List of Steam Boilers: Types, Parts and Operation!
1. Lancashire Boiler:
A Lancashire boiler was first used by about 1840 and is a development of the Cornish pattern. They are still used in considerable numbers in industry, principally on account of their simplicity of design together with reliability and freedom from maintenance costs for repairs and tube renewals which are necessary for multi-tubular and water-tube boilers.
The modern Lancashire boilers are made for working pressure upto 15 bar and having an evaporative capacity upto 8500 kg per hour. They are upto 9 metre in length and have a diameter upto 3.5 metre, though the smaller sizes and pressure are more usual. A Lancashire boiler is a fire-tube type, internally fired, natural circulation, horizontal boiler. 
The Lancashire boiler consists of a horizontal cylindrical shell through which pass two internal circular fire-tubes. The furnaces are accommodated at the front ends of the flues, and they are about 1.8 metre long for hand fired boilers. At the back of the grate is the fire brick bridge. The Lancashire boiler is set in the brickwork settings. Fig. 4-2 gives the passage of flue gases through a Lancashire boiler and settings.
ADVERTISEMENTS:
The hot gases produced in the furnace due to combustion of the fuel pass over the fire brick bridge towards the other end of the internal flue tube at the back of the boiler. The gases leaving the internal flues enter a down take chamber which is also the position for superheater in this boiler and then passes into the bottom flue to the front of the boiler.
At the front of the boiler they divide into two streams and return by way of the side flues to the dampers and thence enter into the main flue which leads them to the chimney. If an economiser is fitted to the boiler, it is placed in the enlargement of the main flue and so hot gases from the main flue pass through it before passing up the chimney.
The hot gases, as they progress in their passage, transfer heat to the water in the boiler through the metal walls and as the heat is abstracted by water from the gases its temperature is continuously reduced.
The fire brick bridge is so placed that it closes the through passage of air in the internal flue and so air has to pass through fire grate bars before it can enter the furnace and thereby complete combustion of fuel takes place. The portion of internal flue below the fire bars is called the ash pit.
ADVERTISEMENTS:
The Sliding dampers are situated at the back of each side flue. They are operated by chains and ropes, which pass over sheaves, from the front of the boiler. They are balanced by balance weights suspended to the ropes at the front of the boiler and so they can be operated easily. The dampers regulate the rate of flow of air through the furnace and therefore enable the rate of steam generation to be controlled.
A Lancashire boiler can take overloads and it meets the demands at falling pressure. If overload persists for a long time the boiler will finally stop. For short duration overloads, the pressure can be maintained by reducing the feed water supply, Lancashire boiler is more sluggish in reacting to variation in firing rate and therefore more time is required for raising steam from cold boiler.
2. Cornish Boiler:
A Cornish boiler differs from a Lancashire boiler in two respects: one, it has only one internal flue-tube and secondly it is smaller in size. Like a Lancashire boiler, a Cornish boiler also has external flues of brickwork.
The internal flue may be situated on the vertical centre line of the boiler. But the other arrangement in which the flue is placed somewhat to one side of the centre, has the advantage that this non-symmetrical position causes vigorous circulation throughout the water space, thus giving a heat transfer better than with a symmetrical internal flue-tube. The construction of a Cornish boiler is similar to that of a Lancashire boiler.
ADVERTISEMENTS:
Lancashire and Cornish type boilers are known as shell type boilers with internal straight flues. Shell type boilers have large water and steam capacity as compared with multi-tubular and water tube type. Shell type boilers have a relatively large heat storage capacity.
This enables these boilers to cope with sudden demands of moderate duration, and heat reserve can be drawn upon without a fall in steam pressure by reducing the rate of feed until the fires have been sufficiently increased to meet the increased demand and to re-gain the correct water level in the gauges.
Shell type boilers require much larger space than tubular boilers of the same evaporation. On account of large water capacity and brick work setting these boilers are more sluggish in reacting to variations in firing rate, and much slower in raising steam from cold than are self-contained multi-tubular and water tube boilers.
