Unit 1

A steam boiler or steam generator is a closed vessel in which water is heated, vaporised

and converted into steam at a pressure higher than the atmospheric pressure. The heat

energy required for steam generation is produced by burning fuel in the furnace.

INDIAN BOILER REGULATION (IBR, Act 1923): Steam boiler means any closed vessel

exceeding 22.75 litres (other pressure vessel) in capacity and which is used expressively for

generating steam under pressure. It includes any mountings or other fittings attached to

such a vessel, which is wholly or partly under pressure when steam is shut off.

Functions

The function of a steam boiler or generator is to convert chemical energy of a fuel by

combustion into heat and transfer this heat to water and thus to produce the steam.

1.2 Applications of Steam Generated

The steam thus generated is used for:

i. Power Generation: Mechanical work or electric power may be generated by expanding steam in the steam engine or steam turbine.

ii. Heating: The steam is utilized for heating the residential and industrial buildings in cold weather and for producing hot water for hot water supply.

iii. Utilisation of Steam: For industrial processes such as for sizing and bleaching etc. in textile industries. Steam is also used in many other industries like Sugar Mills and Chemical Industries.

Boilers Classification:

There are a large number of boiler designs, but boilers can be classified according to the following criteria:

1. According to Relative Passage of water and hot gases:

1. Water Tube Boiler: A boiler in which the water flows through some small tubes which are surrounded by hot combustion gases, e.g., Babcock and Wilcox, Stirling, Benson boilers, etc.

2. Fire-tube Boiler: The hot combustion gases pass through the boiler tubes, which are surrounded by water, e.g., Lancashire, Cochran, locomotive boilers, etc.

2. According to Water Circulation Arrangement:

1. Natural Circulation: Water circulates in the boiler due to density difference of hot and water, e.g., Babcock and Wilcox boilers, Lancashire boilers, Cochran, locomotive boilers, etc.

2. Forced Circulation: A water pump forces the water along its path, therefore, the steam generation rate increases, Eg: Benson, La Mont, Velox boilers, etc.

3. According to the Use:

1. Stationary Boiler: These boilers are used for power plants or processes steam in plants.

2. Portable Boiler: These are small units of mobile and are used for temporary uses at the sites.

3. Locomotive: These are specially designed boilers. They produce steam to drive railway engines.

4. Marine Boiler: These are used on ships.

4. According to Position of the Boilers:

Horizontal, inclined or vertical boilers

5. According to the Position of Furnace

1. Internally fired: The furnace is located inside the shell, e.g., Cochran, Lancashire boilers, etc.

2. Externally fired: The furnace is located outside the boiler shell, e.g., Babcock and Wilcox, Stirling boilers, etc.

6. According to Pressure of steam generated

Low-pressure boiler: a boiler which produces steam at a pressure of 15-20 bar is called a low-pressure boiler. This steam is used for process heating.

Medium-pressure boiler: It has a working pressure of steam from 20 bars to 80 bars and is used for power generation or combined use of power generation and process heating.

High-pressure boiler: It produces steam at a pressure of more than 80 bars.

Sub-critical boiler: If a boiler produces steam at a pressure which is less than the critical pressure, it is called as a subcritical boiler.

Supercritical boiler: These boilers provide steam at a pressure greater than the critical pressure. These boilers do not have an evaporator and the water directly flashes into steam, and thus they are called once through boilers.

7. According to charge in the furnace.

Pulverized fuel,

Supercharged fuel and

Fluidized bed combustion boilers.

simple vertical boiler is one whose axis of orientation is vertical with respect to the ground. it is a fire tube or a water tube boiler.

Working of simple vertical boiler

In a simple vertical boiler fuel is added through the fire hoke into the grate which burn there to produce the hot gases. fuel when converted into ash is collected into the ash pit. Hot gases rises above and pass their heat to the water in the cross box and go out of the boiler through the chimney. Water heats up and steam production starts. steam which produce as a result of water heating is collected at the steam space of the boiler. steam is collected until a certain pressure is attain and then steam is passed out for use like running turbine or engine.

Application of simple vertical boiler

Simple vertical boiler have may application is railway locomotives for example railway steam engine

Simple vertical boiler are used in the road vehicles like steam wagon (steam lorry or steam waggon)

Simple vertical boiler have a very famous application that steam tractor

There are number of boats specially smaller one which uses the simple vertical boiler to power the engine

In some parts of the world simple vertical boiler are used in steam donkeys

Simple vertical boilers are also used in the steam cranes and steam Steam shovels

Advantages of simple vertical boiler

low initial cost because of lesser parts

Low maintenance cost

Simple working

Easy to install and replace

Occupy small space on ground

Simple vertical boiler have water level tolerance

Disadvantages of simple vertical boiler

Vertical design limits its working in many places

Because of the limited grate area steam production is limited

Impurities settle down at the bottom thus prevent water from heating

Boiler tubes must be kept short to minimise height. As a result, much of the available heat is lost through the chimney, as it has too little time to heat the tubes.

