BoilersBoilers

BoilersBoilers Boiler is an apparatus to produce steam. Thermal energy

released by combustion of fuel is used to make steam at the desired temperature and pressure.

The steam produced is used for: (i) Producing mechanical work by expanding it in steam engine or

steam turbine. (ii) Heating the residential and industrial buildings. (iii) Performing certain processes in the sugar mills, chemical and

textile industries.

Boiler properties-

a). Safety: The boiler should be safe under the operating conditions.

b). Accessibility: the various part of boiler should be accessible for repair & maintenance.

c). Capacity: Should be capable of supplying Steam according to the requirements.

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

e). It should be simple in construction.f). Its initial cost and maintenance cost is low.g). It should be capable of quick starting and loading.

CLASSIFICATION OF BOILERS1. According to flows of water and hot gases a) Water tube b) Fire tube2. Method of firing a) Internally fired boilers b) Externally fired boilers 3. Pressure of steama) High pressure boilers( above 80 bar)b) Low pressure boilers (below 80 bar)4. Method of circulation of watera) Natural circulation b) Forced circulation5. Nature of service to be performed a) Land boilersb) Mobile boilers

Fire – Tube BoilersFire – Tube Boilers

Water tube boilers

Comparison between Fire-tube & water-tube boilersS no.

Particulars Fire tube boilers Water tube boilers

1. Mode of firing Internally fired Externally fired

2. Rate of steam production

lower Higher

3. construction Difficult Simple

4. transportation Difficult Simple

5. Treatment of water Not so necessary More necessary

6. Operating pressure Limited to 16 bar Under high pressure as 100 bar

7. Floor area More floor area Less floor area

8. Shell diameter Large for same power Small same power

9. explosion Less More

10. Risk of bursting lesser More risk

MERITS OF WATER TUBE BOILERS OVER MERITS OF WATER TUBE BOILERS OVER FIRE TUBE BOILERSFIRE 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.

7. Damage due to the bursting of water tube is less serious. Therefore, water tube

boilers are sometimes called safety boilers.

8. All parts of the water tube boilers are easily accessible for cleaning, inspecting

and repairing.

9. The water tube boiler's furnace area can be easily altered to meet the fuel

requirements.

Demerits :

1. 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.

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

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

1. Fire tubes boilersa) Simple vertical boilerb) Cochran boilersc) Lancashire boilerd) Locomotive boilers

2. Water tube boilersa) Babcock and Wilcox boilerb) Stirling Bent-tube boilers

a) SIMPLE VERTICAL BOILER

• It consist of a cylindrical shell, the greater portion of which is full of water & remaining is the steam space.

• At the bottom of the fire box is grate on which fuel is burnt and the ash from it falls in the ash pit.

• A simple vertical boiler is self-contained & can be easily transported.

Heating area- 7-10 times grate areaSteam generation capacity- 1000 kg/hrMax steam pr.- 10 barEfficiency- 50%

b) Cochran b) Cochran BoilerBoiler

The outstanding features of this boiler are :

• Fire tube of vertical type boiler• Heating area- 10-25 times grate area• It is very compact and requires minimum floor area• Any type of fuel can be used with this boiler like wood, oil

fuel etc.• Well suited for small capacity requirements• Steam pressure -20 bar• Steam generation capacity – 20 kg/hr to 3000 kg/hr• It gives about 70% thermal efficiency with coal firing and

about 75% with oil firing

Main parts.....

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

2. Water Level Indicator: This indicates the water level in the boiler .

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

4. Fusible Plug: If the water level in the boiler falls below a predetermined level, the boiler shell and tubes will be overheated.

5. 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.

6. 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.

7. 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.

c) Lancashire c) Lancashire BoilerBoiler

• It is stationary fire tube, internally fired, horizontal, natural circulation boiler. This is a widely used boiler because of its good steaming quality and its ability to burn coal of inferior quality. These boilers have a cylindrical shell 2 m to 3 m in diameters and its length varies from 6 m to 10 m. It has two large internal flue tubes having diameter between 80 cm to 100 cm in which the grate is situated.

•Working pressure – 0.7 MPa to 2 MPa •Efficiency – 65-70%

LancashirLancashire Boilere Boiler

AdvantagesAdvantages• The feed pipe projecting into the boiler is perforated to ensure

uniform water distribution.• Its heating surface area per unit volume at the boiler is considerably

large.• Its maintenance is easy.• It is suitable where a large reserve of hot water is needed. This

boiler due to the large reserve capacity can easily meet load fluctuations.

• Super-heater and economizer can be easily incorporated into the system, therefore; overall efficiency of the boiler can be considerably increased (80-85%).

d) LOCOMOTIVE BOILERd) LOCOMOTIVE BOILER• Locomotive boiler is a horizontal fire tube type mobile

boiler. The main requirement of this boiler is that it should produce steam at a very high rate. Therefore, this boiler requires a large amount of heating surface and large grate area to burn coal at a rapid rate. In order to provide the large heating surface area, a large number of fire tubes are setup and heat transfer rate is increased by creating strong draught by means of steam jet.