3. Multi-Tubular Fire Tube Boilers:
These are known as economic boilers. These boilers consist of a cylindrical shell in which is one or two internal longitudinal flues of which approximately half the front portion forms the furnace or fire box. Hot gases resulting from the combustion of the fuel in the furnace leave the internal flues and enter a refractory lined combustion chamber which may either be internal with the boiler itself or built on separately as its back.
ADVERTISEMENTS:
From this chamber the hot gases pass into the fire tubes running the full length of the boilers. In case of self-contained boilers, without external flues, the gases leave by way of a smoke box across the front of the tube nest. The stack is placed directly above the smoke box.
In economic boiler of this type, baffles are set into external brick work flues through which the gases pass from the smoke box beneath the boiler shell to a damper at the rear end. This external brick flue extends over the entire under surface of the shell. Suitable arrangement of brick work baffles distribute the gases as uniformly as possible over the under surface of the shell.
One variation of simple two pass economic boiler employs tubes of longitudinally sinuous section. It is claimed that stratification of the gases is broken up and increased heating surface is obtained from a given length of tube.
In super economic boiler, the smoke tubes are arranged at the side of flue tube or tubes. Each boiler tube nest extends well down towards the boiler bottom thus giving increased temperature in this position and more satisfactory circulation than with the tube nest above the flue tubes.
ADVERTISEMENTS:
Another modification of super economic boiler is three pass boiler. In this type of boiler flue gases leave the smoke box and enter into a second nest of fire tubes, which are of somewhat smaller bore than those in the lower nest of the fire tubes. This reduction in tube bore serves to maintain the velocity of flue gases because the volume and temperature of flue gases are reduce.
Another multi-tubular boiler unit is known as Super Lancashire boiler. Similar to an ordinary Lancashire boiler, this boiler has two internal flues but they are placed higher in the shell than in an ordinary Lancashire boiler. Beneath these internal flue tubes is a nest of smoke tubes leading from the combustion chamber at the back end to a transverse smoke box below the furnace mouth at the front end of the boiler.
From this smoke box the gases divide to pass through air heaters which are situated at each side of and externally to the boiler shell. Flue gases are discharged to the atmosphere by an induced draught fan. The economic boiler is only about half the size of an equivalent Lancashire boiler and it has higher thermal efficiency.
The following advantages can be claimed for multi-tubular boilers such as economic, super economic and super Lancashire boilers:
(1) These boilers have high steaming capacity for the space occupied.
(2) These boilers respond easily to changes in firing rate, together with quick and safe steam raising from cold.
(3) Economic boilers have high ratio of heating surface to their grate area and hence do not require feed water economizers as the flue gases leave the boiler at an average low temperature of 180°C.
(4) Combustion efficiency can be improved by installing air preheater and induced draught fan.
Economic boilers are not suitable where steam demand is subject to heavy fluctuations due to limited water and steam spaces. However by employing thermal storage system such as an accumulator, the effect of variable load and firing rate will be ironed out.
4. Horizontal Return Tubular Boilers:
The Horizontal Return Tubular (HRT) boilers have been more widely used in United States of America. It is characterized by simplicity and cheapness. The boiler consists of the combination of grate, fire brick bridge wall, fire and ash doors, ash pit, combustion space, cylindrical shell, fire tube and smoke box.
As the furnace is external to the cylindrical shell, any kind of combustion equipment can be accommodated. The pressure parts consist of a long cylindrical shell with flat end sheets which are provided with holes to receive the fire tubes. The inside surfaces of the furnace walls are lined with the bricks.
The boiler shell is usually suspended from overhead girders and brick setting built around it. Fire tube acts as stays for the end sheets, but where there are no tubes, the manufacturers provide stay braces to resist deformation of the ends by the steam pressure. Normal water level is such that all the fire tubes are kept immersed in water.
Hot gases pass over the bridge wall, along the bottom of the boiler shell and then reverse direction and pass back through the fire tubes to the front of the boiler, where a cylindrical metal extension of the shell serves to guide them into the stack.
Heating surface is partly the shell and partly the tubes. In Economic type HRT boilers two pass arrangement is provided.
