An Introduction To Boiler Types , Principle And Efficiency
Introduction
A-Low-Pressure Boiler Definition
In a low-pressure boiler (figure 2) the pressure does not exceed 15 psi, and hot water heating boilers are not designed to exceed over 260 psig. The temperature in a low-pressure boiler will not rise above 250 degrees F. Because these types of boilers operate at lower pressures, they don't need to be monitored regularly and only have to be checked when the appliance begins to break down.
A boiler is called a high-pressure boiler (figure 1) when it operates with a steam pressure above 80 bars. The high-pressure boilers are widely used for power generation in thermal power plants.
In a high-pressure boiler, if the feed-water pressure increases, the saturation temperature of water rises and the latent heat of vaporization decreases. The feed water can be heated to saturation of temperature in the economizer with the help of waste heat. Recovery from the exhaust gases is escaping to the chimney. Then the boiler supplies only latent heat of vaporization and superheat. Thus, a boiler operation at high pressure will require less heat addition for steam generation.
Low-Pressure Boiler Uses
Low-pressure boilers are often used in buildings and designed to heat rooms through radiators. Types of buildings may include restaurants, hospitals, office buildings and schools. The boilers are able to heat the water used in bathrooms and use steam to heat the different rooms throughout the building, allowing them to become warm.
High-Pressure Boiler Uses
High-pressure boilers are used in industries and designed to generate the steam found in power plants, dry cleaners and laundromats. These boilers were also used in locomotive steam engines to give them the necessary power to run. Individuals who maintain these boilers need to follow certain precautions to avoid burns, shocks and other hazardous conditions.
2-Hydronic Boiler
Hydronic boilers are frequently applied in residential & commercial buildings for heating purposes.
They are usually manufactured in small sized portable units for domestic applications whereas large sized units are put into use for industrial applications.
These boilers are extremely durable and provide a long service life. Furthermore, their use is less complicated than other heating systems. However, the hydronic boiler installation procedure is quite expensive
The fuel is inserted into a pressurized tank where the combustion process takes place in a typical hot water boiler system.
The system includes a temperature control device called a thermostat that keeps a fuel temperature check. Within the pressurized tank, the water is supplied in combination with a regulated amount of air that starts the process of burning the fuel.
Hydronic Boiler System (Hot Water )
A boiler is used to heat water that is circulated through a closed loop piping system for general facility and service water heating. Low-temperature systems generally operate below 200° Fahrenheit Medium-temperature systems generally operate at temperatures between 200 and 250° Fahrenheit.
A feature of hot water systems (Figure 4 ) is an expansion tank to accommodate the expansion of the water in the system as the water is heated. The expansion tank, when piped into the system on the suction side of the circulating pumps, also pressurizes the system to prevent flashing in the circulating pump, piping, and piping components. In many low- and medium-pressure systems, pressurization is maintained by flash steam in the expansion tank. In a few hot water systems, pressurization is maintained by maintaining a compressed gas blanket above the water level in the expansion tank.
High-temperature hot water systems, which operate above 250° Fahrenheit, are basically the same as hot water systems that operate below 250°F. High-temperature systems are generally installed when a process requires the higher temperature, a number of locations require small quantities of low-pressure steam that the high-temperature hot water can generate in a local converter, or high-temperature drop equipment can be used at end use points to minimize the size of water circulation piping required.
Most facility boiler systems are fired using a combustible gas (typically natural gas or propane) or fuel oil. In many facilities, the boilers are designed to fire both a combustible gas fuel and a fuel oil. In these facilities, the combustible gas fuel is generally natural gas that is considered the primary fuel, and fuel oil is considered to be the backup fuel.
Water tube boilers are classified as follows.
1. Horizontal straight tube boilers
(a) Longitudinal drum
3- Cast iron boilers
Cast iron boilers are fabricated from a number of cast iron sections that are bolted together. The
design of each section includes integral water and combustion gas passages. When fully assembled, the interconnecting passages create chambers where heat is transferred from the hot combustion gases to the water. These boilers generally produce low-pressure steam (15 psig) or hot water (30 psig) and burn either oil or natural gas.