Cochran Boiler is a vertical drum axis, natural circulation, natural draft, low pressure, multi-tubular, solid fuel fired, fire tube boiler with internally fired furnace. It is the modified form of simple vertical boiler. In this boiler, the fire tubes are placed horizontally. The efficiency of this boiler is much better than the simple vertical boiler.

1. Shell: It has a vertical axis cylindrical drum with hemispherical dome type shell at the top.

2. Grate: It is the platform on which the solid fuel is burnt.

3. Combustion Chamber: The burning of fuel takes place in the combustion chamber.

4. Fire Tubes: Cochran boiler has multi tubular fire tubes. The hot flue gases from the combustion chamber travels to the smoke box through these fire tubes. The fire tubes helps in the exchange of heat from the hot flue gases to the water. 5. Fire Hole: It is the hole provided to fire the fuel inside the furnace.

6. Furnace: It lies at the bottom of the boiler. Furnace is the place where all the fuel is burnt. Without furnace the working of this boiler is not possible.

7. Chimney: The chimney is attached to the smoke box. It transfer smoke to the environment. The size of chimney is small as compared with other boiler.

8. Fire Brick Lining. The fire brick lining is present in the combustion chamber and helps in the combustion of the fuel.

9. Manhole: A manhole is provided for the cleaning and inspecting of the boiler from inside.

10. Flue Pipe: It is a small passage connecting the fire box and combustion chamber. The hot gases enters into combustion chamber through the flue pipe.

Other Boiler mountings and accessories attached to the this boiler are:

1. Pressure Gauge: It measures the pressure of steam inside the boiler. 2.

Safety Valve: It blows off the extra steam when the steam pressure inside the boiler reaches above safety level.

3. Water level Indicator: The position of the water level in the Cochran boiler is indicated by the water level indicator.

4. Stop Valve: Stop valve is used to transfer steam to the desired location when it is required. Otherwise it stops the steam in the boiler.

5. Blow off Cock: It is used to blow off the settle down impurities, mud and sediments present in the boiler water.

Working

In Cochran boiler first the fuel is inserted into the fire box and placed on the grate. The fuel is ignited through the fire hole provided at the right bottom of the boiler.

The fuel is burnt in the fire box and due to the burning of the fuel, smoke and hot flue gases emerges out. The hot flue gases enter into the combustion chamber through flue pipes.

From the combustion chamber hot gases enters into the fire tubes. The fire tubes are surrounded by water. The hot flue gases inside the tubes exchange the heat from the hot gases to the water. Due to the exchange of heat, the temperature of the water start increasing and it gets converted into steam. The steam produced rises upward and collected at top of the boiler in the hemispherical dome. An anti-priming pipe is installed at top of the boiler which separates the water from the steam and makes it dry steam. This dry steam is then transfer to the turbines through the steam stop valve.

The hot flue gases and smoke after exchanging heat moves to the smoke box. From the smoke box the burnt gases and smoke is discharge to the atmosphere through the chimney.

Burnt fuel is transferred to the ash pit. Blow off cock is preset at left bottom of the boiler and is used to blow of the impurities, mud and sediment from the boiler water.

A fusible plug is also provided at top of the combustion chamber. When the temperature of the combustion chamber crosses the permissible level, the fusible plug melts and the water through the combustion chamber enter into the furnace of the boiler and stop the fire. In this way a big fire accident can be prevented to take place and also protects the boiler from damage.

Various boiler mounting and accessories are attached to the boiler for its efficient working.

Advantages and Disadvantages

Advantages

1. Low initial installation cost. 2. It requires less floor area. 3. Easy to operate and handle. 4. Transportation of Cochran boiler is easy. 5. It can use all types of fuel.

Disadvantages

1. Low rate of steam generation. 2. Inspection and maintenance is difficult. 3. High room head is required for its installation due to the vertical design. 4. It has limited pressure range.

Babcock and Wilcox Boiler

It is a Horizontal drum axis, natural draft, natural circulation, multitubular, stationary, high pressure, solid fuel fired, externally fired water tube boiler.

It was discovered by George Herman Babcock and Stephen Wilcox in the year 1967. And if was named after its discoverer as Babcock and Wilcox boiler.

The various main parts of Babcock and Wilcox Boiler are as follows

1. Drum: It is horizontal axis drum which contains water and steam.

2. Down Take Header: It is present at rear end of the boiler and connects the water tubes to the rear end of the drum. It receives water from the drum.