Return tubular boilers are not suitable for large power plants because of their small capacities, limited steam pressures and low rates of steaming capacity. However, these boilers are much common for small industrial plants because of their compactness, ease of cleaning, low cost and water storage capacity.
HRT boilers cannot be built for high pressures and capacities for the reasons that the diameter of the shell increases with the capacity in order to accommodate large number of tubes. The thickness of the shell must be increased for higher pressures, or for greater diameters at the same pressure. Practical limitations for diameter are about 2400 mm and about 20 mm for shell thickness. Greater shell thickness than this may lead to difficulties in fabrication and operation.
The locomotive boiler is a straight fire-tube boiler having an internal fire-box.
The principal merits claimed for boilers of the locomotive fire-box type are:
(a) Compactness,
(b) Great steaming capacity,
(c) Fair economy and
(d) Mobility.
The most prominent objectionable features in fire-box boilers are:
(a) The great extent of flat surface which requires bracing
(b) The sluggishness of water circulation which the arrangement of their parts induce
(c) The liability of corrosion in the water legs on account of sedimental deposits and
(d) The difficulty of reaching the inside for cleaning.
Fig. 4-8(a) and fig. 4-8(b) provide the details of locomotive boiler. It consists of a cylindrical shell to one end of which is fitted, a fire-box and to the other a smoke-box. The fire-box forms the chamber within which the fuel is burnt in the grate which is supported in the fire-box at the bottom.
The outer box surrounding the fire-box leaves a space forming a water leg, the lower end of which is closed by a steel ring called mud or foundation ring. The best heating surface of the boiler is that which surrounds the fire-box. Coal is supplied in the fire-box through the fire-hole which is provided with a door.
The shell is traversed by many flue-tubes of small diameter. They connect the rear and front tube sheets of the boiler. The smoke-box is riveted to the shell and a stack is attached to the top of it. A large door, in front of the smoke-box, gives access to it and tubes for examination and cleaning purposes.
The coal burns on the grate and the products of combustion pass from the fire-box to the smoke-box through the flue tubes and from the smoke-box they pass to atmosphere through the stack.
The locomotive boilers used for railways slightly differ in construction from the land type boiler. The grate of the boiler is inclined. A steam dome is placed on the top of the shell and in front of the fire-box. A steam dome is placed on the top of the shell and in front of the fire-box. A stop valve called the regulator is placed in the steam dome.
The steam is taken from this elevated part of the boiler in order that it may contain as small an amount of moisture as possible. The steam pipe from the regulator leads to a superheater header placed in the smoke-box and from the super-heater steam is sent to the cylinder by pipes passing out from the smoke-box to the cylinder. The superheater elements are arranged in the tubes traversing the shell above the flue-tubes.
Draught is produced by the exhaust steam from the cylinders, which is discharged through the blast pipe, placed in the smoke-box, to the stack. A movable cap is attached to the mouth of blast orifice and thus suits the conditions as to work being done by the engine.
A steam blower is also provided for use when the steam supply to the engine is shut off. It consists of a perforated hollow ring which is placed round the mouth of the blast pipe and is supplied with steam direct from the boiler.
6. Vertical Boilers:
These type of boilers are self-contained and are particularly useful on account of its portability, the small amount of floor space required for installation and the rapidity with which these boilers can be put to work. These boilers are largely used on cranes, excavators and for general contract work where temporary supplies of steam are required.
Vertical boilers can be broadly divided into two classes:
(i) The plain vertical and
(ii) The multi-tubular vertical.
The plain vertical boiler comprises of a cylindrical shell containing a parallel or slightly tapered fire-box varying in height from about 0.5 to 0.6 of that of the shell and fitted in most cases with cross water tubes whose number may range from one to six. Fig. 4-13 shows the plain vertical boiler in which three large diameters cross water tubes are shown.
Multi-tubular vertical boilers are constructed with a cylindrical shell but contain a fire-box and a combustion chamber designed for various arrangements of vertical, horizontal or inclined smoke or water tubes of small diameter.
Fig. 4-14 shows the multi-tubular vertical boiler.
