heat.

Feedwater check valve or clack valve: A non-return stop valve in the feedwater line. This may be fitted to the side of the boiler, just below the water level, or to the top of the boiler.

Top feed: A check valve (clack valve) in the feedwater line, mounted on top of the boiler. It is intended to reduce the nuisance of limescale. It does not prevent limescale formation but causes the limescale to be precipitated in a powdery form which is easily washed out of the boiler.

Desuperheater tubes or bundles:
A series of tubes or bundles of tubes in the water drum or the steam drum designed to cool superheated steam. Thus is to supply auxiliary equipment that doesn't need, or may be damaged by, dry steam.

Chemical injection line: A connection to add chemicals for controlling feedwater pH.


Economisers & Superheaters


The flue gases, having passed through the main boiler and the superheater, will still be hot. The energy in these flue gases can be used to improve the thermal efficiency of the boiler. To achieve this the flue gases are passed through an economiser.

The economiser is a heat exchanger through which the feedwater is pumped. The feedwater thus arrives in the boiler at a higher temperature than would be the case if no economiser was fitted. Less energy is then required to raise the steam. Alternatively, if the same quantity of energy is supplied, then more steam is raised. This results in a higher efficiency. In broad terms a 10°C increase in feedwater temperature will give an efficiency improvement of 2%.


Whatever type of boiler is used, steam will leave the water at its surface and pass into the steam space. Steam formed above the water surface in a shell boiler is always saturated and cannot become superheated in the boiler shell, as it is constantly in contact with the water surface.
If superheated steam is required, the saturated steam must pass through a superheater. This is simply a heat exchanger where additional heat is added to the saturated steam.
In water-tube boilers, the superheater may be an additional pendant suspended in the furnace area where the hot gases will provide the degree of superheat required . In other cases, for example in CHP schemes where the gas turbine exhaust gases are relatively cool, a separately fired superheater may be needed to provide the additional heat.


General View of a Boiler

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