APPLIED THERMAL ENGINEEING 15ME52T

UNIT-II

STEAM BOILERS

Steam generators are used to produce high pressure superheated steam. A steam generator is complex combination of economizer, boiler, super heater, reheater and air preheater.

Boiler is a portion of the steam generator but it is used to mean the whole steam generator.

DEFINITION OF BOILER

A steam generator or a boiler is a closed steel vessel which generates the steam by transferring heat, produced by burning of fuel, to water.

FUNCTION OF BOILER

Following are the functions of steam boiler:

1. To transfer the heat produced by combustion of the fuel to water to generate the steam. 2. To supply the steam at the required constant pressure either dry or superheated.

APPLICATION OR USES OF STEAM BOILERS (APPLICATION OR USES OF STEAM)

Applications Steam boilers are as follows:

(1) POWER GENERATION: Mechanical power can be generated by expanding steam either in a steam engine or in a steam turbine. If the engine/turbine is coupled to a generator, electric power can be generated. (2) HEATING: Steam is used in winter air conditioning of residential and industrial buildings. It is also used to heat water for hot water supply. (3) INDUSTRIAL PROCESSES: Steam is used for certain industrial processes such as sizing, bleaching etc., in textile mills, sugar factories, breweries etc.,

CLASSIFICATION OF BOILERS

(1) BASED ON FLOW OF WATER AND HOT GASES

(A) Fire tube boilers (B) Water tube boilers

(2) BASED ON USE

(A) Stationary boiler (B) Mobile boiler

(3). BASED ON POSITION OF FURNACE

(A) Internally fired boilers (B) Externally fired boilers

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APPLIED THERMAL ENGINEEING 15ME52T

(4). BASED ON AXIS OF BOILER SHELL

(A) Horizontal boilers (B) Vertical boilers

(5). BASED ON NUMBER OF TUBES

(A) Single tube boiler (B) Multi tubular boiler

(6). BASED ON METHOD OF CIRCULATION OF STEAM AND WATER

(A) Natural circulation boiler (B) Forced circulation boiler

(7). ACCORDING TO THE SOURCE OF HEAT

Boilers are classified based on the nature of source of heat as follows:

(A) Boilers which uses solid, liquid or gaseous fuels (B) Boilers which uses nuclear energy (C) Boilers which used electrical energy

FIRE TUBE BOILERS: In fire tube boilers, the hot flue gases produced by combustion of fuels are passed through a tube or group of tubes around which water circulates.

These boilers operate at moderate pressure (16 bars) and are suitable to generate 3-8 tons of steam per hour which is used in process heating.

Eg., Cochran boiler, Lancashire boiler, Cornish boiler, Locomotive boiler etc.,

WATER TUBE BOILERS: In water tube boilers, water circulates inside the tubes while the hot flue gases produced by combustion of fuel pass over them externally.

These boilers operate at very high pressure and are used for power generation.

Eg., Babcock and Wilcox boiler, Stirling boiler, Yarrow boiler etc.,

STATIONARY BOILERS: If the boilers are used at one place only they are termed as stationary boilers.

These boilers are used for either process heating in industries or for power generation in steam power plants.

Eg., Babcock and Wilcox boiler, Fluidized bed combustion(FBC) boiler etc.,

MOBILE BOILER: If the boilers are portable they are known as mobile boilers. Such boilers are use in locomotive and ships.

Eg., Locomotive boiler, Scotch marine boiler etc.,

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APPLIED THERMAL ENGINEEING 15ME52T

INTERNALLY FIRED BOILERS: If the furnace is located inside the boiler shell, then the boilers is termed as internally fired boiler. Most of the fire tube boilers are internally fired.

EXTERNALLY FIRED BOILERS: If the furnace is located outside the boiler shell, then the boilers is termed as externally fired boilers. Water tube boilers are always externally fired.

HORIZONTAL BOILERS: If a boiler shell is placed with its axis horizontal, then it is called horizontal boiler.

Eg., Cornish boiler, Locomotive boiler, Scotch marine boiler, Babcock and Wilcox boiler etc.,

VERTICAL BOILERS: If a boiler shell is placed with its axis vertical, then it is called vertical boiler.

Eg., Cochran boiler

SINGLE TUBE BOILER: In a single tube boiler there is only one fire or water tube.

Eg., Cornish boiler

MULTI TUBULAR BOILER: In a multi tubular boiler there are two or more fire tubes or water tubes.

