Fire Tube BoilerFire tube boiler is one of the most basic types of boiler and the design is also very old. It waspopular in 18th century. It was mainly used for steam locomotive engines.

Operation of fire tube boilerOperation of fire tube boiler is as simple as its construction. In fire tube boiler, the fuel is burntinside a furnace. The hot gases produced in the furnace then passes through the fire tubes. The firetubes are immersed in water inside the main vessel of the boiler. As the hot gases are passedthrough these tubes, the heat energy of the gasses is transferred to the water surrounds them. As aresult steam is generated in the water and naturally comes up and is stored upon the water in thesame vessel of fire tube boiler. This steam is then taken out from the steam outlet for utilizing forrequired purpose. The water is fed into the boiler through the feed water inlet.

As the steam and water is stored is the same vessel, it is quite difficult to produce very high pressuresteam from. General maximum capacity of this type of boiler is 17.5 kg/cm2 and with a capacityof 9 Metric Ton of steam per hour. In a fire tube boiler, the main boiler vessel is under pressure,so if this vessel is burst there will be a possibility of major accident due to this explosion.

Types of fire tube boilerAccording to the location of furnace there are two types of fire tube boiler and these are externalfurnace and internal furnace type.

There are mainly three types of external furnace fire tube boiler

1) Horizontal return tubular fire tube boiler

2) Short fire box fire tube boiler

3) Compact fire tube boiler

There are also two types of internal furnace fire tube boiler

1) Horizontal tubular and

2) Vertical tubular fire tube boiler.

Working principle of Horizontal return fire tube boiler

Horizontal return fire tube boiler is most suitable for low capacity thermal power plant. Themain constructional features of this boiler are one big size steam drum which lies horizontallyupon supporting structures. There are numbers of fire tubes come from furnace and also aligned

horizontally inside the drum. When the drum is filled with water these tubes are submerged inwater.

The fuels (normally coal) burnt in the furnace and combustible gasses move into the fire tubes,travel through these tubes from rear to front of the boiler drum and finally the gases come into thesmoke box. The hot gasses in the tubes under water transfer heat to the water via the tube walls.Due to this heat energy steam bubbles are created and come up upon the water surface. As theamount of steam is increased in that closed drum, steam pressure inside the drum increases whichincrease significantly the boiling temperature of the water and hence rate of production of steamis reduced. In this way a fire tube boiler controls its won pressure. In other words this is a selfpressure controlled boiler.

Advantage of fire tube boiler

1) Compact in construction.

2) Fluctuation of steam demand can be met easily.

3) Cheaper than water tube boiler.

Disadvantage of fire tube boiler

1) Due to large water the required steam pressure rising time quite high.

2) Output steam pressure cannot be very high since the water and steam are kept in same vessel.

3) The steam received from fire tube boiler is not very dry.

4) In a fire tube boiler, the steam drum is always under pressure, so there may be a chance of hugeexplosion which resulting to severe accident.

Water Tube BoilerA water tube boiler is such kind of boiler where the water is heated inside tubes and the hot gassessurround them. This is the basic definition of water tube boiler. Actually this boiler is just oppositeof fire tube boiler where hot gasses are passed through tubes which are surrounded by water.

Advantages of Water Tube BoilerThere are many advantages of water tube boiler due to which these types of boiler are essentiallyused in large thermal power station.

1) Larger heating surface can be achieved by using more numbers of water tubes.

2) Due to convectional flow, movement of water is much faster than that of fire tube boiler; hencerate of heat transfer is high which results into higher efficiency.

3) Very high pressure in order of 140 kg/cm2 can be obtained smoothly.

Working principle of Water Tube BoilerThe working principle of water tube boiler is very interesting and simple.

let us draw a very basic diagram of water tube boiler. It consists of mainly tow drums, one isupper drum called steam drum other is lower drum called mud drum. These upper drum and lowerdrum are connected with two tubes namely down-comer and riser tubes as shown in the picture.Water in the lower drum and in the riser connected to it, is heated and steam is produced in themwhich comes to the upper drums naturally. In the upper drum the steam is separated from waternaturally and stored above the water surface. The colder water is fed from feed water inlet at upperdrum and as this water is heavier than the hotter water of lower drum and that in the riser, thecolder water push the hotter water upwards through the riser. So there is one convectional flow ofwater in the boiler system.

