BOILER AND ITS TYPES:-

Introduction

Boiler is a closed vessel in which the heat produced by the combustion of fuel is transferred to water for its conversion into steam at the desired temperature and pressure. Broadly speaking, a boiler is a device used for generating, steam for power generation and hot water for heating purpose.

According to A.S.M.E a boiler is defined as a combination of apparatus for producing ,furnishing or recovering heat together with the apparatus for transferring the heat so made available to water which could be heated and vaporized to steam form.

Classification of Boiler:

Boilers can be classified as follows:
1. According to the flow of water and hot gases – fire tube (or smoke tube) and water tube boilers.

In fire tube boilers, hot gases pass through tubes which are surrounded with water. Examples: Vertical, Cochran, Lancashire and Locomotive boilers. There may be single tube as in case of Lancashire boiler or there may be a bank of tubes as in a locomotive boiler.

In water tube boilers, water circulates through a large number of tubes and hot gases pass around them. Eg., bobcock & Wilcox boiler.

2. According to the axis of the shell – vertical and horizontal boilers.

3. According to location or position of the furnace. Externally and internally fired boilers.

In internally fired boilers, the furnace forms an integral part of the boilers structure. The vertical tubular, locomotive and the scotch marine boilers are well known examples.

Externally fired boilers have a separate furnace built outside the boiler shell and usually below it. The horizontal return tube (HRT) boiler is probably the most widely known example of this type.

4. According to the application – stationery and mobile boilers. A stationary boilers is one of which is installed permanently on a land installation.

A marine boiler is a mobile boiler meant for ocean cargo and passenger ships with an inherent fast steaming capacity.

5. According to steam pressure – low, medium and high pressure boilers.

Merits and demerits of fire tube boiler over water tube boiler:

Merits:

—  1)The fire tube boiler have greater reliability and low fire cost  because of simple and rigid construction.

—  2)Due to large cylindrical drums of fire tube boiler, there is ample water surface from which the stream can be quickly raised. Simple antipriming device serve the purpose very well.

—  3)fire tube boiler are excellent for engines operating with rapid changes in load like locomotive boiler.

Demerits:

1)Fire tube boiler have larger ratio of water to steam and therefore makes the boiler slow in reaching the operating           pressure

2)Larger diameter of shell 2.4 m and limit of maximum thickness 3cm and stress consideration limit the pressure to 20 bar.

3)The maximum generating capacity of these   boilers is about 9000kg/hr.

Merits and Demerits of water tube boiler over fire tube boiler:

—  Merits

—  1)generation of steam is much quicker due to small ratio of water convert to steam content .this also help In reaching the steaming temperature in short  time.

—  2)its evaporating capacity is considerably larger and the steam pressure range is also high as 200 bar.

—  3)heating surface are more effective as the hot gases travel at right angles to the direction of water flow.

—  Demerits

—  1)it is less suitable for impure and sedimentary water, as a small deposit of scale may cause the overheating and bursting of tubes.therefore use of pure feed water is essential.

—  2)they require careful attention ,the maintenance costs are higher.

—  3)failure in feed water supply even for a short period is liable to make the boiler overheated.  

Cochran Boiler

It is a multi-tubular vertical fire tube boiler having a number of horizontal fire tubes. T is the modification of a simple vertical boiler where the heating surface has been increased by means of a number of fire tubes.

It consists of

1. Shell
2. Crate
3. Fire box
4. Flue pipe
5. Fire tubes
6. Combustion chamber
7. Chimney
8. Man-hole

Shell
It is hemispherical on the top, where space is provided for steam.

Grate
It is placed at the bottom of the furnace where coal is burnt.

Fire box (furnace )
It is also dome-shaped like the shell so that the gases can be deflected back till they are passed out through the flue pipe to the combustion chamber.

Flue pipe:
It is a short passage connecting the fire box with the combustion chamber.

Fire tubes:
A number of horizontal fire tubes are provided, thereby the heating surface is increased.

Combustion chamber:
It is lined with fire bricks on the side of the shell to prevent overheating of the boiler. Hot gases enter the fire tubes from the flue pipe through the combustion chamber.

Chimney:
It is provided for the exit of the flue gases to the atmosphere from the smoke box.

Manhole:
It is provided for inspection and repair of the interior of the boiler shell.

Normal size of a Cochran boiler:
Shell diameter – 2.75 meters:
Height of the shell – 6 meters.

