steam boiler - What are the types of steam boiler?

What are steam boilers used for?

Boilers are used to produce steam.the steame is used to rorate the terbine to produce mechanical power.

What are the types of steam boiler?

Cornish Fire Tube Boiler,Lancashire Fire Tube Boiler,Locomotive Fire Tube Boiler,Vertical Fire Tube Boiler

Cochran Fire Tube Boiler,Immersion Fire Tube Boiler,Water Tube Boilers.

COCHRAN BOILER

This boiler consists of a cylindrical shell with its crown having a spherical shape. The furnace is on hemispherical in shape. The grate is also placed at the bottom of the furnace and the ash-pit is noted below the grate. The coal is fed into the grate through the fire door and ash formed is collected

the ash-pit located just below the grate and it is removed manually. The furnace and the combustion chamber are connected through a pipe. The back of the combustion chamber is lined with firebricks. The hot gases from the combustion chamber flow through the nest of horizontal fire tubes (generally 625 cm in external diameter and 165 to 170 in number). The passing through the fire tubes transfers a large portion of the heat to the water by convection. The flue gases coming out of fire tubes are finally discharged to the atmosphere through chimney.

 

The spherical top and spherical shape of firebox are the special features of this boiler. These shapes require least material for the volume. The hemi spherical crown of the boiler shell gives maximum strength to withstand the pressure of the steam inside the boiler. The hemi-spherical crown of the fire box is advantageous for resisting intense heat. This shape is also advantageous for the absorption of the radiant heat from the furnace.

Coal or oil can be used as fuel in this boiler. If oil is used as fuel, no grate is provided but the bottom of the furnace is lined with firebricks. Oil burners are fitted at a suitable location below the fire door. A manhole near the top of the crown of shell is provided for cleaning. In addition to this, a number of hand-holes are provided around the outer shell for cleaning purposes. The smoke box is provided with doors for cleaning of the interior of the fire tubes.

The airflow through the grate is caused by means of the draught produced by the chimney. A damper is placed inside the chimney (not shown) to control the discharge of hot gases from the chimney and thereby the supply of air to the grate is controlled. The chimney may also be provided with a steam nozzle (not shown; to discharge the flue gases faster through the chimney. The steam to the nozzle is supplied from the boiler.

The outstanding features of this boiler are listed below:

1. It is very compact and requires minimum floor area.

2. Any type of fuel can be used with this boiler.

3. It is well suited for small capacity requirements.

4. It gives about 70% thermal efficiency with coal firing and about 75% with oil firing

5. The ratio of grate area to the heating surface area varies from 10: 1 to 25: 1.

It is provided with all required mountings. The function of each is briefly described below

1. Pressure Gauge. This indicates the pressure of the steam in the boiler.

2. Water Level Indicator. This indicates the water level in the boiler The water level in boiler should not fall below a particular level otherwise the boiler will be overheated and the tubesburn out.

3. Safety Valve. The function of the safety valve is to prevent the increase of steam pressur the holler above its design pressure. When the pressure increases above design pressure, the valve onen and discharges the steam to the atmosphere. When this pressure falls just below design pressure the valve closes automatically. Usually the valve is spring controlled.

4. Fusible Plug. If the water level in the boiler falls below a predetermined level, the boiler shell and tubes will be overheated. And if it is continued, the tubes may burn, as the water cover will be removed. It can he prevented by stopping the burning of fuel on the grate. When the temperature of the shell increases above a particular level, the fusible plug, which is mounted over the grate, melts and forms an opening. The high-pressure steam pushes the remaining water through this hole on the grate and the fire is extinguished.

5. Blow-off Cock. The water supplied to the boiler always contains impurities like mud, sand and, salt Due to heating, these are deposited at the bottom of the boiler, and if they are not removed, they are accumulated at the bottom of the boiler and reduces its capacity and heat transfer rates. Also the salt content will goes on increasing due to evaporation of water. These deposited salts are removed with the help of blow off cock. The blow-off cock is located at the bottom of the boiler as shown in the figure and is operated only when the boiler is running. When the blow-off cock is opened during the running of the boiler, the high-pressure steam pushes the water and the collected material at the bottom is blown out, Blowing some water out also reduces the concentration of the salt. The blow-off cock is operated after every 5 to 6 hours of working for few minutes. This keeps the boiler clean.

6. Steam Stop Valve. It regulates the flow of steam supply outside. The steam from the boiler first enters into an ant-priming pipe where most of the water particles associated with steam are removed.

7. Feed Check Valve. The high pressure feed water is supplied to the boiler through this valve This valve opens towards the boiler only and feeds the water to the boiler. If the feed water pressure less than the boiler steam pressure then this valve remains closed and prevents the back flow of steam through the valve.

 

LANCASHIRE BOILER

It is stationary fire tube, internally fired, horizontal, natural circulation boiler. This is a widely used boiler because of its good steaming quality and its ability to burn coal of inferior quality. These boilers have a cylindrical shell 2 m in diameters and its length varies from 8 m to 10 m. It has two large internal flue tubes having diameter between 80 cm to 100 cm in which the grate is situated. This boiler is set in brickwork forming external flue so that the external part of the shell forms part of the heating surface.

