Thermodynamics and Heat Engine4 (3+1)

LESSON - 28 COCHRAN BOILER, LOCOMOTIVE BOILER AND BABCOCK-WILCOX BOILER

Uses

The most common applications of this boiler are steam roller, pile drivers, portable hoisting rigs, steam shovels and other mobile applications.

Construction Details

It consists of a vertical cylindrical shell having dome shaped top where the steam space is provided. The hemispherical shaped furnace provided at the bottom most part of the boiler in which the fuel is burnt on a grate. The ash pit is provided below the grate for collecting ash after regular intervals. The hemispherical shape furnace without a single weld or seam makes the furnace strongest structure under compression and suitable to resist the intense heat produced by the combustion of fuel.

Fig. 28.1. Cochran Boiler

In the furnace, a fire door and a damper is provided for feeding coal to the grates and controlling the amount of air entering the grate, respectively. Adjacent to the furnace, the boiler has a fire brick lined combustion chamber which is connected to the furnace through a small flue pipe. The furnace and a small flue pipe are surrounded by water on all sides. After the combustion chamber a number of horizontal, equally spaced fire tubes are provided.The fire tubes are also completely surrounded by water. After horizontal fire tubes smoke box is provided. After horizontal fire tubes, a smoke box and a chimney are provided for the discharge of the gases to the atmosphere. The smoke box is fitted with a door for cleaning and inspectingof fire tubes.

There are connections provided on the shell at appropriate places for fixing the usual boiler mounting such as pressure gauge, safety valve, feed check valve, blow off cock, steam stop valve, and water level indicator etc. Different accessories are also located at their proper place.

Working

(i) Path of Flue gas:The hot flue gases produced from the burning of the fuel in furnace rise up and enter through the small flue pipe into the combustion chamber. There after the hot flue gases pass through the horizontal fire tubes and the smoke box before finally discharged to the atmosphere through the chimney. The flue gases during their travel from fire box to the chimney give its heat to the surrounding water to generate steam. The path of hot flue gas in the boiler is shown by arrows in Fig. 28.1.

(ii) Path of steam flow: The feed water is supplied uniformly to the shell controlled by a feed check valve. When the boiler is heated, the cold water in the shell courses down and hot water rises up between the fire tubes by natural circulation due to convection current and the steam is generated and collected in the steam space over the water surface of the shell. The circulation of water in the shell is shown by arrows in Fig. 28.1. The steam collected in the steam space then passes through the antipriming pipe where most of the water particles present in the steam are removed.

       (iii) Draft system: In this the draft is produced by natural circulation of air and gas usually done naturally with the help of a chimney.

28.1.2.  Locomotive Fire-Tube Boiler

Features and characteristics

• The locomotive boiler is shown in Fig. 28.2.

  • internally fired,

  • horizontal,

  • multi-tubular,

  • fire tube,

  • natural circulation,

  • artificial draft,

  • portable boiler.

  • It is so designed that it is capable of meeting the sudden and fluctuating demands of steam which may be imposed on it because of variation of power and speed.

  • The steam pressure ranges from 12.6 bar to 21 bar and the capacity varies from 900 to 4000 kg per hour.

Uses

It is mainly used in locomotives though it may also be used for stationary power service where semi-portability is desired.

Construction

It consists of a cylindrical steel shell or barrel with a rectangular fire-box/furnace at the back end and a small smoke box at the front end. The fire-box has a combustion chamber with fire grate at the bottom. The fire box is water cooled on all the three sides except the bottom. A fire door is provided in the fire box through which coal is introduced into the grates. The fire brick arch in the fire box is provided to deflect the flames and hot flue gases upwards so that they come in close contact with the heating surface of the fire box. Ash pit is provided under the grate for depositing the ash.

Fig. 28.2. Locomotive Boiler

The horizontal fire tubes are placed longitudinally inside the shell through which the hot gases pass from the furnace to the smoke box. These tubes are always immersed in water. Some of these tubes are of larger diameter and others of smaller diameter. The superheater tubes of small diameter are placed inside the fire-tubes of larger diameter. A smoke box door in front of the smoke box gives access to fire tubes for cleaning, inspection and repairing of the boiler.

A short chimney is provided on the smoke box to discharge hot gases from the smoke box into the atmosphere. The height of the chimney is kept low so that when the locomotive is passing through a tunnel or under a bridge, the chimney does not strike against the top. Moreover, a high chimney is likely to create unbalancing and friction while the engine is in motion.

The steam dome is located at the centre of the shell from which steam is supplied to superhaeters to steam engine. The function of dome is to increase the steam release capacity and to increase the distance of steam from water line which reduces priming.

The various mountings and accessories such as feed check valve, safety valve, whistle etc., are attached to the boiler.