Eg., Lancashire boiler, Locomotive boiler, Cochran boiler, Babcock and Wilcox boiler etc.,

NATURAL CIRCULATION BOILER: In a natural circulation boiler, circulation of water is by natural convection currents which are set up during heating of water.

All conventional boilers are natural circulation boilers.

FORCED CIRCULATION BOILER: In a forced circulation boiler, the circulation of water is effected by mechanical pump which forces the water through the tubes. Since water is circulated rapidly under pressure, the heat transfer is maximum and steam generation is faster.

Modern high pressure boilers employ the forced circulation system.

Eg., Lamont boiler, Benson boiler, Velox boiler etc.,

COCHRAN BOILER

It is a multi tubular, internally fired, vertical, natural convection, portable, fire tube boiler. It has boiler shell of about 3 meter diameter and 6 meter high. It has a steam generation capacity of 3500 – 4000 Kg/Hr and can generate steam at a pressure of about 10 to 20 bars.

CONSTRUCTION:

The boiler consists of a vertical, cylindrical shell and a fire box. Both the shell and fire box are hemi- spherical. The hemi-spherical crown of the shell gives maximum strength to withstand steam

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APPLIED THERMAL ENGINEEING 15ME52T

pressure inside the shell. The hemi-spherical crown of the furnace is useful to with stand the intense heat in the boiler furnace and to deflect the hot flue gases. The fire box and combustion chamber is connected by means of an elliptical flue tube. The flue gases from the combustion chamber flow to the smoke box through a number of fire tubes. These tubes are generally have 60 – 80 mm diameter and are 160 – 170 in numbers. The gases from the smoke box pass to the atmosphere through a chimney. The combustion chamber is lined with fire bricks on the shell side. A manhole near the top of the crown in the shell is provided for cleaning.

The boiler is also fitted with mounting like steam stop valve, blow down valve, feed check valve, water level indicators, fusible plug, pressure gauge etc.,

WORKING:

The boiler shell is filled with water using feed pump up to the required level. Water fills the shell completely covering the furnace, combustion chamber and the flue tubes. Coal is burnt in the furnace. The products of combustion rise in the fire box and get deflected towards the combustion chamber through elliptical flue tube. The hot flue gases from combustion chamber enter the smoke box via flue tubes. The gases from the smoke box are discharged to atmosphere through chimney. The hot gases while passing through the flue tubes transfer their heat to the water which is converted into steam. The steam thus formed is accumulated in the steam space. GOVERNMENT POLYTECHNIC KAMPLI U-II : STEAM BOILERS Page 4

APPLIED THERMAL ENGINEEING 15ME52T

MERITS:

1. It occupies less floor space 2. It is portable and easy to transport and install. 3. It is simple in design and construction. 4. Due to flexibility in burning, any type of fuel can be used.

DEMERTIS:

1. Rate of steam generation is low. 2. Cleaning and inspection is difficult due to vertical design. 3. It is suited only for small capacity requirement.

BABCOCK AND WILCOX BOILER

It is a horizontal, externally fired, natural circulation, stationary, water tube boiler. It has a steam generation capacity of 2 – 20 tons/Hr and can generate steam at a pressure of about 40 – 44 bars.Since it is capable of generating large quantities of steam at high pressure, therefore they are extensively used in thermal power plants.

CONSTRUCTION:

It mainly consists of four parts 1) Water and steam drum 2) Water tubes 3) chain grate stoker 4) Super heater tubes.

Water and steam drum is suspended from iron girders (supporting beam) resting on the iron column (not shown in figure). A number of inclined water tubes, of about in 100 mm diameter at relatively low inclination, are connected at right angle to end boxes called ‘headers’. A mud box is provided just below the down take header. Any sediment in the water will settle down in the mud box and is blown off from time to time through blow off valve.

The chain grate stoker is provided at the front end below the uptake header. The coal, fed on to one end of the grate, is burned on the moving grate in the furnace and the residual ash falls at the other end of the grate into ash pit.

The boiler is provided with a super heater. The super heater consists of a number of U tubes secured at each end to the horizontal connecting boxes and placed in the combustion chamber below the boiler drum. The upper box of the super heater tubes is connected to a T (Tee) tube and upper branches of the T being situated in the steam space in the drum. The lower box of the super heater tubes is connected to steam stop valve mounted over the drum through a vertical tube passing outside the drum.

