A boiler is a closed vessel in which fluid (generally water) is heated. The fluid does not necessarily boil. The heated or vaporized fluid exits the boiler for use in various processes or heating applications, including water heating, central heating, boiler-based power generation, cooking, and sanitation.
Types of Boiler
Different types of boilers are available in the market, which are manufactured by different manufacturers. The different types of boilers find their use in different applications. As the time has passed, more efficient types of boilers have replaced the old and inefficient ones. This article elaborates on different types of boilers and the boiler classification. Boilers can be broadly classified as shell tube and water tube boilers.
Shell and Tube Boiler
Shell and tube boilers are also referred to as fire tube or smoke tube boilers. Fire tube boilers; contain long steel tubes through which the hot gasses from a furnace pass and around which the water to be converted to steam circulates. Fire tube boilers, typically have a lower initial cost, are more fuel efficient and easier to operate. Their capacities are up to 25 tons/hr and 17.5 kg/ cm2.
These are the earliest form of high pressure fire tube boiler. These consist of long horizontal cylinder with single large flue containing fire. Fuel is added in the grate area where it burn to produce hot gases. The hot gases transfer the heat to the water.
Water takes heat and after some time it starts boiling to produce steam. Hot gases upon reaching at the end of the fire tube, divided into two section and each move into the one of two side flue which take them once again at the front section of the boiler where they are move into the bottom flue and bottom flue take them toward the chimney.
Here, water partially fills a boiler barrel with a small volume left above to accommodate the steam (steam space). This is the type of boiler used in nearly all steam locomotives. The heat source is inside a furnace or firebox that has to be kept permanently surrounded by the water in order to maintain the temperature of the heating surface below the boiling point. The furnace can be situated at one end of a fire-tube which lengthens the path of the hot gases, thus augmenting the heating surface which can be further increased by making the gases reverse direction through a second parallel tube or a bundle of multiple tubes (two-pass or return flue boiler); alternatively the gases may be taken along the sides and then beneath the boiler through flues (3-pass boiler). In case of a locomotive-type boiler, a boiler barrel extends from the firebox and the hot gases pass through a bundle of fire tubes inside the barrel which greatly increases the heating surface compared to a single tube and further improves heat transfer. Fire-tube boilers usually have a comparatively low rate of steam production, but high steam storage capacity. Fire-tube boilers mostly burn solid fuels, but are readily adaptable to those of the liquid or gas variety. Fire-tube boilers may also be referred to as “scotch-marine” or “marine” type boilers.
In this type, tubes filled with water are arranged inside a furnace in a number of possible configurations. Often the water tubes connect large drums, the lower ones containing water and the upper ones steam and water; in other cases, such as a mono-tube boiler, water is circulated by a pump through a succession of coils. This type generally gives high steam production rates, but less storage capacity than the above. Water tube boilers can be designed to exploit any heat source and are generally preferred in high-pressure applications since the high-pressure water/steam is contained within small diameter pipes which can withstand the pressure with a thinner wall. These boilers are commonly constructed in place, roughly square in shape, and can be multiple stories tall
A flash boiler is a specialized type of water-tube boiler in which tubes are close together and water is pumped through them. A flash boiler differs from the type of mono-tube steam generator in which the tube is permanently filled with water. In a flash boiler, the tube is kept so hot that the water feed is quickly flashed into steam and superheated. Flash boilers had some use in automobiles in the 19th century and this use continued into the early 20th century.
Sometimes the two above types have been combined in the following manner: the firebox contains an assembly of water tubes, called thermic siphons. The gases then pass through a conventional firetube boiler. Water-tube fireboxes were installed in many Hungarian locomotives,but have met with little success in other countries.
Sectional boiler. In a cast iron sectional boiler, sometimes called a “pork chop boiler” the water is contained inside cast iron sections. These sections are assembled on site to create the finished boiler.
Chimney throws these gases out of the boiler into the atmosphere. Maximum heat transfer is taken place at fire tube and shell section then taken place at side flue and at last at bottom flue.
For efficiency, the boiler was commonly encased beneath by a brick-built chamber.
The Lancashire boiler is similar to the Cornish, but has two large flues containing the fires. Pressure range of the boiler is about 0.7 MPa to 2 MPa and efficiency is 65 to 70%. Fuel in these boilers is added into the grate which heats the gases.
Hot gases enter the front section of the boiler and leave the boiler from back and then enter the bottom flue and start moving to front section of boiler. At front section hot gases leave the bottom flue and enter in side flue and move again towards the back of the boiler and enter the main outlet. 85% of heat is transferred when hot gases are in fire tube while 15% is transferred when they are in bottom and side fluid.
A locomotive boiler has three main components
- Double-walled firebox
- Horizontal, cylindrical “boiler barrel” containing a large number of small flue-tubes
- Smokebox with chimney, for the exhaust gases.
Fuel is burned to produce the hot gases. Fuel is feed through fire hole. Hot gases are diverted to fire tube with the help of fire brick arch. Steam is collected in the steam drum which is placed at the top of the shell.
The wet steam goes through inlet headers of super heater and after passing through tubes, it returns to the outlet header of super heater and is taken out for steam engine.
Locomotive-type boilers are also used in traction engines, steam rollers, portable engines etc.
On the basis of construction these can be classified wet back boilers and dry back boilers.
Reversal Chamber Boiler
This is the posterior portion of the combustion chamber through which the flue gases travel from the first pass (furnace) to the second-pass tubes
Wet Back Boiler
In wet back boiler, as the name suggests the reversal chamber is completely surrounded by water. The combustion reversal chamber is surrounded by water and therefore the heat in the flue gases is optimally utilized. Radiation losses are reduced as none of the parts of the combustion chamber are open to atmosphere instead they are surrounded by water. That means fewer losses, and lesser fuel bills. Most efficient modern boilers supplied are wetback type
Dry Back Boiler
The reversal chamber in dry back boilers is not completely surrounded by water. The posterior part is exposed to the atmosphere. This leads to the increased radiation losses, as the radiant heat is lost to the atmosphere instead of going to the water as in wet back boilers. Earlier generation boilers used to be dry back thus wet back boilers ensure lesser radiation losses and hence save fuel.
The layout of the tubes involves the number of passes the tube will make to pass the heat from the boiler furnace before being discharged. These can be two- pass and three pass boiler.
Depending upon the layout of tubes boilers can be two pass or three pass boilers.
In two pass boiler, the combustion gases travels twice in the boiler. Combustion gases should be cooled before entering the reversal chamber.
A three pass boiler design provides three opportunities for heat transfer. The stack temperature of 3 pass will be lower than that of 2 pass boiler, of the same design and operating pressure. It has more efficiency than of two pass boiler.
Each pass in boiler should be designed with cross sectional area to achieve optimal flue gas velocity, which in turn maximizes heat transfer while also minimizing performance robbing soot build up within the tubes.