Cast iron boilers (Figure 8 ) are made in three general types: horizontal-sectional, vertical-sectional, and one-piece
4-Package Boiler
These boilers come as complete package. It requires only the steam, water pipe work, fuel supply and
electrical connections to be made for it to become operational.
The applications of this boiler mainly include process steam for industrial, chemical or used as power generator with a steam turbine. The package boiler can be used as a peak-load boiler because of an addition to another power that kicks in while other supplies not succeed.
Package boilers are generally shell type with fire tube design so as to achieve high heat transfer rates
The packaged boiler is so called because it comes as a complete package. Once delivered to site, it
requires only the steam, water pipe work, fuel supply and electrical connections to be made for it to
become operational. Package boilers are generally of shell type with fire tube design so as to achieve
high heat transfer rates by both radiation and convection
The features of package boilers are:
1. Small combustion space and high heat release rate resulting in faster evaporation.
2. Large number of small diameter tubes leading to good convective heat transfer.
3. Forced or induced draft systems resulting in good combustion efficiency.
4. Number of passes resulting in better overall heat transfer.
5. Higher thermal efficiency levels compared with other boilers
5- Longitudinal drum boiler.
11- Stoker Fired Boiler
The stoker fired boiler is most capable in common range and cogeneration systems in sugar factories. These boilers have membrane design and operate completely automatic. These Boilers have intrinsic features for the operation of trouble-free and high-thermal eiciency. These are classified based on the technique of supply fuel to the boiler & with the grate type. The stoker-fired boilers are classified into two types namely chain-gate or traveling grate stoker and spreader stoker.
12-Natural circulation boiler
Boilers in which motion of the working fluid in the evaporator is caused by thermosiphon effect on heating the tubes are called “natural circulation boilers. circulation of water in natural circulation boilers depends on the difference between the density of a descending body of relatively cool and steam-free water and an ascending mixture of hot water and steam. The difference in density occurs because the water expands as it is heated, and thus, becomes less dense. All natural circulation boilers are drum-type boilers.
13-Forced/assisted circulation boiler
the density difference between the saturated liquid and saturated vapor starts diminishing at 18 MPa or higher fluid pressure, thus it is difficult to maintain natural circulation of fluid flow in boiler tubes. In such cases fluid flow is ensured with the help of forced/assisted circulation using pumps. The forced/assisted circulation principle applies equally in both supercritical and subcritical ranges.
14- Babcock and Wilcox boiler
Babcock and Wilcox boiler is a natural circulation, externally fired medium pressure, stationary horizontal water tube boiler in which water is flows in the inclined tubes. The furnace is located outside of the drum. • This boiler is invented by Stephen Wilcox and George Herman Babcock in the year 1967. So this is named as Babcock and Wilcox.
15- Benson Boiler
Benson boiler is also known as super critical steam generator which is developed by Mark Benson in the year 1922. This boiler can generate high pressure steam, which is further used in production of electricity and other industrial processes. It is a water tube boiler.
In the early stage water tube boiler used to generate steam at the pressure up to 10MPa, which is known as sub critical boiler.
It works on the principle that the pressure of the water is increased to the supercritical pressure (i.e. above critical pressure of 225 bar). When the pressure of water is increased to the super critical level, the latent heat of water becomes Zero and due to this, it directly changes into steam without boiling. And this prevents the formation of bubbles at tube surface.
16- Once-through boiler
This type of boiler does not have a drum. Simply put, a once-through boiler is merely a length of tube through which water is pumped, heat is applied, and the water is converted into steam. In actual practice, the single tube is replaced by numerous small tubes arranged to provide effective heat transfer, similar to the arrangement in a drumtype boiler. The fundamental difference lies in the heat-absorbing circuit. Feedwater in this type of boiler enters the bottom of each tube and discharges as steam from the top of the tube. The working fluid passes through each tube only once and water is continuously converted to steam. As a result there is no distinct boundary between the economizing, evaporating, and superheating zones. The circulation ratio of this type boiler is unity . These boilers can be operated either at subcritical or at supercritical pressures.
