BOILERS

BOILERS

• A Boiler is a closed vessel having a capacity of more than 22.75 litres and used for generating steam under pressure.

• It includes mountings and other accessories attached to such a vessel for its safe operation.

BASIC SPECIFICATIONS OF BOILERS

• Steam Pressure (bar)• Steam Temperature (K, ºC)• Steam Flow Rate (Tonnes/hr, kg/s)

THERMAL POWER STATION

BOILER PROPERTIES

(i) Safety. The boiler should be safe under operating conditions.

(ii) Accessibility. The various parts of the boiler should be accessible for repair and maintenance.

(iii) Capacity. Should be capable of supplying steam according to the requirements.

(iv) Efficiency Should be able to absorb a maximum amount of heat produced due to burning of fuel in the furnace.

(v) It should be simple in construction.

(vi) Its initial cost and maintenance cost should be low.

(vii) The boiler should have no joints exposed to flames.

(viii) Should be capable of quick starting and loading.

CLASSIFICATION OF BOILERS

Fire Tube Boilers

In the Fire Tube Boilers the hot gases from the furnace pass through the tubes which are surrounded by water. The hot gases heat the water. Fire tube boilers are low pressure boilers. In process industries like Paper, Leather, Textile, Chemical and Sugar industries fire tube boilers are used.• e.g., Cochran boiler, Locomotive boiler.

Water Tube Boilers

In Water Tube Boilers the water circulates inside the tube and is heated from outside by the hot gases that are produced through combustion in the furnace. Normally for high pressure boilers water tube boilers are used. e.g., Babcock and Wilcox boiler

• Packaged Boilers are compact in size which can be transported by trucks. These are low capacity boilers mostly used in process industries.

• Field Erected Boilers are the ones which are erected in the site. Individual components are transported to the site where erection will take place. These boilers are used for power generation.

• Wet, Dry and Super Heated Steam

Steam can be classified as wet steam, dry steam and super heated steam. In wet steam some moisture is present, in dry steam no moisture is present and in superheated steam the dry steam will further be heated.

• Boiler Mountings The valves and gauges which are

necessary for the safe operation of boiler are known as boiler mountings. Water gauge, pressure gauge, safety valve, blow off valve and fusible plug are some of the mountings.

Fusible Plug

If the water level in theboiler falls below a predetermined level, the boiler shell and tubes will be overheated.

Main parts ……

Water Level Indicator

This indicates the water level in the boiler .

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

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

Safety Valve.

The function of the safety valve is to prevent the increase of steam pressure in the boiler above its design pressure.

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.

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, they have to be removed using blow off cock.

• Boiler Accessories Equipment like economizer, air pre

heater and super heater are attached to the boiler to increase its efficiency. These are used to extract maximum heat from the flue gases.

Air Pre-heater

COCHRAN BOILER – FIRE TUBE BOILER

COCHRAN BOILER – FIRE TUBE BOILER

• Cochran boiler is a Vertical Fire Tube Boiler• It is a small capacity steam generation boiler

having a number of horizontal fire tubes• The boiler consists of a cylindrical shell with its

crown having a Hemispherical Shape• The Hemispherical crown of the boiler shell gives

maximum strength to withstand the pressure of the steam inside the boiler.

• The flue gases coming out of fire tubes are finally discharged to the atmosphere through chimney.

BABCOCK AND WILCOX BOILER (WATER TUBE BOILER)

BABCOCK AND WILCOX BOILER (WATER TUBE BOILER)

• This is a water tube boiler used for High Pressure Steam Production

• The Hot gases from the furnace pass through Water Tubes, Baffles, Super Heater and then go to chimney through damper.

• The Damper controls the rate of burning and the steam generation.

MERITS OF WATER TUBE BOILERS OVER FIRE TUBE BOILERS

1. Generation of steam is much quicker due to small ratio of water content to steam content. This also helps in reaching the steaming temperature in short time.

2. Its evaporative capacity is considerably larger and the steam pressure range is also high-200 bar.

3. Heating surfaces are more effective as the hot gases travel at right angles to the direction of water flow.

4. The combustion efficiency is higher because complete combustion of fuel is possible as the combustion space is much larger.

5. The thermal stresses in the boiler parts are less as different parts of the boiler remain at uniform temperature due to quick circulation of water.

6. The boiler can be easily transported and erected as its different parts can be separated.

DEMERITS :

• It is less suitable for impure and sedimentary water, as a small deposit of scale may cause the overheating and bursting of tube. Therefore, use of pure feed water is essential.

• They require careful attention. The maintenance costs are higher.

• Failure in feed water supply even for short period is liable to make the boiler over-heated.

UTILITY BOILERS

• Utility Boilers are used in thermal power plants to generate electric power in large quantities

• These are water tube boilers that produce high pressure (175 bar or more) and high temperature (540 ºC or more) steam in large quantities with a single unit power from 210 to 1000 MW

• BHEL Trichy is involved in the manufacture of such boilers of capacity upto 660 MW

THE BOILER CAN BE DESCRIBED TRACING THE FOLLOWING CIRCUITS.

• FUEL – AIR – COMBUSTION PRODUCTS• WATER / STEAM CIRCUIT• TURBINE / GENERATOR