3. Up Take Header: it is present at front end of the boiler and connected to the front end of the drum. It transports the steam from the water tubes to the drum.

4. Water Tubes: They are the tubes in which water flows and gets converted into steam. It exchanges the heat from the hot flue gases to the water. It is inclined at angle of 10-15 degree with the horizontal direction. Due to its inclination the water tubes do not completely filled with water and the water and steam separated out easily.

5. Baffle Plates: Baffle plates are present in between water tubes and it allows the zigzag motion of hot flue gases from the furnace.

6. Fire Door: It is used to ignite the solid fuel in the furnace.

7. Grate: It is a base on which the burning of the solid fuel takes place.

8. Mud Collector: It is present at the bottom of down take header and used to collect the mud present in the water.

9. Feed Check Valve: it is used to fill water into the drum.10. Damper: It regulates the flow of air in the boiler.

The various boiler mounting and accessories used in this type of boiler are:

1. Superheater: It increases the temperature of saturated steam to the required temperature before discharging it from steam stop valve.

2. Pressure Gauge: It is used to check the pressure of steam within the boiler drum.

3. Water Level Indicator: It shows the level of water within the drum.

4. Safety Valve: It is a valve which acts when the pressure of steam within the boiler drum increase above the safety level. It opens and releases the extra steam in the environment to maintain the desired pressure within the boiler.

Working:

Now let’s discuss the working of Babcock and Wilcox boiler step by step.

1. First the water starts to come in the water tubes from drum through down take header.

2. The water present in the inclined water tubes gets heated up by the hot flue gases. The coal burning on the grate produces hot flue gases and it is forced to move in zigzag way with the help of baffle plates.

3. As the hot flue gases come in contact with water tubes, it exchanges the heat with water and converts it into steam.

4. The steam generated is moved upward and through up take header it gets collected at upper side in the boiler drum.

5. An anti-priming pipe is provided in the drum. This anti-priming pipe filters the water content from the steam and allows only dry steam to enter into superheater.

6. The superheater receives the water free steam from the anti-priming pipe. It increases the temperature of steam to desired level and transfers it to the steam stop valve.

7. The superheated steam from the steam stop valve is either collected in a steam drum or made to strike on the steam turbine for electricity generation.

Application

The Babcock and Wilcox boiler are generally used to produce high pressure steam in power generation industries. The high pressure steam so generated is used to produce electricity.

Advantages and Disadvantages

Advantages

Steam generation capacity is high. It is about 2000 to 40000 kg/hr.

It occupies less space.

Replacement of defective tubes is easy.

It is the only boiler that is used to generate large quantity of heat in power stations.

The draught loss is minimum.

Inspection of this types of boiler can be done anytime during its working.

Disadvantages

High maintenance cost.

It is not much suitable for impure and sedimentary water. In case of impure and sedimentary water, scale may deposit in the tubes and this leads to overheating and bursting of tubes. That’s why water treatment is must before feeding into the boiler.

Continuously supply of feed water is required for the working. In the case if feed water is not continuously supplied even for a short period of time, the boiler gets overheated. Water level must be carefully watched during the operation of the Babcock and Wilcox boiler.

Lancashire Boiler:

Principle of Lancashire Boiler:

This boiler works on the basic principle of heat ex-changer. It is basically a shell and tube type heat ex-changer in which the flue gases flow through the tubes and the water flows through shell. The heat is transfer from flue gases to the water through convection. It is a natural circulation boiler which uses natural current to flow the water inside the boiler.

Construction:

As we discussed, this boiler is similar a shell and tube type heat ex-changer. It consist a large drum of diameter up to 4-6 meter and length up to 9-10 meter. This drum consist two fire tube of diameter up to 40% of the diameter of shell. The water drum is placed over the bricks works.

Three spaces create between the drum and the bricks, one is at bottom and two are in sides as shown in figure. Flue gases passes through the fire tubes and side and bottom space. The water level inside the drum is always above the side channels of flue gases, so more heat transfer to the water. The drum is half filled with water and the upper half space for steam. The Furnace is located at one end of the fire tubes inside the boiler. The low brick is situated at the grates (space where fuel burns) which does not allow to un-burned fuel and ash to flow in fire tubes. The boiler also consist other necessary mountings and accessories like economizer, super heater, safety valve, pressure gauge, water gauge, etc. to perform better.

Working:

The Lancashire boiler is a shell and tube type heat ex-changer. The fuel is burn at the grate. The water is pumped into the shell through the economizer which increases the temperature of water. Now the shell is half filled with water. The fire tube is fully immersed into the water. The fuel is charged at the grate which produces flue gases. These flue gases first passes through the fire tube from one end to another. This fire tubes transfer 80-90% of total heat to the water. The backward flue gases passes from the bottom passage where it transfer 8-10% heat to water.