More and moresteam is produced the pressure of the closed system increases which obstructs this convectionalflow of water and hence rate production of steam becomes slower proportionately. Again if thesteam is taken trough steam outlet, the pressure inside the system falls and consequently theconvectional flow of water becomes faster which result in faster steam production rate. In this waythe water tube boiler can control its own pressure. Hence this type of boiler is referred as selfcontrolled machine.

Types of Water Tube BoilerThere are many types of water tube boiler .They can be sub-categorized in three types, namely,

1) Horizontal Straight Tube Boiler

2) Bent Tube Boiler

3) Cyclone Fired Boiler

Horizontal Straight Tube Boiler again can be sub-divided into two different types, such as

i. Longitudinal Drum Boiler

ii. Cross Drum Boiler

Bent Tube Boiler also can be sub-divided into four different types, such as

i. Two Drum Bent Tube Boiler

ii. Three Drum Bent Tube Boiler

iii. Low Head Three Drum Bent Tube Boiler

iv. Four Drum Bent Tube Boiler

Horizontal or Longitudinal or Babcock Wilcox Water TubeBoilerConstruction of Babcock Wilcox Boiler

Babcock Wilcox Boiler is also known as Longitudinal Drum Boiler or Horizontal TubesBoiler . In this type of boiler, one cylindrical drum is longitudinally placed above the heat chamber.In rear of the drum down comer tube is filled and in front of the drum riser tube is fitted as shownin the figure. These down-comer tube and riser tube are connected to each other by 5o to 15 o straightwater tubes as shown in the figure.

Working principle of Longitudinal Drum Water Tube Boiler

working principle of Babcock Wilcox Boiler depends upon thermonyphon principle. Thelongitudinally placed drum as mentioned in the construction of longitudinal drum boiler, is fed bycolder mater at its rear feed water inlet. As the colder water is heavier it falls down through down-comer fitted at the rear part of the drum. From down-comer the water enters in to horizontal water

tube where it becomes hot and lighter. As the water becomes lighter, it passes up through theseinclined horizontal tubes and ultimately comes back to the boiler drum through riser. Duringtravelling of water through inclined water tubes, it absorves heat of the hot gases, surrounds thewater tube, consequently steam bubbles are created in these tubes. These steam bubbles then cometo the steam drum through riser and naturally separated from water and occupies the space abovethe water surface in the longitudinal drum of Babcock Wilcox Boiler

Cross Drum Water Tube BoilerConstruction of Cross Drum Water Tube Boiler

Cross Drum Boiler is essentially a variant of the Longitudinal Drum Boiler. In Cross DrumBoiler the steam drum is placed at cross ways to the heat source as shown in the figure. Here, thedown-comer is fitted on the bottom of the drum and riser is fitted on the top of the drum via ahorizontal tube as shown in the figure. 5o to 15 o inclined water tubes are connected with down-comer and riser tubes in same manner of Babcock-Wilcox boiler.

Working principle of Cross Drum Boiler

The working principle of cross drum boiler is same as longitudinal drum boiler .

1) The feed water is fed to the cross drum through feed water inlet.

2) Then this water comes down through the down-comer pipe and enters into inclined water tubeplaced in hot chamber.

3) Here, the water becomes hot and steam is produced in the water which comes into steamchamber.

4) Here in the steam drum steam is separated from water in natural way

Bend Tube Boiler or Sterling BoilerBend Tube Boiler or Sterling Boiler is the developed version of water tube boiler. The workingprinciple of bend tube boiler is more or less same as other water tube boilers, but it utilizes fourdrums.

Construction of Bend Tube Boiler

Three drums are placed on the heat source as shown on the figure. Fourth drum is placed insidethe heat chamber and this fourth drum is connected to upper three steam drums with help of bendwater tube. Upper three drums are connected with equalizer tubes as shown in the figure. Steam istaken from equalizer tubes.

Working principle of Bent Tube or Stirling Boiler

The feed water first enters into right most upper drum. Due to more density this water comes downin the lower water drum. The water within that water drum and the connecting pipes to the other

two upper drums, are heated up and consequently steam bubbles are produced. This is the mostbasic working principle of bent tube boiler .

Capacity of Stirling Boiler

Capacity of Stirling Boiler is much higher than Babcock Wilcox boiler as high as 50,000kgper hour and pressure as high as 60kg/cm