Lancashire Boiler

LANCASHIRE BOILER

Lancashire boiler is a stationary, horizontal, internally fired, natural circulation, fire tube boiler. It can generate steam at the rate of about 9000 kg / hour at a pressure of 15 bar.

Description (Fig. 4)

Lancashire boiler consists of the following parts:

1. Horizontal Cylindrical Shell

2. Internal Flue Tubes (Furnace Tubes)

3. Side Flues and Bottom Flue

4. Grate

5. Furnace Door

6. Fire Bridge

7. Dampers

1. Horizontal 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 internal flue tubes. A Lancashire boiler may have cylindrical shell of diameter 2 – 3 meters and of length 8 – 10 meters.

2. Internal Flue Tubes (FT): Internal Flue Tubes are also known as Furnace Tubes. Two internal tubes extend from one end to the other end of the shell. These flue tubes are connected to the grate at their front end. An Enclosed Chamber (EC) is built for each of the flue tubes at the rear end of the boiler shell.

3. Side Flues (SF) and Bottom Flue (BF): There are two side flues and one bottom flue outside the shell. These flues are made of ordinary brick lines with fire bricks on their inner faces. Side Flues are connected at their rear end to a common rear passage, which is connected to the chimney flue. Internal flue tubes are connected to the Bottom Central Flue. Bottom flue, in turn, is connected to the side flues.

4. Grate (G): The grate is provided at the front end of the internal flue tubes.

5. Furnace Door (FD): Coal is fed on to the grate through the furnace door.

6. Fire Bridge (FB): It is made of brick work. It is built at the end of the grate. Its purpose is to prevent the flow of coal and ash particles into the interior of the flue tubes. Otherwise, the coal and ash particles carried with gases may settle down as deposits on the interior of the flue tubes. Thus, these deposits reduce the heat transfer from the flue tubes to the water surrounding the tubes.

7. Dampers (D₁ and D₂): Dampers are in the form of sliding doors. These are provided at the end of the side flues. Their function is to regulate the flow of gases from the side flues to the chimney flue. Thus, the dampers regulate the combustion rate and thereby the quantity of steam generated.

Note: For examination purpose, only sectional front view and sectional side view need be drawn.

Working

Water Circulation

Boiler shell is filled with water to three-fourth of its volume so as to submerge both the internal flue tubes. The remaining space above water surface is the steam space.

Path of hot flue gases

See Fig. 4. The fuel (coal) is fed through the fire door on to the grate and is burnt. The hot flue gases leaving the grate move along the length of the internal flue tubes from the front end upto the rear end of the shell.

As these hot gases pass through the flue tubes, heat transfer takes place from the hot gases to the water through the walls of the flue tubes. Note that the hot gases emerge from both the flue tubes into the respective rear enclosed chambers.

Then, from the rear enclosed chambers, the hot gases flow downwards to the bottom flue from its rear end to the front end. This results in the heat transfer from the hot gases to the water through the bottom portion of the boiler shell which is exposed to the bottom flue.

Then, the flue gases divide into two streams at the front end of the shell and pass to the side flues. Thus, the two sides of the boiler shell exposed to the side flues are heated.

Passing along the two side flues, the hot gases travel upto the rear end of the boiler to the chimney. Then, the flue gases are discharged into the atmosphere through the chimney.

The above arrangement of the flow passages of the hot flue gases increases the heating surface of the boiler to a large extent. The path of the flow of the flue gases has been shown by arrows in the Figure.

Dampers (D₁ and D₂)

Dampers control the flow of hot flue gases and regulate the combustion rate of the fuel as well as the steam generation rate of the boiler. Dampers control the draught. The object of draught is to ensure complete combustion of coal by supplying sufficient quantity of air through the fuel.

Boiler Mountings

The boiler is fitted with necessary mountings. Water level indicator and pressure gauge are fitted at the front. Steam stop valve, Safety valve, High steam and low water safety valve and Manhole are fitted on the top of the shell.

High Steam and Low Water Safety Valve: It is mounted over the low water alarm apparatus. It is a combination of two valves. One valve blows off steam when the working pressure of steam exceeds the design value. The other valve blows off steam when the level of water falls below the normal level.

Blow-off Cock: It is placed below the front end of the shell for the removal of mud and sediments. It is also used to empty water in the boiler during inspection.

Fusible Plug: It is placed at the top of the internal flues just above the grate. It prevents overheating of the boiler tubes by extinguishing the fire when the water level falls below the required level.

Manhole: It is provided at the top of the shell for the purpose of periodical inspection and repair.

 

1. SALIENT FEATURES

• The arrangement of flow of flue gases in Lancashire boiler increases the heating surface of the shell to a large extent.

• It has a very good steaming capacity.

• Coal of inferior quality can be used in this boiler without any operational defects.

• Superheater can be easily incorporated into the system at the end of the internal flue tubes. Thus, overall efficiency of the boiler can be increased.

• Feed water used does not require strict treatment before use in the boiler shell.

• Low initial cost. Its maintenance is easy.

• Due to moderate working pressure and slow evaporation rate, it is excellent to supply steam to process industries like paper mills, sugar mills, chemical industries, etc.

 

2. ADVANTAGES AND DISADVANTAGES OF WATER TUBE BOILERS OVER FIRE TUBE BOILERS

Advantages

1. Steam Pressure

Steam can be generated at very high pressures in water tube boilers.

2. Heating Surface

In water tube boilers, water is contained in a large number of tubes. Hence, heating surface is more than that of fire tube boilers. Thus, evaporation rate increases.

3. Water Circulation

A good and rapid circulation of water can be made.

4. Variation of Load

Circulation of water is more positive in water tube boilers. Steam can be raised quickly than is possible with a fire tube boiler. Hence, it can be used for variations of load.

5. Heat Transfer

The hot gases flow almost at right angles to the direction of water flow. Hence, maximum amount of heat is transferred to water.

6. Safety Boilers

Bursting of one or two water tubes does not affect the boiler very much with regard to its working. Hence, water tube boilers are sometimes called as Safety Boilers. But, bursting of flue tubes in a fire tube boiler causes serious problems.

7. Transportation

The different parts of a water tube boiler can be separated. Hence, it is easy to transport.

8. Space Required

For a given power, water tube boiler occupies less space than that of fire tube boiler.

9. Thermal Power Plants

It is suitable for use in thermal power plants (because of various advantages listed above).

Disadvantages

1. Water Treatment

Water tube boiler is not suitable for impure and sedimentary water, as a small deposit of scale may cause the overheating and bursting of tubes. Hence, water treatment is very essential for water tube boilers.

2. Feed Water Supply

Failure in feed water supply even for a short period is liable to make the boiler overheated. Hence, the water level must be watched very carefully during operation of a water tube boiler.

3. Maintenance Cost

Maintenance cost of water tube boiler is high.

4. Initial Cost

Initial cost of water tube boiler is more than that of fire tube boiler.

5. Mobile Purpose

Water tube boilers are not suited for mobile purpose.