AdvantagesAdvantages1. Large rate of steam generation per square metre of heating

surface. To some extent this is due to the vibration caused by the motion.

2. It is free from brickwork, special foundation and chimney. This reduces the cost of installation.

3. It is very compact.

The pressure of the steam is limited to about 20 bar.

Water tube boilers

a) BABCOCK WILCOX a) BABCOCK WILCOX BOILERBOILER

Features1. The evaporative capacity of this boilers is high compared with other

boilers (20,000 to 40,000kg/hr). The operating pressure lies between 11.5 to 17.5 bar.

2. The draught loss is minimum compared with other boilers.3. The defective tubes can be replaced easily.4. The entire boiler rests over an iron structure, independent of brick

work, so that the boiler may expand or contract freely. The brick walls which form the surroundings of the boiler are only to enclose the furnace and the hot gases.

Stirling Bent-Tube Boiler-

• It consists more than 2 steam Water drum.• This gives greater steam and • Water storage capacity. • Drum interlinked with a bank of bent tube- -To allow free expansion and contraction of tubes -Tube replacement becomes easier -Flexibility in design Pr - 60 barTemperature – 450CCapacity – 50,000 kg/hr

BOILER MOUNINGS • Water level indicator• Safety valves• Fusible plug• Pressure gauge• Stop valve• Feed check valve• Blow off cock• Man hole and mud box

BOILER ACCESSORIES • Super heater • Economizer• Air preheater • Feed pump

Water Level Indicator

• This indicates the water level in the boiler.

• It is fitted in front of the boiler from where it easily visible to the operator.

• Top of the mounting is connected to steam Space and bottom to water space.

• Two control clock is provided for regulating Steam and water.

• In case of breakage of glass tube two balls are provided, to prevent accident.

Fusible Plug• It is a safety device used for preventing the level of water from going down below a critical point andThus avoid overheating.

• Mounted at crown plate of combustion chamber.

• Made of fusible metal at interface of copper plug and gun metal body.

• Melting point is low of fusible metal so Copper plug melt down and goes into the furnace, thus quenches the fire.

Safety valve• Its function is to prevent the steam pressure from exceeding a

limiting maximum pressure. • It operate automatically by releasing excess steam and bring

pressure down with in safe limit.

These are-• Dead weight safety valve• Spring loaded safety valve• Lever safety valve

Dead weight safety valve

Stop valve• It regulate the flow of steam from Boiler.

• It mounted on highest part of the boiler shell.

• It operated by rotating the hand wheel which Causes lifting or lowering of spindle, thus causing opening and closing of valve.

Feed check valve• It is non return valve at the end of delivery pipe from feed

water pump• It is mounted on boiler shell slightly below normal water

level• Valve opening and closing is regulated by hand wheel

rotation • It permit unidirectional flow of water from feed pump to

boiler shell

Blow off cock

• It is used for periodical cleaning by discharging water and sediments from bottom of boiler.

• It has conical type plug for opening and closing the flow.

Super Heater

• Its purpose is to super heat steam.

• It is a type of heat exchanger in which steam flows inside the tubes and hot gases surround it.

Economizer

• It is a heat recovery device • Feed water heated from heat available with exhaust gases.• It lowers the fuel requirement• Minimize tendency of corrosion and pitting, reduce thermal stresses• Location depends upon boiler structure• Scrappers used for cleaning and removal of soot Deposit on tubes.

Air Preheater • It is used for recovering the heat going along with exhaust gases

by the air before being sent to furnace.• Heat recovered by passing exhaust gases through air• It placed after economizer and before chimney• Air pre heating helps in- Faster rate of combustion Possibility of burning interior quality of coal/fuel Increased rate of evaporation from boiler

Feed Pump• It used for sending water into boiler at the pressure at which steam

generation takes place.

• Types of feed pump- Centrifugal pump Reciprocating pump

Now a days multistage centrifugal type pumps are used, which have favorable pressure/ volume characteristics.

•La-Mont Boiler

Boiler PerformanceBoiler Performance

1) Evaporative capacity can be expressed in terms of:

a) kg of steam/hr

b) kg of steam/kg of fuel fired

Equivalent EvaporationEquivalent Evaporation

• But different boilers give out steam at different temperature and pressure

• It refers to the quantity of dry saturated steam generated per unit time from feed water at 100ºC to steam 100ºC at saturated pressure corresponding to 100ºC.

• It is also called equivalent evaporation from and at 100ºC.• Evaporative capacity expressed in terms of mass of water

getting converted to steam-

Equivalent Evaporation = m (h-hw)/538.9

Boiler EfficiencyBoiler Efficiency

• Ratio of heat actually utilized in the generator to the heat supplied by the fuel.

Efficiency= m (h-hw)/mf CV

Where CV is the calorific value