The remaining flue gases passes from the side passage where it transfer 6-8% of heat to water. The brick is the lower conductor of heat, so work as heat insulator. The steam produces in drum shell it taken out from the upper side where it flows through super heater if required. So the steam produce is taken by out for process work.

Advantages & Disadvantages:

Advantage:

1. This boiler is easy to clean and inspect.

2. It is more reliable and can generate large amount of steam.

3. It required less maintenance.

4. This boiler is a natural circulation boiler so lower electricity consumption than other boilers.

5. It can easily operate.

6. It can easily meet with load requirement.

7. Lancashire boiler has high thermal efficiency about 80-90%.

Disadvantages:

1. This boiler required more floor space.

2. This boiler has leakage problem.

3. It requires more time to generate steam.

4. It cannot generate high pressure steam if required.

5. Grates are situated at the inlet of fire tube, which has small diameter. So the grate area is limited in this boiler.

Cornish boiler is a simple horizontal boiler which belong to the shell and tube class of boilers. Cornish boiler is much like the Lancashire boiler. Cornish boiler has the ability to produce steam at the rate of 1350 kg/hr and can take the maximum pressure of about 12 bar. Dimensions of the Cornish boiler shell is 4 m to 7 m in length and 1.2 m to 1.8 m in diameter. Cornish boiler is a fire tube type of boiler that is hot gases flow in tubes and water surround these tubes in shell.

Parts of Cornish boiler

Tubes

These are present inside the shell and hot gases flow through them.

Shell

A big shell which contain water inside it as well as tubes. Heat transfer between hot gases and water is take place here

Side flue

Flue are the tubes which take hot gases after they passes out from the fire tubes. Flue present at side of the horizontal shell are called side flues.

Bottom flue

Flue present at the bottom of the shell is called bottom flue. Bottom flue take the smoke toward the chimney.

Grate

Grate is the place where fuel is added for burning.

Ash pit

Ash pit the one which contain the ashes of fuel aster they are completely burn.

Chimney

Chimney take smoke from the bottom flue and take it out to the atmosphere.

Stop valve

It is used to regulate the flow of steam from boiler

Safety valve

Purpose of this valve is to stop the steam pressure from exceeding the maximum limit

Blow of cock

Blow of cock is used to clean boiler by discharging the water and sediments from bottom of boiler

Working of the Cornish boiler

Fuel is added in the grate area where it burn to produce hot gases. There hot gases move into the fire tube which take it inside the shell where it exchange its heat with surrounding water. Water take heat and after some time it start boiling to produce steam. Hot gases upon reaching at the end of the fire tube, divided into two section and each move into the one of two side flue which take them once again at the front section of the boiler where they are move into the bottom flue and bottom flue take them toward the chimney. Chimney throw these gases out of the boiler into the atmosphere. In this process hot gases travels complete length of boiler thrice that is once in fire tube then in side flue and at last bottom flue. Maximum heat transfer is taken place at fire tube and shell section then taken place at side flue and at last at bottom flue.

Application of Cornish boiler

Cornish boiler are used in many industries like textile, sugar, paper, tyre, chemical etc

They are also used to produce steam to run steam turbine

They are also used in many marines

Advantages of Cornish boiler

Cornish boiler have simple design and easy construction

One of its main advantages is its Compactness and Portability

Low construction and maintenance cost

It has ability to overcome load fluctuation

Disadvantages of Cornish boiler

Low steam production rate = 1350 kg/hr

Low maximum steam pressure range 12 bar

Need more ground because of horizontal structure

Locomotive boiler is a horizontal drum axis, multi-tubular, natural circulation, artificial draft, forced circulation, mobile, medium pressure, solid fuel fired fire tube boiler with internally fired furnace. It is used in railway locomotive engines and in marine. It is a mobile boiler and has high steam generation rate.

Construction The construction or main parts of a locomotive boiler are:

1. Fire hole: It is a hole provided at the rear end of the boiler. The solid fuel is inserted and ignited into the furnace through this hole.

2. Fire box: It is a box in which the burning of the fuel takes place.

3. Grate: Grate is a platform on which the solid fuel is kept and burnt.

4. Fire brick arch: It is a brick arch placed inclined over the grate. It prevents the entry of the ash, dust and burnt fuel particles into the fire tubes. It provides a way to the hot flue gases to travel a definite path before entering into the fire tubes of the boiler.

5. Boiler tubes: They are the fire tubes through which the hot flue gases passes and exchange the heat with surrounding water.