Internal fire-box is frequently made slightly tapering towards the top to allow ready passage of the steam to the surface. The bottom of the fire-box is attached to the bottom of the outer shell by being flanged out as shown in fig. 4-13 or sometimes by means of a solid wrought iron ring, as shown in fig. 4-8, the rivets passing right through the plates and the solid ring.
Cross water tubes are provided to increase the heating surface as well as to improve the circulation as shown in fig. 4-13. In such boilers, the uptake is frequently protected either with fire clay, or with a cast iron liner. In standard vertical multi-tubular fire tube boilers as shown in fig. 4-14, upper ends of the fire tubes are in the steam space.
In such boilers overheating and damage result when the boiler is forced or too hot a fire is maintained when starting up. A slow fire is essential until steam generation starts. Then the upper ends of the tubes may be cooled by the steam.
In order to prevent overheating of the upper ends of the tubes, the submerged head type boiler was developed. In this case the upper tube sheet is below the normal water level.
The following are the advantages of vertical boilers:
(1) Minimum floor area is required.
(2) Boilers are self-contained and hence require no brick-work setting.
(3) Cost of construction is low.
(4) It is semi-portable. It can be moved and set up readily in different locations.
(5) As the vertical fire tubes are all of the same size, one tube for a spare is sufficient for replacement.
The following are disadvantages of the vertical boilers:
(1) The capacity and pressure are limited. The maximum practical capacity of the vertical tubular boiler is 5000 kg and the maximum usual pressure is 14 bars.
(2) The furnace cannot be altered to meet requirement for the change in fuel.
(3) These boilers require a high head room.
7. Cochran Boiler:
A Cochran boiler is one of the best popular types of vertical multi-tubular, fire-tube boiler. The Cochran boiler is made in sizes upto 2.75 metre diameter and 5.8 metre height. It has a maximum evaporative capacity of 3640 kg of steam per hour, from cold feed, when burning 568 kg of coal per hour.
The most cold feed, when burning 568 kg of coal per hour. The most economic rate is about three quarters of the maximum. The heating surface of 2.75 metre diameter boiler is about 120 m2.
The boiler consists of a vertical cylindrical shell, having a hemispherical top. It also has a hemispherical furnace. The fire grate is arranged in the furnace and the ashpit is below the grate. The boiler has a combustion chamber, a number of horizontal smoke-tubes, a smoke-box and a stack. The furnace is surrounded by water on all sides except the opening for fire-door and the combustion chamber. The tubes are also completely surrounded by water.
The hot gases pass from the fire-grate to the combustion chamber through a short flue pipe and thence they pass through horizontal tubes to the smoke-box from which they pass into the stack.
The boiler is self-contained; therefore, it does not require any settings.
8. Scotch Marine Fire-Tube Boiler:
The most common type of marine fire-tube boiler is the Scotch Marine boiler. It is self-contained and it requires low head room. This boiler has a large amount of heating surface for the space, occupied. Scotch Marine fire-tube boiler consists of a cylindrical shell ordinarily varying in diameter from 2 to 5-5 metre and in length 2.5 to 3.5 metre.
The shell contains from one to four furnaces. The front end of the furnace is attached to a flanged opening in the front head end. The rear end is connected to the front wall of the combustion chamber, into which the furnace opens. These furnaces are internally fired and are entirely surrounded by water and therefore do not require a fire-brick lining.
The boiler has a number of tubes passing from front plate of internal combustion chamber to the front plate of the shell. They are surrounded by water. Uptake passages are provided leading from the boiler front end to the stack.
The gases from the furnace pass into the combustion chamber and then return through the tube to the front end of the boiler and enter the uptake and then pass off the stack.
This boiler does not require a setting. It is self-contained. It is supported by a cradle securely fastened to the frame of the ship. Adjustable turn-buckle stays hold the boiler in place in the cradle.
9. Water Tube Boilers:
Water tube boilers may be classified in two parts:
(a) Water tube boilers with straight tubes such as Babcock and Wilcox boilers, Maine boilers, Union boilers, Murray boilers, Keeler boilers, Springfield boilers, etc.