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WORKING:

Water is introduced into the drum through feed valve. Constant water level is maintained in the boiler drum. The water descends at the rear end into the down take header and passes up in the inclined water tubes, uptake header and in the tubes connecting the uptake header and drum. Thus a circuit is established between drum and water tubes for the flow of water.

The products of combustion or flue gases come in contact with the water tubes and heat them. The path of the flue gases is controlled using baffle plates and gases heat the steam drum and super heater tubes, and finally passes out of the boiler through exit door and chimney. During this path of hot gases, the hottest gas coming directly from grate come in contact with highest portions of the water tubes. The water contained in this portion of water tubes gets evaporated. The water and steam mixture from this portion of water tubes ascends through uptake header and reaches the boiler drum. Now due to this flow of water a continuous, rapid circulation of water is established between drum and pipes. The steam gets separated from surface of the water in the boiler drum.

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APPLIED THERMAL ENGINEEING 15ME52T

Steam from the steam space in the boiler drum is led into the branches of T tube and then passes into upper connecting box of super heater tubes, then through U tubes. Since super heater tubes are fitted in combustion chamber and exposed to hot gases, the steam passing through will be super heated. The super heated steam from super heater tubes is passed to steam stop valve through the lower connecting box and the vertical tube fitted outside the drum. From the stop valve, the super heated steam is passed to the prime movers.

HIGH PRESSURE BOILERS

In modern power plants it is very common to use a single boiler unit per turbine. This leads to simpler piping systems and relatively easier boiler and turbine control. There boilers are usually designed to operate at critical pressure (212.2 bars) or above or below the critical pressure. If the boilers are designed to operate above the critical pressure, then they are known as ‘super critical boilers’ or ‘once through boilers’. If the boilers are designed to operate below the critical pressure, then they are known as ‘sub critical boilers’ or ‘drum boilers’.

These boilers employ either natural or forced circulation of water.

The different high pressure boilers are: Lamont boiler, Loeffler boiler, Benson boiler, Velox boiler etc.

LA-MONT BOILER:

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APPLIED THERMAL ENGINEEING 15ME52T

This is an example of externally fired, forced circulation, water tube, sub-critical boiler.

The feed water from the hot well is pumped to the boiler drum through economizer. The feed water is preheated in the economizer by the flue gases. The circulating pump circulates the water into boiler tubes. Water is evaporated into steam when passing through these tubes (Radiant and Convective evaporator).

The water and steam from the tube enters the boiler drum where the steam is separated. This steam is passed through a convective super heater and the steam is superheated by the flue gases. This super heated steam is supplied to the prime mover through steam outlet.

The water level in the drum is kept constant by pumping the feed water into the boiler drum. The air is preheated by the flue gases before entering the combustion chamber to aid the combustion of fuel.

This type of boiler is capable of supplying steam at a rate of 3000 tons per hour with a working pressure of 170 bars.

BENSON BOILER

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APPLIED THERMAL ENGINEEING 15ME52T

It is an example of super critical, drum less, water tube, forced circulation boiler.

In this boiler, the feed water enters at one end and discharges as super heated steam at the other end. The feed pump increases the pressure of water to super critical pressure (i.e., above the critical pressure of 225 bar) and water is directly transforms into steam without boiling.

The feed water passes through the economizer to the radiant heating section. The water receives the heat by radiation and the temperature rises to almost to critical temperature. It then enters convective evaporator and may get super heated to some degree. Finally it is passed through the super heater to obtain desired super heated steam.

Benson boiler is also known as light weight boiler as there is no large water and steam drum. Thermal efficiency of 90% may be achieved with this boiler. The average operating pressure and capacity of such boilers are 250 bars and 135 tons/Hr.

COMPARISON BETWEEN WATER TUBE BOILER AND FIRE TUBE BOILER

WATER TUBE BOILER FIRE TUBE BOILER

1. In water tube boiler, the water circulates inside the In fire tube boiler, the hot gases from the furnaces tubes which are surrounded by hot gases from the pass through the tubes which are surrounded by furnace. water.

2. Water tube boiler can generate steam at a higher But a fire tube boiler can generate steam up to a pressure of 165 bars. pressure of 24.5 bars.

3. In water tube boiler, the rate of steam generation is But in a fire tube boiler, the rate of steam high (up to 4-50 tons/Hr). generation is low (up to 9 ton/Hr).