6. Smoke box: According to its name, it is a box in which the smoke of the burnt fuel after passing through the fire tubes gets collected. From there it is exhausted in the environment by the chimney.

7. Blast pipe: It is pipe provided above the steam engine. The exhaust steam passes through this blast pipe. It is used to create the artificial draft that pushes the smoke out through the chimney and creates suction for the hot flue gases. The suction created allows the hot flue gases to move forward through the fire tubes.

8. Steam pipe: It is a pipe through which the steam passes. We have two steam pipes; one is main steam pipe present in between the superheater header and dome. And second one is that which connects the superheater exit end to the steam engine.

9. Superheater: It superheats the steam to the desired temperature before entering into the cylinder of the steam engine.

10. Super heater element pipes: These are the pipes of superheater through which the steam travels and gets superheated.

11. Dome: It is present at the top and contains the regulator for regulating the steam produced through the steam pipe.

12. Regulator valve: It is a valve that regulates the steam through main steam pipe for superheating.

13. Safety valve: It is used to maintain the safe working steam pressure in locomotive boiler. It blows off steam when the pressure of the steam increases above safety level and prevents blasting of the boiler.

14. Superheater header: It is the head of the superheater which accepts the steam form the steam pipe.

15. Chimney: It is used to throw out the exhaust smoke and gases to the environment. The length of the chimney is very small in this boiler.

Working

In locomotive boiler, first the solid fuel (coal) is inserted on the grate and is ignited from the fire hole. The burning of the fuel starts and it creates hot flue gases. A fire brick arch is provided that makes the flow of hot flue gasses to a definite path before entering into the long tubes (fire tubes). It also prevents the entry of burnt solid fuel particles into the fire tubes.

The hot flue gases passes through the long fire tubes and heats the water surrounding them. Due to the heating the water gets converted into saturated steam and gets collected at the top.

The saturated steam from the dome enters into the main steam pipe through the regulator valve. The steam travels in the main steam pipe and reaches to the superheater header. Form header, the steam enters into super heater element pipes.

Here it is superheated and then the superheated steam enters into the steam pipe of the smoke box.

The steam form the superheater goes to the cylinder containing piston. The superheated steam made the piston moves within the cylinder. The piston is connected to the wheels of the steam engine and the wheels start rotating.

The exhaust steam from the cylinder enters into the blast pipe. The burnt gases and smoke after passing through the fire tubes enters into the smoke box. The exhaust steam coming out from the blast pipe pushes the smoke out of the boiler through the chimney. Here the smoke cannot escapes out form the boiler by its own, so artificial draft is created by exhaust steam coming out from the steam engine. This artificial draft created pushes the smoke out of the smoke box and creates suction for the hot flue gases.

Advantages and Disadvantages

Advantages

It is portable.

It is capable of meeting sudden and fluctuating demands of steam.

It is cost effective boiler.

It has high steam generation rate.

It is compact in size and its operation is easy.

Disadvantages

It faces the problems of corrosion and scale formation.

Unable to work under heavy load conditions because of overheating problems.

Some of its water space are difficult to clean.

Application

Locomotive boilers are mostly used in railways and marines. The efficiency of this boiler is very less. It cannot work in heavy load conditions because this leads to the overheating of the boiler and finally gets damage. They are also used in traction engines, steam rollers, in portable steam engines and some other steam road vehicles.

Lamont boiler is a high pressure, forced circulation, water tube boiler with internally fired furnace. An external pump is used to circulate the water within small diameter water tubes of the boiler. This boiler was invented by Walter Douglas La-Mont in the year 1925. At that time this boiler was invented to use in ships.

Working Principle

It works on the principle of forced circulation of water within the boiler with the help of centrifugal pump. Its working is totally depends upon the pump. The centrifugal pump circulates the mixture of steam and water through the small diameter tubes of the boiler.

Main parts or Construction

The main parts of Lamont boiler are

1.Feed pump: It supply the feed water into the boiler form hot well.

2.Economizer: It increases the temperature of the feed water to some degree.

3.Steam separating drum: As its names indicates, the steam separating drum separates the steam form the water. The steam gets collected at the upper portion and water at the lower portion of the drum.

4.Circulating pump: It is a centrifugal pump driven by the turbine. It circulates the water from the steam separating drum to the small diameter tubes of the radiant superheater, convective superheater and back to the steam separating drum.

5.Radiant evaporator: It evaporates the water steam mixtures with the help of radiation.

6.Convective evaporator: It changes the mixtures of steam and water into saturated steam through convective mode of heat transfer.

7.Superheater: It superheats the steam to the desired temperature for striking on the turbine blades.