Working of the Cochran boiler:

Coal is fed into the grate through the fire hole and burnt. Ash formed during burning is collected in the ashpit provided just below the grate and then it is removed manually.

The host gases from the grate pass through the flue pipe to the combustion chamber. The hot gases from the combustion chamber flow through the horizontal fire tubes and transfer the heat to the water by convection.

The flue gases coming out of fire tubes pass through the smoke box and are exhausted to the atmosphere through the chimney.
Smoke box is provided with a door for cleaning the fire tubes and smoke box.

The following mountings are fitted to the boiler:


Pressure gauge: this indicates the pressure of the steam inside the boiler.

Water gauge: this indicates the water level in the boiler. The water level in the boiler should not fall below a particular level, otherwise the boiler will be over heated and the tubes may burn out.

Safety valve: the function of the safety valve is to prevent an increase of steam pressure in the boiler above its normal working pressure.

Steam stop valve: it regulates the flow of steam supply to requirements.

Blow-off cock: it is located at the bottom of the boiler. When the blow-off cock is opened during the running of the boiler, the high pressure steam pushes (drains) out the impurities like mud, sand, etc., in the water collected at the bottom.

Fusible plug: it protects the fire tubes from burning when the water level in the boiler falls abnormally low.

Salient features of Cochran boiler:

1. The dome shape of the furnace causes the hot gases to deflect back and pass through the flue. The un-burnt fuel if any will also be deflected back.
2. Spherical shape of the top of the shell and the fire box gives higher area by volume ratio.
3. It occupies comparatively less floor area and is very compact.
4. It is well suited for small capacity requirements.

Lancashire Boiler

It is a stationary, fire tube, internally fired boiler. The size is approximately from 7-9 meters in length and 2-3 meters in diameter.

Construction of Lancashire Boiler:

It consists of

1. Cylindrical shell
2. Furnace tubes, bottom flue and side flues
3. Grate
4. Fire bridge
5. Dampers

Cylindrical shell

It is placed in horizontal position over a brick work. It is partly filled up with water. The water level inside the shell is well above the furnace tubes.

Furnace tubes, bottom flue and side flues:

Two large internal furnace tubes (flue tubes) extend from one end to the other end of the shell. The flues are built-up of ordinary brick lined with fire bricks. One bottom flue and two side flues are formed by brick setting, as shown in the figure.

Grate

The grate is provided at the front end of the main flue tubes. Coal is fed to the grate through the fire hole.

Fire bridge:

A brickwork fire bridge is provided at the end of the grate to prevent the flow of coal and ash particles into the interior of the furnace (flue) tubes. Otherwise the coal and ash particles carried with gases form deposits on the interior of the tubes and prevent the heat transfer to the water.

Dampers:

Dampers is in the form of sliding doors are placed at the end of the side flues to control the flow of gases from side flues to the chimney flue.

Working of Lancashire boiler

Coal is fed to the grate through the fire hole and is burnt. The hot gases leaving the grate move along the furnace (flue) tubes upto the back end of the shell and then in the downward direction to the bottom flue. The bottom of the shell is thus first heated.

The hot gases, passing through the bottom flue, travel upto the front end of the boiler, where they divide into two streams and pass to the side flues. This makes the two sides of the boiler shell to become heated. Passing along the two side flues, the hot gases travel upto the back end of the boiler to the chimney flue. They are then discharged into the atmosphere through the chimney.

With the help of this arrangement of flow passages of hot gases, the bottom of the shell is first heated and then its sides. The heat is transferred to water through the surface of the two flue tubes (which remain in water) and bottom and sides of the shell.

The arrangement of flues increases the heating surface of the boiler to a large extent.

Dampers control the flow of hot gases and regulate the combustion rate as well as steam generation rate.

The boiler is fitted with necessary mountings. Pressure gauge and water level indicator provided at the front. Safety valve, steam stop valve, low water and high steam safety valve and man-hole are provided on the top of the shell.

High steam low water safety valve:

It is a combination of two valves. One is lever safety valve, which blows-off steam when the working pressure of steam exceeds. The second valve operates by blowing-off the steam when the water level falls below the normal level.

Blow-off clock:

It is situated beneath the front portion of the shell for the removal of mud and sediments. It is also used to empty the water in the boiler during inspection.

Fusible plug:

It is provided on the top of the main flues just above the grate. It prevents the overheating of the boiler tubes by extinguishing the fire when the water level falls below a particular level. A low water level alarm is mounted in the boiler to give a warning when the water level falls below the preset value.