17- Loeffler Boiler
Loeffler Boiler is a forced circulation, high pressure, and water tube boiler with internally fired furnace. In this boiler, the 2/3 of superheated steam is used to evaporate the water in the evaporating drum and remaining 1/3 of the steam from the superheater is used by the turbine. A steam circulating pump is used to circulate the steam into the boiler.
18-Velox boiler
Velox boiler is a forced circulation water tube boiler. It is mostly used in gas turbine. In this boiler, the velocity of flue gases is greater than the velocity of sound, which causes more heat transfer from gas to the water, which increases the steam generation rate. Due to this, it is most important boiler.
When the velocity of the gas is greater than the speed of sound, its heat transfer rate is also
increases. So more heat is transferred from gas to water as compare when the heat transfer at the subsonic speed. This is the basic principle of it. This boiler can increase the heat transfer rate or can say steam generation rate without increasing boiler size. This is why; Velox boiler is most successful boiler in the gas turbine industries.
19- Wet Back Boilers
In wet back boilers as the name suggests the reversal chamber (the posterior portion of the combustion chamber through which the flue gases travel from the first pass (furnace) to the second-pass tubes) is completely surrounded by water. The combustion reversal chamber is surrounded by water and therefore the heat in the flue gases is optimally utilized. Radiation losses are reduced as none of the parts of the combustion chamber are open to atmosphere instead they are surrounded by water. That means fewer losses, and lesser fuel bills. Most efficient modern boilers supplied are wetback type.
20- Dry Back Boilers
The reversal chamber in dry back boilers is not completely surrounded by water. The posterior part is
exposed to the atmosphere. This leads to the increased radiation losses, as the radiant heat is lost to
the atmosphere instead of going to the water as in wet back boilers. Earlier generation boilers used to
be dry back.
Thus wet back boilers ensure lesser radiation losses and hence save fuel.
The layout of the tubes involves the number of passes the tube will make to pass the heat from the
boiler furnace before being discharged. These can be two- pass and three pass boiler.
21-Two pass boilers
In two pass the combustion gases travels two times in the boiler.
Combustion gases should be cooled before entering the reversal chamber. Excess temperature causes
overheating and cracking of the tube. The heat transfer rate is maximum at the first pass, this rate
decreases with the increasing passes.
22- Three pass boilers
A three pass design provides three opportunities for heat transfer. The stack temperature of 3 pass will be lower than that of 2 pass boiler, of the same design and operating pressure. Efficiency is more than two pass boiler.
Each pass in boiler should be designed with cross sectional area to achieve optimal flue gas velocity,
which in turn maximizes heat transfer while also minimizing performance robbing sooth build up within the tubes.
No. Of pass Area of tube m² Temperature C⁰ Heat transfer
1st 11 1600 65%
2nd 43 400 25%
3rd 46 350 10%
23- Fossil fuel-fired boiler
Coal, fuel oil, and natural gas are the main types of fossil fuel. These fuels can generate a substantial quantity of heat by reacting with oxygen. These fuels consist of a large number of complex compounds comprised of five principal elements: carbon (C), hydrogen (H), oxygen (O), sulfur (S), and nitrogen (N). Any of these three fuels, i.e., coal, fuel oil, and natural gas, can be used in steam power stations, but coal plays a large role in power generation because of the enormous reserves of coal in many countries around the world,. Most of the large steam generating stations in these countries are coal based, and coal is expected to remain a dominant fuel for this purpose for many years. Coal-fired power plants at present account for about 41% of power produced globally.
Coal-based steam-generating stations also dominate in those countries where natural gas and fuel oil are abundantly available, since the cost of natural gas and fuel oil is exorbitantly high compared to the cost of coal. Additionally, natural gas and fuel oil are more economical in other industries. In modern, large-capacity, coal-fired steam generators coal is burnt in suspension.