(b) Water tube boilers with bent tubes such as Stirling boilers, low head Babcock and Wilcox boilers, Riley steam generators, etc.
Straight tube boilers have a parallel group of straight equal length tubes arranged in a uniform pattern and joined at either end to headers. These headers in turn are joined to one or more horizontal drums. According to their construction, headers may be classified as box type or sectional type.
Bent tube boilers are headerless. The drums serve the same function as the headers.
Straight tube types of water tube boilers may be subdivided into three groups:
(a) Sinuous header or box header type
(b) Horizontal or vertical type
(c) Longitudinal or cross drum type.
Water tube boilers have the following advantages:
(1) It possesses the flexibility in starting up. The boiler can be brought upto the steaming temperature in a short time without causing excessive temperature stresses.
(2) It possesses evaporation flexibility due to the multiplicity of paths for circulation of water and gas.
(3) It has a wide range of capacity. The evaporating capacity of bent tube boilers is practically unlimited.
(4) The construction of boilers is such that impurities deposited from the water are received outside the zone of rapid circulation and are removed from the boiler.
(5) As the water space is divided into many tubes, the failure of one tube need not cause a disastrous explosion.
(6) The construction of the furnace and combustion chamber permits of alterations to suit various classes of fuel and system of firing.
(7) The general design permits high operating efficiencies and the carrying of high overloads without damage to the boiler.
(8) All parts are accessible for cleaning, inspection or repair.
We shall now consider Babcock and Wilcox boiler of straight tube type and Stirling boiler of bent tube type.
10. Babcock and Wilcox Water Tube Boiler:
The land type of Babcock and Wilcox boiler is made with one or more horizontal drums, having a cross box connected to a series of headers by short tubes. These tubes, which are called nipples, are expanded into openings in the header and cross box. Each front and rear header is connected with a single row of straight, tubes.
The headers have a serpentine form. This shape of the header arranges the tubes such that they are staggered. The headers may be vertical or inclined. Opposite to the end of each tube an elliptical handhole is located and handhole covers are used to cover the openings.
A mud box is attached to the bottom of the rear headers and each rear header is joined at its bottom end to it. In mud box any matter held in suspension in the water is, to a large extent, precipitated by reason of its greater specific gravity. Blow off valve connection is made with the mud box and a hand-hole is provided for the cleaning of the mud box.
The entire boiler, except the furnace, is suspended from a steel girder frame by steel rods called slings. The slings pass around each end of the drum and are thus entirely independent of the brickwork of the setting. By this arrangement the boiler can expand or contract freely without straining the brickwork setting.
The other method of supporting each end of the upper drum is by employing a heavy U bolt at each end. Each end of the U bolt is threaded and secured by nuts and plate washers to overhead steel cross beam. The cross beams are supported at each end by external to the high temperature zones of the setting and are protected against overheating.
The feed water pipe enters the front head and opens into the boiler several metres from the front head. The water circulation is from front to rear of the drum, downward through nipples to the rear headers; then forward through the tubes to the front headers and up into the drum again.
Sometimes a deflection plate is placed above the header connections at the front to throw back the water carried away with the steam, since the steam formed in the passage through the tubes is liberated when it reaches the front of the drum.
The furnace gases are compelled by baffle plates to pass upwards between the tubes into a combustion chamber under the steam and water drum; thence downwards between the tubes and once more upwards between the tubes and then they pass off to the chimney.
A damper is placed at the rear flue opening to the chimney. It regulates the draught.
The superheater is placed in the combustion chamber just under the steam and water drum. It consists of a number of U-tubes secured at each end to horizontal boxes. Steam formed in the boiler is led to the upper box of the superheater by a vertical T-tube placed in the steam space of the drum.
The T-tube is perforated and works as an anti-priming pipe. The steam from the upper box passes through U-tubes attaining high temperature then it enters the lower connecting box from where it is led into the main steam pipe.
