Babcock and Wilcox boiler

BABCOCK AND WILCOX BOILER

Babcock and Wilcox boiler is a Horizontal, Externally Fired, Water Tube Boiler. In this, water is circulated inside the tubes and hot gases flow over the tubes.

Description [Fig. 3]

The Babcock and Wilcox boiler consists of:

1. Water and Steam Drum (Boiler Shell)

2. Water Tubes

3. Uptake Header and Down-take Header

4. Grate and Furnace

5. Baffles

6. Superheater

7. Mud Box

8. Two Inspection Doors

1. Water and Steam Drum (Boiler Shell)

One half of the drum is filled with water. Steam remains in the top half of the drum. It is about 8 m in length and 2 m in diameter.

2. Water Tubes

Water tubes are placed between the drum and the furnace in an inclined position (at an angle of 12° to 15°) to promote water circulation. These tubes are connected at right angles to the uptake-header and the down-take header as shown.

3. Uptake Header and Down-take Header

Drum is connected at one end to uptake-header by short tubes and at the other end to down take header by long tubes.

4. Grate and Furnace

Grate and Furnace are provided below the uptake-header. Coal is fed to the grate through the fire door.

5. Baffles

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

6. Superheater

The boiler is fitted with a superheater tube of U-shape. It is placed just under the drum and above the water tubes. Its upper box is connected to a vertical tube, the top of which is situated in the steam space. Its lower box is connected to stop valve.

7. Mud Box

Mud box is provided at the bottom end of the down-comer.

8. Two Inspection Doors

These are provided for cleaning and inspection of the boiler.

9. Damper

A damper is fitted as shown.

Working

A constant water level is maintained in the boiler drum. Coal is fed to the grate through the fire door and is burnt. There is a slow moving chain grate on which the coal is fed from the hopper. By using the moving grate, the rate of fuel burning is controlled by changing the thickness of the coal bed and also by changing the speed of the moving grate.

Flow of flue gases

The hot flue gases from the furnace rise upward and pass across the left-side portion of the water tubes. The baffles deflect the flue gases. Hence, the flue gases travel in a 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 the atmosphere through the chimney. A damper is fitted as shown to regulate the flue gas outlet

Water circulation

Water descends into the down-take header. It flows upwards in the inclined water tubes, then in the uptake headers and finally to the drum.

That portion of the 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-take header 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 known as Natural Circulation.

The mud or sediment in the water is collected in the mud box. It is blown-off time to time by means of a blow-off cock.

Superheating

Steam is taken from the steam space of the drum through a tube to the superheater. Steam is superheated in the superheater, as it receives additional heat. From the superheater, the superheated steam goes to the stop valve and finally to the turbine.

Boiler 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.

1. SALIENT FEATURES

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 boilers.

5. Steam generation capacity and operating pressure are high compared with fire tube boilers.

6. Normally, the furnace is provided with a moving chain grate. By changing the speed of the moving chain grate, the rate of fuel burning can easily be controlled.

7. The water tubes are kept inclined at an angle of 10°-15° to promote water circulation.