Salient features of Lancashire Boiler

The arrangement of flues in this boiler increases the heating surface of shell to a large extent.
It is suitable where a large reserve of steam and hot water is needed.
Its maintenance is easy.
Superheated can be easily incorporated into the system at the end of the main flue tubes. Thus overall efficiency of the boiler can be increased.

Note : The simple vertical Boiler, Cochran and Lancashire Boilers discussed till this post are Fire tube boilers. In the upcoming posts, I will write about water tube boilers namely Babcock and Wilcox Boiler.

Bacock and Wilcox

It is a water tube boiler used in steam power plants. In this, water is circulated inside the tubes and hot gases flow over the tubes.

Construction of Babcock and Wilcox Boiler

The Babcock and Wilcox Boiler consists of

1. Steam and water drum (boiler shell)
2. Water tubes
3. Uptake-header and down corner
4. Grate
5. Furnace
6. Baffles
7. Super heater
8. Mud box
9. Inspection door
10. Damper

Steam and water drum (boiler shell):
One half of the drum which is horizontal is filled up with water and steam remains on the other half. It is about 8 meters in length and 2 meter in diameter.

Water tubes:
Water tubes are placed between the drum and furnace in an inclined position (at an angle of 10 to 15 degree) to promote water circulation. These tubes are connected to the uptake-header and the down-comer as shown.

Uptake-header and down-corner (or downtake-header)
The drum is connected at one end to the uptake-header by short tubes and at the other end to the down-corner by long tubes.

Grate: Coal is fed to the grate through the fire door.

Furnace : Furnace is kept below the uptake-header.

Baffles: The fire-brick baffles, two in number, are provided to deflect the hot flue gases.

Superheater: The boiler is fitted with a superheater tube which is placed just under the drum and above the water tubes

Mud box: Mud box is provided at the bottom end of the down comer. The mud or sediments in the water are collected in the mud box and it is blown-off time to time by means of a blow –off cock.

Inspection doors: Inspection doors are provided for cleaning and inspection of the boiler.

Working Babcock and Wilcox Boiler:

Coal is fed to the grate through the fire door and is burnt.

Flow of flue gases:

The hot flue gases rise upward and pass across the left-side portion of the water tubes. The baffles deflect the flue gases and hence the flue gases travel in the zig-zag manner (i.e., the hot gases are deflected by the baffles to move in the upward direction, then downward and again in the upward direction) over the water tubes and along the superheater. The flue gases finally escape to atmosphere through chimney.

Water circulation: 


That portion of water tubes which is just above the furnace is heated comparatively at a higher temperature than the rest of it. Water, its density being decreased, rises into the drum through the uptake-header. Here the steam and water are separated in the drum. Steam being lighter is collected in the upper part of the drum. The water from the drum comes down through the down –comer into the water tubes.

A continuous circulation of water from the drum to the water tubes and water tubes to the drum is thus maintained. The circulation of water is maintained by convective currents and is known as “natural circulation”.

A damper is fitted as shown to regulate the flue gas outlet and hence the draught.

The boiler is fitted with necessary mountings. Pressure gauge and water level indicator are mounted on the boiler at its left end. Steam safety valve and stop valve are mounted on the top of the drum. Blow-off cock is provided for the periodical removed of mud and sediments collected in the mud box.

Salient features of Babcock and Wilcox Boiler:

1. Its overall efficiency is higher than a fire tube boiler.
2. The defective tubes can be replaced easily.
3. All the components are accessible for inspection even during the operation.
4. The draught loss is minimum compared with other boiler.
5. Steam generation capacity and operating pressure are high compared with other boilers.
6. The boiler rests over a steel structure independent of brick work so that the boiler may expand or contract freely.
7. The water tubes are kept inclined at an angle of 10 to 15 degree to promote water circulation.

Locomotive Boiler

It has basically three parts i.e. smoke box, shell and fire box.

Inside fire box the fuel is burnt over the grate .For feeding fuel the fire hole is used .hot Gases produced in fire box are diverted By fire brick arch and enter into the fire Tubes surrounded with steam produced gets collected in a stream drum fitted on top of the shell. Arrangement for super heating is there in these boilers. As shown the wet steam goes through inlet header of super heater and after passing through tubes ,it return to the outlet header of super heater and is taken out for steam engine. A very large door is provided at the end of smoke box so as to facilitate cleaning and maintenance of complete boiler.

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