11. Stirling Boiler (Bent Tube Type Water Tube Boiler):
The Stirling boiler, patented in 1888 by Alan Stirling, was the first bent tube boiler: A bent tube water tube boiler is suitable for the highest steam pressures, and for capacities ranging from 500 kg to 500000 kg of steam per hour. Twin units are used for the higher capacities. Solid, Liquid or gaseous fuels can be burnt economically and any method of firing can be used.
Bent tube boilers are multi-drum boilers. They consist of one or more steam, drums in the upper part of the setting and one or more mud drums in the lower part of the setting. The upper drums may not be at the same elevation. In very small sizes the boiler has two steam drums and one mud drum. Boilers having larger heating surface have three steam drums and two mud drums.
Modern moderate size Stirling boiler has four drums. Three upper drums are set at about the same level, and are connected to the mud drum by banks of bent tubes arranged to give ample space for a superheater and with alternate wide-and narrow spaces to permit removal of tubes without removing and other tubes or the brickwork.
The ends of the steam drum are dished and each drum has a manhole on the end. The upper drums called steam drums contain water and steam and have both their water and steam spaces connected by tubes called water circulating and steam circulating tubes respectively. Some tubes of rear bank are bent forward to enter the middle upper drum and some of the tubes of the middle bank are bent forward to enter the front drum.
The tubes are expanded into drilled holes in the drums into which they enter radially.
The tubes are bent for the following reasons:
(i) A bent tube allows free expansion and contraction
(ii) Bent tubes enter the drum in approximately a radial direction.
(iii) Use of bent, tube permits great flexibility in design, particularly as regards drum arrangement.
(iv) It is extremely difficult to install a straight tube boiler.
The rear steam and water drum carries the safety valves, the main steam outlet and a dry pipe to ensure dry steam at the boiler outlet. The feed water pipe enters this drum and discharges into a removable wrought iron trough that distributes the feed water over a relatively large portion of the area of the drum. The water column is attached to one end of the centre drum and blow off pipe to the bottom of the mud drum.
The water which is fed into the rear upper drum passes downward through the rear bank of tubes, to the front drum and it amounts to 75 per cent of the steam generated in the tubes, thus confining the greatest turbulence to this drum.
The middle drum thus acts as a dryer and there is little moisture in the steam leaving the second drum and this is effectively removed by the baffle arrangement in the rear drum from which it passes to the dry pipe into the superheater. The water from the front drum and thence downward through the middle bank of tubes to the mud drum and then retraces its course.
The steam from the dry pipe is led into the superheater top box. The top box is connected to the bottom box of the superheater by tubes bent zigzag. There is a partition in the middle of the length of the box and the steam which enters the box fills half of it and passes to the bottom box by half of the tubes and returns by the other half of the superheater top box and then it goes into the pipe leading to the stop valve.
A coking arch is sprung over the front of the grate with the grate directly under the arch when the boiler is fired by hand stoking (solid fuels). The large triangular space above this arch between the boiler front and the first bank of tubes is available as a combustion chamber.
The arch absorbs heat from the fire becoming an incandescent radiating body which heats the air required for combustion, ignites the gases distilled from coal and prevents the chilling of the boiler by an inrush of cold air when the furnace door is opened.
The furnace gases are directed by fire-brick baffles to pass from the grate along the first bank of tubes; through the smoke connection at the rear. This arrangement, known as three pass design assures economical operation with various fuels and draught conditions. Gas flow is generally in opposite direction to that of the feed water.
It is customary to support the upper drums (generally steam drums) of bent tube boilers by the steel structure and lower drums (mud drums) by the tubes. This arrangement permits free expansion and contraction of the tubes and results in the mud drum rising and falling slightly on temperature changes.
Generally upper drums overhang the setting for attachment of their supports, whereas the mud drum is contained within the width of the setting. Consequently the mud drum is the shortest.
The following three methods are adopted for supporting each end of the upper drums for bent tube boilers:
(i) A heavy U-bolt
(ii) A cast cradle on a cross beam
(iii) A flanged-steel L pad riveted to the lower part of the drum head and resting on an I or H beam.
The entire boiler is surrounded by four walls having doors in the front, side and rear walls for access to the interior of the settings and for inspection, cleaning and repairs. The brick works of the combustion chamber are faced with fire-brick.
