Water tube boiler: A water tube boiler is a type of boiler of boiler in which water circulates in tubes heated ext ernally by the fire. Fuel is burned inside the furnace, creating hot gas which heats water in the steam-generating tubes. In smaller boilers, additional generating tubes are separate in the furnace, while larger utility boilers rely on the water-filled tubes that make up the walls of the furnace to generate steam. The heated water then rises into the steam drum. Here, saturated steam is drawn off the top of the drum. In some services, the steam will reenter the furnace through a valve to become superheated. Superheated steam is defined as steam that is heated above the boiling point at a given pressure. Superheated steam is a dry gas and therefore used to drive turbines, since water droplets can severely damage turbine blades. Cool water at the bottom of the steam drum returns to the feed water drum via large-bore 'down comer tubes', where it pre-heats the feed water wate r supply. (In 'large utility boilers', the feed water is supplied to the steam drum and the down comers supply water to the bottom of the water walls). To increase economy of the boiler, exhaust gases are also used to pre-heat the air blown into the furnace and warm the feed water supply. Such water tube boilers in thermal power station are also called steam generating units. The older fire-tube boiler design – in which the water surrounds the heat source and the gases from combustion pass through tubes through the water space – is a much weaker structure and is rarely used for pressures above 350 psi (2.4 MPa). A significant advantage of the water tube boiler is that there is less chance of a catastrophic failure: there is not a large volume of water in the boiler nor are there large mechanical elements subject to failure.
Advantages of Water Tube Boiler: There are many advantages of water tube boiler due to which these types of boiler are essentially used in large thermal power station.
Larger heating surface can be achieved by using more numbers of water tubes. Due to convectional flow, movement of water is much faster than that of fire of fire tube boiler; hence rate of heat transfer is high which results into higher efficiency. Very high pressure in order of 140 kg/cm2 can be obtained smoothly.
Working principle principle of Water Tube Boiler: The 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 two drums, one is upper drum c alled steam drum other is lower drum called mud drum. These upper drum and lower drum 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 them which comes to the upper drums naturally. In the upper drum the steam is separated from water naturally and stored above the water surface. The colder water is fed from feed water inlet at upper drum and as this water is heavier than the hotter water of lower drum and that in the riser, the colder water push the hotter water upwards through the riser. So there is one convectional flow of water in the boiler system. More and more steam is produced the pressure of the closed system increases which obstructs this convectional flow of water and hence rate production of steam becomes slower proportionately. Again if the steam is taken trough steam outlet, the pressure inside the system falls and consequently the convectional flow of water becomes faster which result in faster steam production rate. I n this way the water tube boiler can control its own pressure. Hence this type of boiler is referred as self controlled machine.
Types of Water Tube Boiler: There 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 Tube Boiler: Construction
of Babcock – Wilcox Boiler:
Babcock – Wilcox Drum Boiler or of boiler, one placed above the down comer tube tube is fitted as comer tube and by 5o to 15 o
Boiler is also known as Longitudinal Horizontal Tubes Boiler . In this type cylindrical drum is longitudinally heat chamber. In rear of the drum is fitted and in front of the drum riser shown in the figure. These downriser tube are connected to each other
Working Principle Of Longitudinal Water Tube Boiler (Babcock – Wilcox Boiler) : Principle of Babcock-Wilcox Boiler depends upon thermosyphon principle. The longitudinally placed drum as mentioned in the construction of longitudinal drum boiler, is fed by colder 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 these inclined horizontal tubes and ultimately comes back to the boiler drum through riser . During travelling of water through inclined water t ubes, it absorbers heat of the hot gases, surrounds the water tube, consequently steam bubbles are created in these tubes. These steam bubbles then come t o the steam drum through riser and naturally separated
from water and occupies the space above the water surface in the longitudinal drum of Babcock –
Wilcox Boiler
Cross Drum Water Tube Boiler: Construction of Cross Drum Water Tube Boiler: Cross Drum Boiler is essentially a variant of the Longitudinal Drum Boiler . In Cross Drum Boiler the steam drum is placed at cross ways to the heat source as shown in the figure. Here, the down-comer is fitted on the bottom of the drum and riser is fitted on the top of the drum via a horizontal tube as shown in the figure. 5 o 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 wate r tube placed in hot chamber. 3) Here, the water becomes hot and steam is produced in the water which comes into steam chamber. 4) Here in the steam drum steam is separated from water in natural way
Bend Tube Boiler or Sterling Boiler: Bend Tube Boiler or Sterling Boiler is the developed version of water t ube boiler. The working principle of bend tube boiler is more or less same as other water tube boilers, but it utilizes four drums.
Construction of Bend Tube Boiler:
Three drums are placed on
the heat source as
shown on the figure. Fourth
drum is placed inside
the heat chamber and this
fourth drum is
connected to upper three
steam drums with help
of bend water tube. Upper
three drums are
connected with equalizer
tubes as shown in the
figure. Steam is taken from
equalizer tubes.
Working principle of Bent Tube or Stirling Boiler: The feed water first e nters into right most upper drum. Due to more density this water comes down in the lower water drum. The water within that water drum and the connecting pipes to the other two upper drums, are heated up and co nsequently steam bubbles are produced. This is the most basic working principle of bent tube boiler .
Capacity of Sterling Boiler: Capacity of Sterling Boiler is much higher than Babcock – Wilcox boiler as high as 50,000kg per hour
and pressure as high as 60kg/cm
2