24-Fluidized-bed boiler
Fluidized-bed combustion ensures burning of solid fuel in suspension, in a hot inert solid-bed material of sand, limestone, refractory, or ash, with high heat transfer to the furnace and low combustion temperatures (1075-1225 K). The combustor-bed material consists of only 35% coal. Fluidized-bed combustion is comprised of a mixture of particles suspended in an upwardly flowing gas stream, the combination of which exhibits fluid-like properties. Fluidized-bed combustors are capable of firing a wide range of solid fuels with varying heating value, ash content, and moisture content. In this type of boiler, pollutants in products of combustion are reduced concurrently with combustion much of the ash and hence the particulate matter as well as sulfur is removed during the combustion process. Further, lower temperature combustion in the fluidized bed results in lower production of NOX and obviates any slagging problem.
25-Scotch Marine Boiler
Scotch Marine Boiler is a Fire Tube Boiler. It is also a scotch or tank type boiler. It is a common boiler in the marine industry. This boiler has some special advantage like its compactness and operation efficiency. Scotch Marine Boiler can use any type of water.
The general layout of the Scotch Marine Boiler is that of a squat horizontal cylinder. At the lower part of the boiler shell, one or more large cylindrical furnaces are present. Burned gases and smoke from the boiler furnace pass through the back side of this boiler. After that gases and smoke return through the small tubes and up and out through the boiler chimney.
26-Waste-heat recovery boiler
A waste-heat recovery boiler (WHRB) or a heat-recovery steam generator (HRSG) is a heat exchanger that recovers heat from a gas stream and in turn produces steam that can be used in a process or to drive steam turbines. A common application for HRSGs is in a combined cycle power plant (CCPP), where hot exhaust gas from a gas turbine is fed to the HRSG to generate steam.
27- Lamont boiler
Lamont boiler is the first forced convection boiler which is introduced in the year 1925.
A LaMont boiler is a type of forced circulation water-tube boiler in which the boiler water is circulated through an external pump through long closely spaced tubes of small diameter. The mechanical pump is employed in order to have an adequate and positive circulation in steam and hot water boilers. A water circulating pump is used in the boiler to circulate water inside the
boiler. This pump is driven by the steam turbine which uses steam of boiler. It is used in power plant industries to generate electricity.
28- Electric Boilers
Compact wall-mounted electric boilers are used in residential hotwater (hydronic) heating systems (Figure 34 ). Heat is generated by electric heating elements immersed in water housed in a waterproof
cast-iron shell. Although small in size, these boilers are capable of generating as high as 90,000 Btu/h, enough heat for the average eight-room house.
29- Biomass boilers
Biomass boilers work by burning biological matter and outputting the resulting heat for use in heating systems. Wood pellets, chips, logs or other biological materials are fed – automatically, semi-automatically, or by hand – into a combustion chamber where they are ignited. The hot gas and air produced by this process travel through a flue, and are then passed through a heat exchanger, which transfers the heat to the water used in the property’s central heating system. The excess heat is also stored in a thermal tank (also called a buffer vessel).
30- Nuclear Boiler
The Nuclear Boiler System (NBS) produces steam from the nuclear fission process, and directs this steam to the main turbine. The NBS is comprised of (1) the reactor vessel, which serves as a housing for the nuclear fuel and associated component, (2) the recirculation system, (3) the control rod drive system, (4) the main steam system and (5) the reactor building portion of the feedwater system.
REQUIREMENTS OF A GOOD BOILER
A good boiler must possess the following qualities:
1. The boiler should be capable to generate steam at the required pressure and quantity as quickly as possible with minimum fuel consumption.
2. The initial cost, installation cost and the maintenance cost should be as low as possible.
3. The boiler should be light in weight, and should occupy small floor area.
4. The boiler must be able to meet the fluctuating demands without pressure fluctuations.
5. All the parts of the boiler should be easily approachable for cleaning and inspection.
6. The boiler should have a minimum of joints to avoid leaks which may occur due to expansion and contraction.
7. The boiler should be erected at site within a reasonable time and with minimum labor.
8. The water and flue gas velocities should be high for high heat transfer rates with minimum pressure drop through the system.