The main features of the Lancashire boiler with its brickwork shelling are shown in figure. The boiler consists of a cylindrical shell and two big furnace tubes pass right through this. The brick setting forms one bottom flue and two side flues. Both the flue tubes, which carry hot gases, lay below the water level .

The grates are provided at the front end of the main flue tubes of the boiler and the coal is fed to the grates through the fire doors. A low firebrick bridge is provided at the end of the grate, to prevent the flow of coal and ash particles into the interior of the furnace tubes. Otherwise, we ash and coal particles carried with gases form deposits on the interior of the tubes and prevent the heat transfer to the water. The firebrck bridge also helps in deflecting the hot gases upward to provide better heat transfer:

The hot gases leaving the grate pass up to the back end of the tubes and then in the downward direction. They move through the bottom flue to the front of the boiler where they are divided into two and pass to the side flues as shown in the figure. Then they move along the two-side flues and come to the chimney as shown in the figure.

With the help of this arrangement of the flow passages of the gases, the bottom of the shell is the heated and then its sides. The heat is transferred to the water through surfaces of the two flue tubes (which remain in water) and bottom part and sides of the main shell. This arrangement increases the heating surface to a large extent.

Dampers in the form of sliding doors are placed at the end of side flues to control the flow gases. This regulates the combustion rate as well as steam generation rate. These dampers are opera by chains passing over a pulley at the front of the boiler. This boiler is fitted with usual mountings. pressure gauge and water level indicator are provided at the front whereas steam stop valve, sarcas valve, low water and high steam safety valve and manhole are provided on the top of the shell.

The blow off cock is situated beneath the front portion of the boiler shell for the removal of sediments and mud. It is also used to empty the water in the boiler whenever required for inspection. The fusible plugs are mounted on the top of the main flues just over the grates as shown in

ubes by extinguishing the fire when the water level falls particular level. A low water level alarm is usually mounted in the boiler to give a warm case the water level going below the precast value.

A feed check valve with a feed pipe is fitted on the front end plate. The feed pipe projecting into hoiler is perforated so that the water is uniformly distributed into the shell.

The outstanding features of this boiler are listed below:

1. Its heating surface area per unit volume at the boiler is considerably large.

2. Its maintenance is easy.

3. It is suitable where a large reserve of hot water is needed. This boiler due to the large reserve capacity can easily meet load fluctuations.

4. Super-heater and economizer can be easily incorporated into the system, therefore, overall efficiency of the boiler can be considerably increased (80-85%).

The super-heater is placed at the end of the main flue tubes. The hot gases before entering the bottom flue are passed over the super-heater tubes as shown in the figure and the steam drawn through the steam stop-valve are passed through the super-heater. The steam passing through the super-heater absorbs heat from hot gases and becomes superheated.

The economizer is placed at the end of side flues before exhausting the hot gases to the chimney. The water before being fed into the boiler through the feed check valve is passed through the economizer. The feed water is heated by absorbing the heat from the exhaust gases, thus leading to better boiler efficiency. Generally, a chimney is used to provide the draught.

 the boiler whenever required for inspection. The fusible plugs are mounted on the top of the main flues just over the grates as shown in

ubes by extinguishing the fire when the water level falls particular level. A low water level alarm is usually mounted in the boiler to give a warm case the water level going below the precast value.

are of larger diameter (13 cm diameter) fitted at the upper part of the shell and others (nearly 160 tub of 4.75 cm in diameter are fitted into the lower part of the shell. The shell contains water surrounding the tubes. The top tubes are made of larger diameter to accommodate the super-heater tubes. Absorb heat from the hot gases flowing over the tubes superheats the steam passing through the super-hea tubes. The steam generated in the shell is collected over the water surface. A dome-shaped cham known as steam dome, is fitted on the top of the shell. The dome helps to reduce the priming as distance of the steam entering into the dome and water level is increased. The steam in the shell fl through a pipe mounted in the steam dome as shown in the figure into the steam header which is divid into two parts. One part of the steam header is known as saturated steam header and the other part known as superheated steam header. The saturated wet steam through the steam pipe enters into the saturated steam header and then it is passed through the super-heater tubes as shown in the figure. The superheated steam coming out of super-heater tubes is collected in the superheated header and then fed to the steam engines. A stop valve serving also as a regulator for steam flow is provided inside cylindrical steam dome as shown in the figure. This is operated by the driver through a regulator shaft passing from the front of the boiler.

