industrial boilers-What does an industrial boiler do?

INDUSTRIAL BOILERS

What does an industrial boiler do?

Ans: A boiler incorporates a chamber or chamber thus on burn the fuel and generate heat. The generated heat is transferred to water to form steam, the tactic of boiling. This produces saturated steam at a rate which can vary keep with the pressure beyond the boiling water.

 

The boilers are generally required in chemical industries, paper industries, pharmaceutical indusmany others. Efficiency, reliability and cost are major factors in the design of industrial boilers

central stations. Boiler's capacity varies from 100 to 400 tons of steam per hour. Industrial conies in foreign countries with large steam demands have considerable interest in cogeneration,

multaneous production of steam and electricity because of federal legislation. High temperature wd high pressure boilers 350°C and 75 ata) are now-a-days used even though high pressure and temarture are rarely, needed to process requirement but they are used to generate electricity to surging prices of the oil, most of the industrial boilers are designed to use wood, municipal - pulverized coal, industrial solid waste and refinery gas few industrial boilers which are in common use are discussed below.

Packaged Water-tube Boilers. The boilers having a capacity of 50 tons/hr are generally designed with water cooled furnaces. Advantages of this design include minimum weight and maintenance as vell rigidity and safety. Presently the boilers are also designed to burn coal, wood and process waste also. The much larger furnace volumes required in units designed for solid fuels restrict the capacity of packaged units to about 40 tons/hr or about one-third of a oil-gas fired unit that can be shipped by railroad

 

MERITS AND DEMERITS OF WATER TUBE BOILERS OVER FIRE TUBE BOILERS MERITS

1.      Generation of steam is much quicker due to small ratio of water content to steam content. is 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 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 car

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

 

REQUIREMENTS OF A GOOD BOILER

 

A good boiler must possess the following qualities :

 

1. The boiler should be capable to generate steam at the required pressure and quantity as quick as possible with minimum fuel consumption.

2. The initial cost, installation cost and the maintenance cost should be as low as possible.

3. The boiler should be light in weight, and should occupy small floor area.

 4. The boiler must be able to meet the fluctuating demands without pressure fluctuations.

5. All the parts of the boiler should be easily approachable for cleaning and inspection.

6. The boiler should have a minimum of joints to avoid leaks which may occur due to expansion and contraction

7. The boiler should be erected at site within a reasonable time and with minimum labour.

8. The water and flue gas velocities should be high for high heat transfer rates with minimum pressure drop through the system.

9. There should be no deposition of mud and foreign materials on the inside surface and soot deposition on the outer surface of the heat transferring parts.

10. The boiler should conform to the safety regulations as laid down in the Boiler Act.

 

 HIGH PRESSURE BOILERS

 

In all modern power plants, high pressure boilers (> 100 bar) are universally used as they offer the following advantages.

In order to obtain efficient operation and high capacity, forced circulation of water through boiler tubes is found helpful. Some special types of boilers operating at super critical pressures and using forced circulations are described in this chapter.

1. The efficiency and the capacity of the plant can be increased as reduced quantity of Steam required for the same power generation if high pressure steam is used.

2. The forced circulation of water through boiler tubes provides freedom in the arrangement furnace and water walls, in addition to the reduction in the heat exchange area.

3. The tendency of scale formation is reduced due to high velocity of water. 4. The danger of overheating is reduced as all the parts are uniformly heated.

5. The differential expansion is reduced due to uniform temperature and this reduces the possibility of gas and air leakages.

6. Some special types of high pressure supercritical boilers are described in this chapter.

 

LA MONT BOILER

A forced circula forced circulation boiler was first introduced in 1925 by La Mont. The arrangement of water on and different components are shown in Fig. 5.5.

 the feed water from hot well is supplied to a storage and separating drum (boiler) through the per Most of the sensible heat is supplied to the feed water passing through the economizer.

Adulates the water at a rate 8 to 10 times the mass of steam evaporated. This water is circulated the evaporator tubes and the part of the vapor is separated in the separator drum. The large of water circulated (10 times that of evaporation) prevents the tubes from being overheated.

