Water tube boilers
Class of Steam boiler in which water flows inside the tubes and hot gases surround the tubes are called as water tube boilers. Unlike fire tube boilers in higher pressures and higher steam capacities can be attained. This is due to reduced circumferential or tangential stress on tubes also known as hoop stress (pressure inside the tube*internal diameter /thickness of tube) because of smaller tube diameters higher pressures can be tolerated.
Water tube boiler operation
Typically Water tube boiler works on the principle of natural water circulation also called as thermo siphoning. The water tube boiler basically consists of two drums which are
- steam drum
- mud or lower drum
The steam drum and mud drum are connected through riser tube and down comer tube as shown in the schematic diagram of water tube boiler. At the beginning feed water is pumped into the steam drum using a feed water pump. When fuel is burnt hot gases are produced which are allowed to flow in the shell side of water tube boiler. These hot gases exchange heat with the water and the water gets converted into steam. As the temperature of water increases, density decreases. Obviously the density of steam will be lower than that of water. So inside the steam drum the water and steam gets separated naturally due to difference in density. Steam travel upwards due to less density and water will travel downwards due to high density. The hot water at the bottom of steam drum flows into mud drum through down comer tube and heats the water in the mud drum. When cooler feed water is introduced into steam drum because of higher density of cold water it descends in the down comer tube towards mud drum, displacing the hot water in the mud drum into the steam drum through riser tubes. This results in natural circulation of water in the water tube boiler. When more steam is produced in the boiler the pressure inside the steam drum increases and the feed water flow into the steam drum decreases which tends to decrease the steam flow. Similarly when the rate of production of steam decreases the pressure inside the water tube boiler decreases and the feed water inflow into the boiler increases which tends to increase the rate of steam production. In this way water tube boiler self controls the steam production.
Types of water tube boiler
The Water tube boilers are divided into two types based on whether the tubes are horizontal or bent as
1. Horizontal straight water tube boilers
- Longitudinal drum
2. Bent tube boilers
- Two drum
- Three drum
- Low head three drum
- four drum.
There is another classification of water tube boilers known as Cyclone fired boilers.
Cyclone fired boilers:
Cyclone water tube boilers are optimized to use the coal grades which are not suitable for pulverized coal (powdered coal) combustion. In the cyclone water tube boilers coal is fed in a spiral manner into combustion chamber to attain higher combustion efficiencies. This kind of water tube boilers are mainly used in industrial boilers.
Babcock and Wilcox water tube boiler design
Babcock and Wilcox water tube boiler was designed by an American company Babcock and Wilcox. Babcock and Wilcox water tube boiler is a horizontal straight tube boiler. It has a single drum (steam drum), with feed water drawn from the bottom of the steam drum. The steam drum is made of high quality steel. The front end and rear ends of the drum are connected to a series of front end and rear end headers by short riser tubes. The riser tubes are inclined at angle of 15o to the principle axis (horizontal axis) of steam drum. The inclined position of tubes helps in flow of water. The level of water in the steam drum is indicated by a level indicator and the level is maintained slightly above the centre of the drum. The fuel is supplied through the fire door located at the bottom and is burnt in a grate. The hot gases produced from the burning of fuel in the grate are forced to flow upwards by providing baffle plates. Mud collector is provided at the bottom of boiler to remove the mud particles through blow down cock.
Babcock and Wilcox water tube boiler working principle
Babcock and Wilcox water tube boiler as the name suggests is a water tube boiler and hence works on the principle of thermo siphoning (natural circulation of water). The water at the front portion of steam drum comes in contact with hot gases a=t higher temperatures and so its temperature increases and density decreases. Due to decrease in density it rises upward into stream drum via up take header. As explained in the operation of water tube boiler, in steam drum water and steam are separated due to difference in density. The water at the bottom of steam drum flows in to water tubes through the down take header at the rear end of the drum. Thus, a continuous circulation of water from the steam drum to the water tubes and water tubes to the drum is maintained. The circulation of water is maintained by convective currents due to difference in density and is known as natural circulation.
Advantages of water tube boilers
The advantages of water tube boilers are as follows
- Large heating surface can be obtained using a large number of smaller tubes because of which steam can be produced rapidly. Also the steam quality will be better than that of fire tube boilers. Because of this reason Water tube boilers are capable of producing superheated steam
- The steam can be produced easily due to less volume of water on tube side and higher volumes of hot gases on shell side.
- High pressure of the order of 140 kg/cm2 can be obtained. Higher steam
- Because of fast movement of water in the tubes the rate of heat transfer is large and consequently the efficiency of boiler will be high.
- Water tube boilers have additional advantage of small shell and tube diameters. So it occupies less space for a predefined steam capacity and pressure.
- Water tube boilers due to presence of less water volume it will respond faster to load changes.
- Thermal stresses are reduced due to rapid circulation of coolant (water) in tubes due to convection.
Disadvantages of water tube boiler
- More complex control systems with very faster response times are required to control the water level in the steam drum due to the less volume of water in water tube boilers. In the event of loss of water (coolant) the temperature in the water tube boilers rises quickly.
- Water tube boilers are complex in construction.
- High quality feed water is required and little allowance for corrosion.
- Water tube boilers are expensive.