9. There should be no deposition of mud and foreign materials on the inside surface and soot deposition on the outer surface of the heat transferring parts.
10.The boiler should conform to the safety regulations as laid down in the Boiler Act.
Boiler Efficiency
Boiler Efficiency
where :
m = mass of water actually evaporated
hfg = latent heat of steam at boiler pressure in kJ
h1 = specific enthalpy of feed water at the generation temp[erature , kJ
Heat Exported from the Boiler to the Fluid
If a fluid like water is used to transfer heat from the boiler - the heat transfer can be expressed as:
For a steam boiler the heat exported as evaporated water at saturation temperature can be expressed as:
Heat Provided by Fuel
Introduction
A boiler is a device that converts the chemical energy of a fuel into a useful heat output, such as
steam or hot water. Boilers are used to supply steam or hot water for heating, processing, or power purposes. Many fossil and nonfossil fuels are fired in boilers, but the most common types of fuel include coal, oil, and natural gas. During the combustion
Boilers are manufactured in many different sizes and configurations depending on the
characteristics of the fuel, the specified heating output, and the required emissions controls. Some boilers are only capable of producing hot water, while others are designed to produce steam.
1- Steam Boilers
A boiler is a closed vessel in which water is heated, steam is generated, superheated or any combination thereof under pressure or vacuum by the direct application of heat from combustible fuels or electricity. The steam produced is used for:
(i) Producing mechanical work by expanding it in steam engine or steam turbine.
(ii) Heating the residential and industrial buildings
(iii) Performing certain processes in the sugar mills, chemical and textile industries.
(ii) Heating the residential and industrial buildings
(iii) Performing certain processes in the sugar mills, chemical and textile industries.
Boiler types According to steam Pressure
1-Low pressure Boilers ,
1-Low pressure Boilers ,
2-Medium Pressure Boilers,
3- High Pressure boilers.
According to ASME there every boiler have pressure above 15 PSI is called high pressure boiler. but generally, Low pressure boilers having pressure below 300 psi (Bulk Boiling). Medium pressure Boilers having a pressure range between 300 psi to 800 psi (mixed Nucleate or bulk Boiling). Boilers having Pressure above 800 psi (film Boiling) are called High pressure Boilers.
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| FIG 1 |
A-Low-Pressure Boiler Definition
In a low-pressure boiler (figure 2) the pressure does not exceed 15 psi, and hot water heating boilers are not designed to exceed over 260 psig. The temperature in a low-pressure boiler will not rise above 250 degrees F. Because these types of boilers operate at lower pressures, they don't need to be monitored regularly and only have to be checked when the appliance begins to break down.
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| FIG 2 |
B- High-Pressure Boiler Definition
A boiler is called a high-pressure boiler (figure 1) when it operates with a steam pressure above 80 bars. The high-pressure boilers are widely used for power generation in thermal power plants.
In a high-pressure boiler, if the feed-water pressure increases, the saturation temperature of water rises and the latent heat of vaporization decreases. The feed water can be heated to saturation of temperature in the economizer with the help of waste heat. Recovery from the exhaust gases is escaping to the chimney. Then the boiler supplies only latent heat of vaporization and superheat. Thus, a boiler operation at high pressure will require less heat addition for steam generation.
C- Steam and Condensate Boiler System
In steam and condensate systems (Figure 3), heat is added to water in a boiler causing the water to boil and form steam is piped to points requiring heat, and as the heat is transferred from the steam to the building area or process .the steam condenses to form condensate. In some very low-pressure saturated steam heating applications, the steam piping may be sized to slope back to the boiler so that the steam distribution piping also acts as the condensate return(single-pipe system).