 

The centrifugal pump delivers the water to the headers at a pressure of 2.5 bar above the drum ressure. The distribution headers distribute the water through the nozzle into the evaporator. The steam separated in the boiler is further passed through the super-heater. Secure a uniform flow of feed water through each of the parallel boiler circuits a choke is fitted entrance to each circuit. 120 bar and ten These boilers have been built to generate 45 to 50 tonnes of superheated steani at a pressure of and temperature of 500°C. Recently forced circulation has been introduced in large capacity power?

 

BENSON BOILER

 

 the main difficulty experienced in the La Mont boiler is the formation and attachment of bubbles on the inner surface of the heating tubes. The attached bubbles reduce the heat flow and steame generation as it offers higher thermal resistance compared to water film and steam generaoffers higher thermal resistance compared to water film .

1. . Benson in 1922 argued that if the boiler pressure was raised to critical pressure (225 atm.), the steam and water would have the same density and therefore the danger of bubble formation can be completely.

Steam and water w

2. Natural circulation boilers require expansion joints but these are not required for Benson as te welded. The erection of Benson boiler is easier and quicker as all the parts are welded at Workshop iob of tube expansion is altogether avoided.

3. The transport of Benson boiler parts is easy as no drums are required and majority of the par are carried to the site without pre-assembly.

4. The Benson boiler can be erected in a comparatively smaller floor area. The space problem does not control the size of Benson boiler used.

5. The furnace walls of the boiler can be more efficiently protected by using small diameter and close pitched tubes

6. The superheater in the Benson boiler is an integral part of forced circulation system, therefore no special starting arrangement for superheater is required.

7. The Benson boiler can be started very quickly because of welded joints,

8. The Benson boiler can be operated most economically by varying the temperature and pres sure at partial loads and overloads. The desired temperature can also be maintained constant at any pressure.

9. Sudden fall of demand creates circulation problems due to bubble formation in the natural circulation boiler which never occurs in Benson boiler. This feature of insensitiveness to load fluctus tions makes it more suitable for grid power station as it has better adaptive capacity to meet sudden load fluctuations.

10. The blow-down losses of Benson boiler are hardly 4% of natural circulation boilers of same capacity.

11. Explosion hazards are not at all severe as it consists of only tubes of small diameter and has very little storage capacity compared to drum type boiler.

During starting, the water is passed through the economiser, evaporator, superheater and back to the feed line via starting valve A. During starting the valve B is closed. As the steam generation starts and it becomes superheated, the valve A is closed and the valve B is opened.

During starting, first circulating pumps are started and then the burners are started to avoid the overheating of evaporator and superheater tubes.

 

LOEFFLER BOILER

 

The major difficulty experienced in Benson boiler is the deposition of salt and sediment on the inner surfaces of the water tubes. The deposition reduced the heat transfer and ultimately the generating capacity. This further increased the danger of overheating the tubes due to salt deposition as it has high thermal resistance.

The difficulty was solved in Loeffler boiler by preventing the flow of water into the boiler tubes Most of the steam is generated outside from the feedwater using part of the superheated steam coming out from the boiler.

The pressure feed pump draws the water through the economiser and delivers it into the evapor tor drum as shown in the figure. About 65% of the steam coming out of superheater is passed throug the evaporator drum in order to evaporate the feed water coming from economiser.

The steam circulating pump draws the saturated steam from the evaporator drum and is pas through the radiant superheater and then connective superheater. About 35% of the steam coming from the superheater is supplied to the H.P. steam turbine. The steam coming out from H.P. turbin passed through reheater before supplying to L.P. turbine as shown in the figure.

The amount of steam generated in the evaporator drum is equal to the steam tapped (65%) the superheater. The nozzles which distribute the superheated steam through the water into the evap tor drum are of special design to avoid priming and noise.

This boiler can carry higher salt concentration than any other type and is more compact than thoilers having natural circulation. These qualities fit it for land or sea transport power offler boilers with generating capacity of 94.5 tonnes/hr and operating at 140 bar have been commissioned.