1. Fire box        2. Grate            3. Fire whole           4. Fire bride arch   5. Ash pit

6. Damper           7. Fine tubes      8. Barrel or shell   9. Smoke box     10. Chimney (short)

11. Exhaust steam pipe           12. Steam dome   13. Regulator      14. Lever

15. Superheated tubes                  16. Super heater header                  17. Superheated exist pipe  

18. Smoke box door                     19. Feed check valve                       20. Safety valve

 21. Whistle                                    22. Water gauge

The supply of air to the gate is obtained by discharging the exhaust steam from the engine through a blast pipe which is placed on the bellow of the chimney. The air flow caused by these methods is known as cleaning the smoke box and fire tubes.

The high of the chimney must be bellow to face plate the locomotive to pass through tunnels and bridges. Because the short of the chiming artificial drought has to be created to drive out the hot gases. The drought is created with the help of exhaust steam when locomotive is moving and with the help of live steam when the locomotive is stationary. The motion of the locomotive helps not only to increase the drought but also to increase the heat transfer rate.

 The pressure gauge and water level indicators are located m the driver's cabina at the front of the fire box .the spring lorded safety valve and fusible plug are located.  Blow-off cock is provided at the bottom of the water wall to remove the debris and mud

The outstanding feathers of this boiler are listed below:

1. Large rate of steam generation per square meter of heating surface. To some exten to the vibration caused by the motion.

2. It is free from brickwork, special foundation and chimney. This reduces the cost of interlace

3. It is very compact. The pressure of the steam is limited to about 20 bar. The details of W.G.Type Locomotive Diameter and length of shell Ordinary tubes Large size tubes Pressure and temperature of steam Grate area Heating surface area = 270 m2.

The capacity of this boiler under normal load is 8500 kg/hr at 14.76 bar and 370°C burning 158.5 kg of coal per hour/m- of grate area.

Boiler manufactured at Chittaranjan is listed below: = 208.5 cm and 520.7 cm = 116 and 57.15 mm in diameter = 38 and 114.3 mm in diameter = 14.76 bar and 370°C = 4.27'112

BABCOCK WILCOX BOILER

As classified earlier, in a water tube boiler, the water is inside the tubes and hot gases flow over the tubes. Babcock and Wilcox original model is a straight water tube boiler. A simple stationary boiler of this type is described here.

The boiler with its parts is shown in Fig. 5.4. The boiler shell known as water and steam drum is made of high quantity steel. It is connected by short tubes with the uptake header or riser and by longer tubes to the down take header. The water level in the drum is slightly above the center. The water tubes are connected to the top and bottom header and are kept inclined at an angle of 15° to the horizontal. The headers are provided with hand holes in the front of the tubes and are covered with caps. This arrangement helps in cleaning of the tubes. The inclined position helps the flow of water.

The furnace is arranged below the uptake header. Coal is fed to the grate through the fire door. Two firebrick baffles are arranged in such a manner that the hot gases from the grate are compelled to move in the upward and downward directions. First the hot gases rise upward and then go down and then rise up again and finally escape to the chimney through the smoke chamber.

The outer surface of the water tubes and half of the bottom cylindrical surface of the drum form the heating surface through which heat is transferred from the hot gases to the water.

The front portion of the water tubes come in contact with the hot gases at higher temperature. So the water from this portion rises in the upper direction due to decreased density and passed into the drum through the uptake header. Here the steam and water are separated and the steam being lighter is collected in the upper part of the drum. From the back portion of the drum, the water enters into the water tubes through the down take header. Thus, a continuous circulation of water from the drum to the water tubes and water tubes to the drum is maintained. The circulation of water is maintained by convective currents and is known as natural circulation.

A super-heater is placed between the drum and water-tubes as shown in the figure. During the first turn of the hot gases, the gases are passed over the super-heater tubes and the steam is passed through the super-heater and becomes superheated steam. The steam is taken into the super-heater from the steam space of the drum through a tube as shown in the figure. The superheated steam coming out through super-heater is supplied through steam-pipe and steam stop valve to the turbine. When the steam is being raised from cold boiler, the super-heater is filled with water to the drum water level. This is essential to prevent the overheating of the super-heater tubes. The super-heater remains flooded with water until the steam reaches the working pressure. Once the rated pressure of steam is achieved in the boiler, then the water from the super-heater is drained and steam is fed to it for superheating purposes

A mud box is fitted to the down header as shown in the figure. The impurities and mud particles from the water are collected in the mud box and they are blown-off from time to time by means of a blow off valve as shown in the figure.

The access to the interior of the boiler is provided by the doors. This is necessary for cleaning the tubes and removing the scoots from their surfaces. The drought is regulated by a damper which is provided in the back chamber as shown in the figure. The damper position is controlled with the help of chain connected to it from the pulley as shown in figure.

The outstanding features of this boiler are listed below :

1.   The evaporator capacity of this boilers is high compared with other boilers (20,000 to 40,00 kg/hr). The operating pressure lies between 11.5 to 17.5 bar.

2.   The drought loss is minimum compared with other boilers.

3.   The defective tubes can be replaced easily.

4.   The entire boiler rests over an iron structure, independent of brick work, so that the boiler may end or contract freely. The brick walls which form the surroundings of the boiler are only to enclose the furnace and the hot gases.