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| FIG 3 |
In other low-pressure applications, there may be steam supply piping and condensate return piping (two-pipe system) .the condensate system is open to the steam system. In typical packaged steam boiler operations, the boiler system steam at about 150 psig for distribution throughout the facility and may be lowered to the operating pressure of equal to supplied through point-of-use pressure reducing stations. As heat is transferred from the steam, condensate is formed
in discharge legs until enough condensate is present to operate a trap that isolates the steam distribution system from condensate system. In common facility heating applications, the condensate system is at atmospheric pressure and arranged to drain the condensate to a central condensate receiver, or into local smaller receivers that pump back to the central condensate receiver
Low-Pressure Boiler Uses
Low-pressure boilers are often used in buildings and designed to heat rooms through radiators. Types of buildings may include restaurants, hospitals, office buildings and schools. The boilers are able to heat the water used in bathrooms and use steam to heat the different rooms throughout the building, allowing them to become warm.
High-Pressure Boiler Uses
High-pressure boilers are used in industries and designed to generate the steam found in power plants, dry cleaners and laundromats. These boilers were also used in locomotive steam engines to give them the necessary power to run. Individuals who maintain these boilers need to follow certain precautions to avoid burns, shocks and other hazardous conditions.
Hydronic boilers are frequently applied in residential & commercial buildings for heating purposes.
They are usually manufactured in small sized portable units for domestic applications whereas large sized units are put into use for industrial applications.
These boilers are extremely durable and provide a long service life. Furthermore, their use is less complicated than other heating systems. However, the hydronic boiler installation procedure is quite expensive
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| FIG 4 |
The fuel is inserted into a pressurized tank where the combustion process takes place in a typical hot water boiler system.
The system includes a temperature control device called a thermostat that keeps a fuel temperature check. Within the pressurized tank, the water is supplied in combination with a regulated amount of air that starts the process of burning the fuel.
Hydronic Boiler System (Hot Water )
A boiler is used to heat water that is circulated through a closed loop piping system for general facility and service water heating. Low-temperature systems generally operate below 200° Fahrenheit Medium-temperature systems generally operate at temperatures between 200 and 250° Fahrenheit.
A feature of hot water systems (Figure 4 ) is an expansion tank to accommodate the expansion of the water in the system as the water is heated. The expansion tank, when piped into the system on the suction side of the circulating pumps, also pressurizes the system to prevent flashing in the circulating pump, piping, and piping components. In many low- and medium-pressure systems, pressurization is maintained by flash steam in the expansion tank. In a few hot water systems, pressurization is maintained by maintaining a compressed gas blanket above the water level in the expansion tank.
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| FIG 4 |
High-temperature hot water systems, which operate above 250° Fahrenheit, are basically the same as hot water systems that operate below 250°F. High-temperature systems are generally installed when a process requires the higher temperature, a number of locations require small quantities of low-pressure steam that the high-temperature hot water can generate in a local converter, or high-temperature drop equipment can be used at end use points to minimize the size of water circulation piping required.
Most facility boiler systems are fired using a combustible gas (typically natural gas or propane) or fuel oil. In many facilities, the boilers are designed to fire both a combustible gas fuel and a fuel oil. In these facilities, the combustible gas fuel is generally natural gas that is considered the primary fuel, and fuel oil is considered to be the backup fuel.
Steam and Hot Water Boiler Similarities and Differences
Steam and hot-water (hydronic) space heating boilers are very similar physically, but there are some important differences.
• Steam boilers operate only about three-fourths full of water, whereas hot-water boilers operate completely filled with water.
• Steam boilers in residential steam heating systems operate at 2 psi pressure or slightly more, whereas residential hotwater boilers operate at approximately six times that pressure.
• Steam boilers are equipped with a low-water cutoff device to protect the appliance from burning out if it should run out of water. Only large hot-water space heating boilers with a capacity exceeding 400,000 Btu/h are presently required by code to be equipped with lowwater
cutoffs. (Note: Many HVAC contractors who install the smaller residential hot-water boilers strongly recommend the addition of a low-water cutoff device to these appliances to prevent burnout if the boiler loses its water.)
Boilers Classification
There are a large number of boiler designs, but boilers can be classified according to the following criteria:
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| FIG 5 |
According to Working pressure
(i) 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.
(i) 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.
(ii) 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.
(iii) High-pressure boiler: It produces steam at a pressure of more than 80 bars.