 

SCHMIDT-HARTMANN BOILER

 

The operation of the boiler is similar to an electric transformer. Two pressures are used to effect an interchange of energy.

In the primary circuit, the steam at 100 bar is produced from distilled water. This steam is passed unmerged heating coil which is located in an evaporator drum as shown in the figure. The high pressure steam in this coil possesses sufficient thermal potential and steam at 60 bar with a heat ate of 2.5 kW/m--°C is generated in the evaporator drum.

The steam produced in the evaporator drums from impure water is further passed through the superheater and then supplied to the prime-mover. The high pressure condensate formed in the sobered heating coil is circulated through a low pressure feed heater on its way to raise the feed water temperature to its saturation temperature. Therefore, only latent heat is supplied in the evaporator drum.

Natural circulation is used in the primary circuit and this is sufficient to effect the desired rate of heat transfer and to overcome the thermo-siphon head of about 2 m to 10 m.

In normal circumstances, the replenishment of distilled water in the primary circuit is not required as every care is taken in design and construction to prevent leakage. But as a safeguard against leakage, a pressure gauge and safety valve are fitted in the circuit.

 

Advantages

 

1. There is rare chance of overheating or burning the highly heated components of the primary circuit as there is no danger of salt deposition as well as there is no chance of interruption to the circulation der by rust or any other material. The highly heated parts run very safe throughout the life of the boiler.

2. The salt deposited in the evaporator drum due to the circulation of impure water can be easily med olf just by removing the submerged coil from the drum or by blowing off the water.

3. The wide fluctuations of load are easily taken by this boiler without undue priming or abnorincrease in the primary pressure due to high thermal and water capacity of the boiler.

 4 The absence of water risers in the drum, and moderate temperature difference across the heatoil allow evaporation to proceed without priming.

 

VELOX-BOILER

 

Now, is known fact that when the gas velocity exceeds the sound-velocity, the heat is transde gas at a much higher rate than rates achieved with sub-sonic flow. The advantages of this theory are taken to effect the large heat transfer from a smaller surface area in these boilr.  the large heat transfer from a smaller surface area in this boiler.  

 Air is compressed to 2.5 bar with an help of a compressor run by gas turbine before supply combustion chamber to get super sonic velocity of the gases passing through the combustion chamber and gas tubes tubes and high heat release rates (40 MW/m").

combusion chamber to Chamber and gas tubes and chamber are passed t gases to water while pass thus formed then passes into The burned gases in the combustion sed through the annulus of the tubes as shown in figure. The heat is transferred from me passing through the annulus to generate the steam. The mixture of water and steam passes into a separator which is so designed that the mixture enters with a spiral flow. The centrifugal force thuse produce causes the heavier water particles to be thrown outerward on the walls.This effect separates the steam from water. The separated steam is further passed to superheater and then supplied to the prime-mover. The water removed from steam in the separator is again passed into the water tubes with the help of a pump.

The gases coming out from the annulus at the top are further passed over the superheater where its heat is used-for superheating the steam. The gases coming out of superheater are used to run a gas turbine as they carry sufficient kinetic energy. The power output of the gas turbine is used to run the air compressor. The exhaust gases coming out from the gas turbine are passed through the economiser to utilise the remaining heat of the gases. The extra power required to run the compressor is supplied with the help of electric motor. Feed water of 10 to 20 times the weight of steam generated is circulated through the tubes with the help of water circulating pump. This prevents the overheating of metal walls.

The size of the velox boiler is limited to 100 tons per hour because 400 KW is required to run the air compressor at this output. The power developed by the gas turbine is not sufficient to run the compressor and therefore some power from external source must be supplied as mentioned above.  

 Advantages

1. Very high combustion rates are possible as 40 MJ/m of combustion chamber volume.

2. Low excess air is required as the pressurized air is used and the problem of drought is simplified

 3. It is very compact generating unit and has greater flexibility.

4. It can be quickly started even though the separator has a storage capacity of about 10% of the maximum hourly output.