4. The floor area required is less in water tube boilers. The floor area required is more in fire tube boilers.

5.Overall efficiency is 90% in water tube boilers Overall efficiency is 75% in fire tube boilers

6. Transportation and erection is easy in water tube Transportation and erection is difficult in fire tube boiler. boiler

7. Water tube boilers are preferred for fluctuating But fire tube boilers are preferred for fixed loads. loads.

8. Operating cost is high in water tube boilers. Operating cost is low in fire tube boilers.

9.Bursting chances are more in water tube boiler Bursting chances are less in fire tube boiler.

10.Water tube boilers are used for large power plants Fire tube boilers are used for small power plants

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APPLIED THERMAL ENGINEEING 15ME52T

BOILER MOUNTINGS AND ACCESSORIES

For the efficient operation, maintenance and safety of the boilers, they should be equipped with two kinds of appliances and fitting. They are: (1) boiler mountings (2)boiler accessories

BOILER MOUNTINGS:

These are required for the complete control of steam generation and to provide safety of boilers. They form an integral part of boiler and are usually mounted on the boiler shell.

The essential boiler mountings are:

(1) WATER LEVEL INDICATORS: The function of the water level indicator is to indicate the level of water in the boiler drum. Two water level indicators (one serves as standby) are fitted in front of the boiler from where it is easily visible to operator. (2) PRESSUER GAUGE: The function of the pressure gauge is to indicate the steam pressure inside the boiler drum. It is usually mounted in the front end at the top of the boiler shell so as to be clearly visible to operator. (3) SAFETY VALVE: The function of the safety valve is to prevent the explosion of the boiler due to excessive internal pressure. This allows the steam to escape to atmosphere when pressure in boiler exceeds the working pressure. These are fitted directly on the boiler above the steam space. (4) STEAM STOP VALVE (JUNCTION VALVE): The function of the steam stop valve is to regulate the rate of flow of steam from the boiler to steam engine or steam turbine. Also it is necessary to shut off the supply of steam when steam engine or turbine is not working. This valve is located at the top most part of the steam space of the boiler and it is connected to steam main which supplies steam to a turbine or engine. (5) BLOW OFF VALVE (BLOW OFF COCK): The function of the blow off cock is to remove periodically the sediments collected at the bottom of the boiler while in operation. It is also used to empty the water in the boiler when required for periodical cleaning and inspection. This valve is fitted at the lowest portion of the boiler. (6) FEED CHECK VALVE: The function of the feed check valve is to control the flow of water from the feed pump to the boiler and to prevent the back flow of water from the boiler to the pump when the pump pressure is less than the boiler pressure or when the feed pump ceases to work.

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This valve is fitted in the feed water pipe line very close to boiler.

(7) FUSIBLE PLUG: It is a safety device used to extinguish the fire in the furnace of the boiler when the water level falls too much below the normal level. It is fitted over the crown of furnace or combustion chamber.

BOILER ACCESSORIES:

These are required to improve the efficiency of the steam power plant and to enable for the proper working of the boiler. The boiler accessories are not mounted directly on the boiler.

The essential boiler accessories are:

(1) ECONOMISER: It is an accessory that recovers a portion of heat of the hot flue gases to heat the feed water which is supplied to boiler, before hot flue gases leave the chimney, It is placed in the path of the flue gases in between super heater and air pre-heater. (2) AIR PRE-HEATER: It is an accessory that recovers a portion of heat of the hot flue gases to heat the air which is supplied to furnace, before hot flue gases leave the chimney. It is placed in the path of the flue gases in between economizer and chimney. (3) SUPER HEATER: It is an accessory that converts dry saturated steam into super heated steam by raising steam temperature above the saturation temperature at constant pressure. It is generally an integral part of a boiler, and is placed in the path of flue gases from the furnace (4) FEED PUMP: It is an accessory that delivers feed water at high pressure into boiler. It is placed in between boiler and water supply source (hot well). (5) STEAM SEPARATOR: It is an accessory that separates water particles from steam before it is supplied to a steam engine or turbine. Thus it prevents the damaging of turbine blades due to moisture present in steam. It is located in the supply line near the turbine or engine. (6) STEAM TRAP: It is an accessory that drains off the condensed water accumulating in the steam pipe line without allowing the high pressure steam to escape. It is connected to a small by-pass pipe which branches off from the main steam pipe line. Boiler draught

Boiler draught is the difference of pressure maintained above and below the fire grate in a boiler, which ensures the entry of the fresh air and exhaust of the combustion gases.