Working

In Lamont boiler, the feed pump circulates the water in the economiser of the boiler. The economiser heats the water to some degree. From economiser, water enters into steam separating drum.

From steam separating drum the mixture of water and steam is forced circulated through the radiant evaporator by an external centrifugal pump. In forced circulation, the pressure of circulation of water through the tubes is more as compared with the natural circulation.

Radiant evaporator heats the water and changes it into steam. Form radiant evaporator the water-steam mixture passes through the convective evaporator. Here the temperature of the fluid increase and most the water gets converted into saturated steam. And after that the saturated steam enters into the steam separator drum.

The steam separator drum as names indicates separates the steam from water. The steam gets collected at the upper portion of the drum. From steam separator drum, steam passes through the superheater. The super heater increases the temperature of the steam to the desired level. And finally the superheated steam is either transfer to the steam collecting drum or made to strike on the blades of the turbine.

The working pressure, temperature and capacity of this boiler is 170 bar, 773 K and 50 tonnes/h.

Advantages and Disadvantages of Lamont Boiler Advantages

1. It can be easily started.

2. High steam generating capacity (about 50 tonnes per hour)

3. It has high heat transfer rate.

4. This boiler can be reassembled with the natural circulation boilers.

5. Its design is simple.

Disadvantages

1. There is a bubble formation at surfaces of the tubes in this boiler. This reduces the heat transfer rate to the steam.

Stirling Bent Tube Boiler

This multidrums water tube boiler, has the following distinguishing features

i) It consists of steam water drums, more than two in number. This gives greater steam and water

storage capacity and the unit can meet peaks in load variations with less pressure drops.

ii) The drums are interlinked to each other with a bank of bent water tubes.

The tubes are bent for the following reasons:-

i) to allow free expansion and contraction of the tubes and to prevent the undue thermal stresses which

are often set up with straight tubes.

ii) tubes replacement becomes easier

iii) flexibility in design particularly with regard to the location of drums.

iv) the tubes can enter the drums in approximately radial direction.

The drums 1, 2 and 3 arranged in the upper part of the setting are called steam drums and mud drum

6 is positioned in the lower part of the setting. The mud drum is usually 10 cm to 25 cm larger in diameter

than a steam drum. The steam drums are usually suspended by the steel structure and the mud drum is

supported by tubes to permit free expansion and contraction of tubes. The entire unit is thus independent of

the brick work setting which serves only to provide furnace space and confine heat.

Fig. 1.10 Stirling Bent Water Tube Boiler

Benson Boiler

The Benson boiler employs forced circulation and has a unique characteristics of the absence of steam separating drum. The entire process of heating, steam formation and suerheating is done in a single continuous tube.

We know that the presence of steam bubbles in contact with surface of tubes impairs the heat transmission from the flue gases to water. Moreover the release of these bubbles can cause pulsation in water circulation. By raising the boiler pressure to the critical pressure of steam 225 bar the above mentioned trouble can be got rid off. At the critical pressure, steam and water co-exist at the same density and no bubble formation occurs. Further at areal pressure the latent heat is zero, the water thus transforms into steam without boiling and passes quickly from liquid to vapour without change in volume. Raising of water to a pressure of 225 bar however requires large amount of work to run the feed pump and this reduces the overall efficiency of plant. The efficiency can however be improved by operating at pressures lightly lower than the critical pressure. The average operating pressure and capacity for such boiler and respectively 210 bar and 13100C kg/hour. Fig. shows and explains diagrammatically the essential components of Benson boiler which are

a) The economizer section.

b) Radiant heating section where most of the heat is transmitted and water gets heated almost to the critical temperature.

c) Transition section where evaporation is complete and superheating begins.

d) Superheater section to obtain the final temperatures.

Fig. 1.12 Benson Boiler

Merits of Benson Boiler

1) As there are no drums, the total weight of Benson boiler is 20 % less than other boilers. This also reduces the cost of the boiler.

2) Steam pressure limitations are absent and supercritical pressures can be employed.

3) Circulating pumps and downcomes are dispensed with.

4) The boiler is self-contained unit and thus it can be readily located outdoors.

5) The Benson boiler is with high specific output and high factor of safety.

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

7) As all the parts of Benson boiler are welded at sites, it can be started quickly because of this.

8) The Benson boiler can be operated most economically by varying the temperature and pressure at partial loads and overloads constant at any pressure.

9) Bubble formation can be eliminated.

10) The blow-clown losses are very low.

Demerits of Benson Boiler

1) The evaporation process leaves some salts and solids deposited in the tubes. This necessitates special means to wash and clean the tubes.

2) In the water steam conversion zone, the tubes are vulnerable to corrosion. This may be remedied by locating the conversion zone in a region where flue gases have a low temperature.