Opposite the middle of each bank of tubes there is a door in the side wall, called a sooting door, through which a jet of steam from a steam hose can be introduced to clean the outside of the tubes.
12. Integral Furnace Boiler:
A recent development in the boilers is the integral furnace type of boiler. An oil fired integral furnace type boiler, manufactured by Babcock and Wilcox Ltd., London. The capacity of the boiler is about 90000 kg of steam per hour for steam conditions of 63.0 bar and 485°C. The boiler is compact in design and it provides efficient and economical steam generation for medium and large industrial plants and power stations.
The boiler is essentially of the bi-drum type with an upper drum and a lower drum connected by rows of generating tubes. The integral furnace is formed by an extended screen of water tubes which form the roof and outer wall of the furnace.
This screen is fed from the lower drum through the tubes forming the bottom of the furnace and is connected back into the top drum. The end walls are similarly treated. Thus, the whole furnace envelope forms part of the main circulation.
The main banks of generating tubes are baffled in such a manner that the hot gases leave the combustion chamber by an opening in the partition wall at the end remote from the burners and traverse the generating tubes and the superheater in a series of horizontal passes and finally leave at the burner end. The furnace is a flat bottomed type. The entire unit is supported from the bottom. It is well insulated and is enclosed by removable steel panels that form an air tight casing.
The absorption of radiant heat in the furnace cooling tubes brings the gases down to a suitable temperature before entering the convection bank and reduces wall maintenance to minimum.
13. Waste Heat Boilers:
In many plants such as internal combustion engines, oil refinery stills, open hearth steel furnaces, etc. the exhaust gases leave the plant with a very high temperature. These gases may be sufficient to supply the plant with steam requirement. In many ships such boilers are installed to recover the heat energy from the exhaust of internal combustion engines.
The exhaust from internal combustion engines has a temperature ranging from 150°C to 430°C The heat recovered and converted into steam is directly proportional to the mass of the gases, their temperature drop between entry and exit and their mean specific heat. Approximately 65 per cent of the energy in the gases measured between the inlet temperature of the hot gases and that of the saturated steam is available.
Waste-heat boilers are placed in the path of exhaust gases. They are of both fire tube-and water tube type. The greater portion of the heat transfer in such boilers is due to convection.
Steam output of waste heat boilers ranges from 3000 kg per hour to 10000 kg per hour. Steam characteristics are 9 bar and 250°C for fire tube boilers and 18 bar and 375°C for multiple forced circulation boilers.
The value of heat recovery depends upon the following consideration:
(i) The cost of producing an equivalent amount of heat by other means
(ii) The cost of heat recovery equipment
(iii) The operating and maintenance cost of the waste heat recovery equipment
(iv) Upon demand for the steam that waste heat recovery equipment may generate.
14. Super Critical Boilers:
The market conditions today demand improved performance from fossil fuel fired power plants. The requirements for improved efficiency, lower operating costs with higher availability and reliability are very important for boiler. The reduced emissions from power plants are need for todays operation to control air pollution. The reduction in life cycle cost is also required. These technology have been updated and development of super critical boiler has been a necessity.
The term ‘super critical’ for boilers in dicates that when water is heated in a boiler, the pressure is such that there is no transformation phase between water and steam. The steam is generated directly from water at this pressure.
The pressure is called critical pressure which is 225 N/cm2. The super critical steam cycle uses pressure more than 2250 N/cm2. The sub-critical conventional boilers uses drum in which the water is evaporated to steam. But in supercritical boilers there is no drum required.
These boilers are used in higher capacity range. In addition to higher pressure, it has higher superheater and re-heater temperature used to achieve higher cycle efficiency. The furnaces of these boilers are designed for effective cooling system and SH and RH uses advanced material like T23, T92. The boilers can operate on any type of coal with high volatile pulverized fuel firing system.
The benefits of these boilers are:
(1) Higher cycle efficiency
(2) Less fuel consumption
(3) Lesser pollutants such as CO2, NOx and SOx emission
(4) Quick response to load changes
(5) Shorter start up time.