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The main objects of producing draught in a boiler are:

1. To provide an adequate supply of air for the fuel combustion. 2. To exhaust the gases of combustion from the combustion chamber. 3. To discharge these gases to the atmosphere through the chimney. Classification of draughts:

The draught system may be classified into following two types:

1. Natural draught: it is the draught produced by a chimney due to the difference of densities between the hot gases inside the chimney and cold atmospheric air outside it. 2. Artificial draught: The artificial draught may be a mechanical draught or a steam jet draught. The draught produced by afan or blower is known as mechanical or fan draught whereas the draught produced by a steam jet is called a steam jet draught. the artificial draught is provided when natural draught is not sufficient. The artificial draught may be induced or forced.

Types of draughts:

1. Chimney draught: The draught produced by means of a chimney alone is known as chimney draught. It is a natural draught and has induced effect. Since the atmospheric air (outside chimney) is heavier than the hot gases (inside the chimney), the outside air will flow through the furnace into the chimney. It will push the hot gases to pass through the chimney. The chimney draught varies with climatic conditions, temperature of furnace gases and height of chimney.

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2. Mechanical or fan draught:The draught produced by means of a fan or blower, is known as mechanical draught or fan draught. The fan used is, generally, of centrifugal type and is driven by an electric motor. In an induced draught, a centrifugal fan is placed in the path of the flue gases before they enter the chimney. It draws the flue gases from the surface and forces them up through the chimney. The action of this type of draught is similar to that of the natural draught. In a forced draught, the fan is placed before the grate, air is forced into the grate through the closed ash pit. 3. Steam jet draught:it is a simple and cheap method of producing artificial draught. In a steam jet draught, the exhaust stem, from a non-condensing steam engine, is used for producing draught.

Comparison between Forced draught and Induced draught Sl.No. Forced draught Induced draught 1 The fan is placed before the fire The fan is placed after the fire grate grate 2 The pressure inside the furnace is The pressure inside the furnace is above the atmospheric pressure. below the atmospheric pressure. 3 It forces the fresh air into the It sucks hot gases from the combustion chamber. combustion chamber, and forces them into the chimney. 4 It requires less power as the fan has It requires more power as the fan has to handle cold air only. Moreover, to handle hot air and flue gases. volume of air handled is less Moreover, volume of air handled is because of low temperature of the more because of high temperature of cold air. air and gases. 5 The flow of air through grate and The flow of air through grate and furnace is more uniform. furnace is less uniform. 6 As the leakages are outward, As the leakages are inward, therefore therefore there is serious danger of there is no danger of blow out. But if blow out when the fire doors are the fire doors are opened and fan is opened and the fan is working working there will be a heavy air infiltration.

Balanced draught: it is an improved type of draught, and is a combination of induced draught and forced draught. It is produced by running both induced and forced draught fans simultaneously.

Comparison between Natural draught and Artificial draught Sl.No. Natural draught Artificial draught 1 Draught is obtained by the use of Draught is obtained by the use of fans chimney. or blowers. 2 It depends on height of the It does not depend on height of the chimney. chimney. 3 Flue gases have to discharge with Flue gases need not discharge with high temperature. high temperature. 4 Simple in construction with less Complicated in construction and high cost. initial cost.

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5 No mechanical parts and hence High maintenance cost due to large maintenance cost is less. number of mechanical parts. 6 It does not require any extra power It requires extra power to produce the to produce the draught. draught. 7 It depends on atmospheric It is independent of temperature. atmospherictemperature. 8 Large consumption of fuel. Low consumption of fuel. 9 Low thermal efficiency High thermal efficiency 10 Poor combustion of fuel due to low Proper combustion of fuel due to high velocity of air velocity of air 11 It creates more smoke Prevents smoke creation

Steam jet draught:

The simple and easy way of producing the artificial draught is the steam jet draught.

There are two types of steam jet draught:

i) Induced steam jet draught ii) Forced steam jet draught Induced steam jet draught: Here, the exhaust steam from a non-condensing steam engine is used for producing the draught and it is used, generally, in locomotive boilers, where the exhaust steam from the engine cylinder is discharged thrugh a blast pipe placed at the smoke box and below the chimney as shown in above figure.

Forced steam jet draught: Here, the steam from the boiler, after having been throttled to a pressure of 1.5 to 2 bar is supplied to nozzles installed in the ash pit. The steam with a high velocity drags the air along the fuel bed, furnace, fuel and then to the chimney. Forced draught produced by steam is known as turbine draught.

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