3) The tubes are likely to get superheated when flow of water is insufficient.

4) Small storage capacity requires close co-ordination between steam output, feed water and fuel input to maintain constant temperature.

Loeffler Boiler

The novel feature of the Loeffler boiler is to evaporate water solely means of superheated steam. The furnace is supplied only to economizer and superheater in other words, steam is used both as a heat carrying and heat absorbing medium.

Fig. 1.13 Loeffler Boiler

The boiler drum is housed at any convenient point outside the furnace setting. The feed water is fed into the drum by the feed pump. The water on its way to drum passes through the economiser section and gains some heat of the outgoing gases. The gases circulating pump extracts the steam generated in the drum and forces it through the radiant and convective superheater arranged in series in path of the gases. From the superheater about 1/3rd of steam is trapped off for external use and the remaining 2/3rd passes back to the evaporator drum. The steam gives its superheat to the water contained in the drum and produces an additional supply of saturated steam.

Since poor feed water has no detrimental effect of scale accumulation, Loeffler boilers are best suited for marine transport power generation. Forces circulation boilers of the Loeffler type with steaming capacity of 94500 kg/hour and operating at 140 bar have been commissioned into service.

Velox Boiler

Principle of Operation When the velocity of a gas exceeds the velocity of sound, the heat is transferred from the gas at a much greater rate than the rate achieved with subsonic flow. This fact is used in the velox boiler to achieve the large amount of heat transfer from the given surface area.

Arrangement of Components

In the velox boiler air is compressed to 2.5 bar by an air compressor runs by a gas tubing before supplying to the combustion chamber as shown in figure. 1.14

Fig. 1.14 Velox Boiler

The object of this compression is to secure a supersonic velocity of the gases passing through the combustion chamber and gas tubes. As a result of this high rate of heat release (8 to 10 million Kcal per m3 of combustion chamber volume) is achieved and hence I his boiler is a very compact one.

The capacity of this boiler is limited to about 100 tonnes per hour because large power (about 6,000 bhp) is required to run the air compressor at this output.

Working:

Fuel and air are injected downwards into a vertical combustion chamber which consists of annular gas tubes and annular water tubes ( fire tube principle) on reaching the bottom of the combustion chamber the products of combustion are deflected towards into the evaporator tubes which consist of water annular tubes through which 10 to 20 times the water evaporated is circulated at a high velocity (This prevents the overheating of metal walls) This way heat is transferred from gases to the water at a very high rate. The mixture of water and steam thus formed then passes into a separator is again passed in the water tubes along with preheated feed water coming from economiser.

The gases coming out from the evaporator tubes passed over the supers heater tubes and then to the gas turbine. The power output of the gas turbine is passed through the compressor and the exhaust gases coming out from the gas turbine are passed through the economiser to the atmosphere.

Advantages of this boiler are as follows:

1) very high rates of heat transfer.

2) Compact steam generating unit of great flexibility.

3) Capable of quick starting, even though the separating drum has a storage capacity of about 1,/8 th of the maximum hourly output.

4) Low excess air is required as compressed air is used and the problem of draught is simplified.

5) The control is entirely automatic, and a thermal efficiency of about 90 to 95 % is maintained over a wide range of load.

Boiler Mountings and Accessories

A. Mountings

Different fittings and devices necessary for the operation and safety of a boiler are known as boiler mountings.

The safety valve water level indicator and the fusible plug are the devices used for the safety operation of the boiler. The pressure gauge, feed check valve, blow off cock and steam stop valve fail under the category of fittings and these are essential for the operation of the boiler.

1) Spring Loaded Safety Valve

This type of safety valve is commonly used nowadays for stationary as well as mobile boilers. It is loaded with spring instead of weights. The spring is made from a square steel rod in helical form. A spring loaded safety valve commonly used on Locomotive boiler is shown in figure. It consists of two valves, each of which is placed over a valve seat fixed over a branch pipe as shown in the figure. The two branch pipes are connected to a common block which is fixed on the shell of the boiler. The level as two pivots each of which is placed over each respective valve. The lever is attached with a spring at its middle which pulls the lever in downward direction. The lower end of the spring is attached to the back as shown in the figure. Thus the valves are held tight to their seats by the spring force.

2) High Steam Low Water Safety Valve

It is combined safety arrangement against high steam pressure and low water level in the boiler. This is generally used in Lancashire boiler. It is actually a combination of two valves, one of which is lever safety valve which blows off steam when the working pressure of steam exceeds the preset value. The second valve operate by blowing of the steam when

the water level in the boiler falls below predetermined level. This raises an alarm, so that corrective action can be taken by pumping in water.