Working

(i) Path of Flue gas: The hot gaseous products from fire box pass through the series of fire tubes and pass out to the smoke box from where they go out into the atmosphere through a short chimney. In this boiler the flue gas complete its path only in one pass. During the travel of hot gases from the grate to the chimney, they give heat to the water and generate steam.

(ii) Path of steam flow: The water is pumped into the boiler and heated through heating surface of the boiler until steam is produced. The steam so generated is collected over the water surface in the steam dome. The dry saturated steam from steam dome is then supplied to the steam engine by turning lever fitted in the cab. In order to get superheated steam,the steam from steam dome is directed into the superaheaters tubes through the superheater header with the help of a regulator and lever arrangement. The superheated steam thus formed is supplied to the steam engine by a superheater exit pipe attached to the superheater exit header.

 (iii) Draft system:

Because the chimney is short, the draft produced by natural circulation is not possible so the artificial draft has to the created to drive out the burnt gases.

The artificial draft is created by either motion of steam engine on rails or periodic rush of spent steam from the steam engine or a blower is mounted at the inlet of the chimney.

Advantages 

• The compactness,

• high steaming capacity,

• mobility 

• low cost of installation.

Weaknesses

• Corrosion in the water legs,

• not capable of meeting very high overloads,

• joint leakages,

• sluggishness of water circulation 

• limited maximum steam pressure of 20 bars are the weaknesses in this boiler.

28.2. WATER TUBE-BOILERS

As discussed under boiler classification, in a water-tube boiler, the water flows inside horizontally, vertically or through inclined tubes and flue gasses flow over the tubes. Under this category, the Babcock Wilcox water-tube boiler is discussed as under.

28.2.1.  Babcock Wilcox Water-Tube Boiler

 Features and characteristics

• This boiler is

  • a stationary

  • longitudinal drum,

  • externally fired,

  • natural circulation,

  • water tube boiler.

  • It is suitable for all types of fuels.

  • Evaporative capacity in this boiler ranges form 1800 to 40,000 kg/h.

  • Operating pressures ranges from 11.5 to 17.5 bar. But the operating pressures may be as high as 42 bar.

Uses

As it is suitable for small size thermal power plants, it may be used for stationary or marine purposes.

Construction

The details of a Babcock Wilcox water tube boiler is shown in Fig. 28.3. It consists of a steam water drum and several inclined straight water tubes. The inclined water tubes are connected with steam water drum with the uptake and down-take header. The water tubes mainly contain water and are kept inclined at 5 to 15 degrees to promote water circulation. The whole assembly of water tubes, headers and drum is hung in a room made of masonry work and lined with fire resistance fire bricks.

Below the uptake header the furnace of the boiler is arranged. A fire door is provided in the front brick work of furnace through which the coal is fed to the grates where its combustion takes place. There is bridge wall deflector which deflects the combustion gases upwards. As the position of water tubes and bottom of the drum is above the furnace, the outer surface of the tubes and half of the cylindrical surface of the steam water drum is exposed to flue gases. To have three passes of flue gas on the water tubes, two baffles are arranged across the water tubes to act as deflectors for the flue gases. A chimney with damper at the inlet is provided for the exit of the gases and to regulate the draft. The superheating tubes are arranged above the water tubes for producing superheated steam.

A mud box is attached to the bottom of down-take header to remove foreign matter from the water. A clean outdoor is provided in the brick wall so that access to the interior of boiler can be made to clean outside of the water tubes. Connections for other mountings and accessories are also provided on the steam water drum.

Fig. 28.3. Babcock Wilcox Boiler

Working

(i)     Path of Flue gas: The hot gases from the furnace first rise upwards and then go down and then rise up again out side the water tube before it finally come out in the atmosphere through the chimney. The flow path of hot gases is shown by the arrows outside the tubes. During their travel they give heat to water and steam is formed.

(ii)   Path of steam-water circulation: Feed water is supplied into the drum by a feed water inlet pipe. As the water in the water tube near the uptake header comes in contact with the hot gases at higher temperature, that portion of water gets evaporated. As a result the mixture of hot water and steam from this portion of the tube rises in the uptake header and then arrives in the steam water drum. In the steam drum, the steam vapors escape the water surface and collected in the upper half of the drum. On other hand, the cold water flows from the drum to the water tubes through the down-take header. Thus a continuous natural cycle of water in boiler is completed.

 (iii) Superheater arrangement: When superheated steam is desired to be produced, the steam accumulated in the steam space in the drum is allowed to enter into the superheater tubes via antipriming pipe.  The flue gases passing over the superheater tubes produce superheated steam. The superheated steam from superheater tube is then finally supplied to the work generating device through a steam stop valve.

(iv) Draft system: The supply of air to the grate is usually done naturally with the help of a chimney.