3) Water Level Indicator

It is important fitting which indicates water level inside the boiler to the observer. Usually two water level indicators are fitted in front of the boiler. The water level indicator shows the level of water in the boiler drum and warns the operator if by chance the water level goes below a fixed mark, so that corrective action may be taken in time to avoid any accident.

Fig.1.17 Water Level Indicator

4) Pressure Gauge

A pressure gauge is used to measure the pressure of the Steam inside the boiler. The commonly used pressure gauge known as Bourdon type pressure gauge is shown in figure.

Fig 1.18 Bourdon Pressure Gauge

It consists of an elastic metallic type of elliptical cross section and is bent in the form of circular arc. One end of the tube is fixed and connected to the steam space of the boiler and other end is connected to a sector wheel through a link. The sector remains in mesh with a pinion fixed on a spindle. A pointer as shown in the figure is attached to the spindle to read the pressure on a dial guage.

5) Fusible Plug

The main object of the fusible plug is to put off the fire in the furnace of the boiler when the water level in the boiler falls below an unsafe level and thus avoid the explosion which may take place due to overheating of the tubes and shell. This plug is generally fitted over the crown of the furnace or over the combustion chamber.

Fig 1.19 Fusible Plug

6) Feed Check Valve

The function of the feed check valve is to allow the supply of water to the boiler at high pressure continuously and to prevent the back flow of water from the boiler when pump pressure is less than boiler pressure or when pump fails.

Fig 1.20 Feed Check Valve

7) Blow-off cock

The blow-off cock is used for dual functions:

1. To empty the boiler when necessary for cleaning, repair and inspection

2. To discharge the mud and sediments carried with the feed water and accumulated at the bottom of the boiler.

By periodic blow-of, the salt concentration in the boiler is also reduced. Even with a small amount of dissolved salt, over a period of time, due to the evaporation of water, the salt accumulates in the boiler raising the salt concentration.

It is fined to the lowest part of the boiler either directly with the boiler shell or to a pipe connected with the boiler.

Fig. 1.21 Blow-off Cock

8) Steam Stop Valve

It is the largest valve on the steam boiler and usually fitted to the highest part of the boiler shell. The function of the stop valve is to regulate the flow of steam from the boiler to the engine as per requirement and shut off the steam flow when not required.

B. Accessories

Boiler accessories are auxiliary parts required to increase the overall efficiency of the plant, water feeding equipment, air preheaters, economizers and superheater are some of the essential accessories of the boiler.

Economiser:

An economizer is a specially constructed heat exchanger for harnessing the heat energy of outgoing flue gases and utilizing it in preheating of boiler feed water. It saves the heat energy and so the fuel and decreases the operating cost of boiler by increasing its thermal efficiency.

It is employed for boilers of medium pressure range. Here a number of vertical tubes made of cast iron are connected to common headers at the bottom and top. Feed water flow into the bottom header and then through the vertical tubes flow out from the top header. Hot flue gases escaping from the boiler are directed to flow across the outside surface of tubes thus indirectly heating the feed water flowing inside. To avoid deposit of soot over the tube surface, tubular scrapers are fitted over the tubes. These are operated by chain and pulley system and while moving up and down slowly scrap the soot over the wall of tubes and so increase the heat transfer rate. An internal tube economizer is fitted inside the boiler and is an integral part of it.

Advantages of Economizer

1. Stresses produced in the boiler material due to temperature difference of boiler material and feed water are reduced because of increase in feed water temperature.

2. Evaporative capacity of boiler increases as less heat will be required to generate steam if feed water temperature is already high due to preheating.

3. Overall efficiency of boiler increases because of more steam produced per kg of fuel burnt.

Air preheater

Air preheater, like economiser, recovers some portion of the waste heat of the flue gases. Air supplied to the combustion chamber is preheated by using the heat in the waste flue gases. Air preheaters are placed after the economiser and before the gases enter the chimney.

The preheating of air offers the following advantages:

1) Waste heat from the flue gases is recovered for heating air and reduces the fuel consumption by about 1.5% for each 100° C drop in gas temperature.

2) Inferior grades of coal can be burnt efficiently with preheated air.

3) Combustion can be more efficient and an intense flame can be achieved in the furnace. This increases the evaporation rate of the boiler.

Air preheaters can be classified as tubular type, plates type and generative type. In tubular type, the hot flue gases pass through the tubes and air is forced to flow over the tubes. The plate type consists of a number of parallel plates. Air flows through the alternate spaces of the plates and flue gas passes through the remaining passages. In regenerative type, a wire mesh rotor is alternately heated by the flue gases and then cooled by the air and as a result the air is heated.

Superheater