(iv)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.
(v)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.
According to Relative Passage of water and hot gases:
There are two main types of boiler
(i) Fire tube boilers The hot combustion gases pass through the boiler tubes, which are surrounded by water, e.g., Lancashire, Cochran, locomotive boilers, etc.
(ii)Water tube boilers.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
Type of fuel
(i) Fire tube boilers The hot combustion gases pass through the boiler tubes, which are surrounded by water, e.g., Lancashire, Cochran, locomotive boilers, etc.
(ii)Water tube boilers.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
According to position of furnace.
(i) Internally fired like Cochran, Lancashire boilers
Internal furnace:
(i) Horizontal tubular
(a) Short fire box
(b) Locomotive
(c) Compact
(d) Scotch.
(ii) Vertical tubular.
(a) Straight vertical shell, vertical tube
(b) Cochran (vertical shell) horizontal tube.
(ii) Externally fired like Babcock and Wilcox, Stirling boilers
. External furnace:
(i) Horizontal return tubular
(ii) Short fire box
(iii) Compact.
In internally fired boilers the grate combustion chamber are enclosed within the boiler shell. Whereas in case of extremely fired boilers and furnace and grate are separated from the boiler shell.
According to the position of principle axis(Position of the Boilers).
(i) Vertical If the axis of boiler is vertical, it is known as vertical boiler.like Cochran boiler
(ii) Horizontal If the axis of boiler is horizontal, it is known as horizontal boiler. like Cornish boiler, Lancashire boiler, and Locomotive Boiler
(iii) Inclined.
According to application(use).
(i) Stationary,
(ii) Mobile, (Marine, Locomotive).
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.
According to Fbrication
(i) packaged boiler
(ii) shop-assembled boiler
According to the circulating water.
(i) Natural circulation (ii) Forced circulation.
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.
The natural circulation limit is 140 Kg/cm2 above this pressure forced circulation is adopted.
According to the type of fuel used or fuel burning methods
Fuel burning method:
•Stoker-fired Boiler
•Pulverized fuel,
•Supercharged fuel and
•Fluidized bed combustion boilers.
(i) Internally fired like Cochran, Lancashire boilers
Internal furnace:
(i) Horizontal tubular
(a) Short fire box
(b) Locomotive
(c) Compact
(d) Scotch.
(ii) Vertical tubular.
(a) Straight vertical shell, vertical tube
(b) Cochran (vertical shell) horizontal tube.
(ii) Externally fired like Babcock and Wilcox, Stirling boilers
. External furnace:
(i) Horizontal return tubular
(ii) Short fire box
(iii) Compact.
In internally fired boilers the grate combustion chamber are enclosed within the boiler shell. Whereas in case of extremely fired boilers and furnace and grate are separated from the boiler shell.
According to the position of principle axis(Position of the Boilers).
(i) Vertical If the axis of boiler is vertical, it is known as vertical boiler.like Cochran boiler
(ii) Horizontal If the axis of boiler is horizontal, it is known as horizontal boiler. like Cornish boiler, Lancashire boiler, and Locomotive Boiler
(iii) Inclined.
According to application(use).
(i) Stationary,
(ii) Mobile, (Marine, Locomotive).
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.
According to Fbrication
(i) packaged boiler
(ii) shop-assembled boiler
According to the circulating water.
(i) Natural circulation (ii) Forced circulation.
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.
The natural circulation limit is 140 Kg/cm2 above this pressure forced circulation is adopted.
According to the type of fuel used or fuel burning methods
Fuel burning method:
•Stoker-fired Boiler
•Pulverized fuel,
•Supercharged fuel and
•Fluidized bed combustion boilers.
•Fossil Fuel (Gas/Oil/Coal)-fired Boiler
•Waste Heat Recovery Boiler
• electric boilers
•Biomass boilers,
•Nuclear Steam generators.
According to steam and water spaces
• Drum Type Boiler
• Once-through Boiler
According to construction
According to construction
• wet back boilers
• dry back boilers.
According to the layout of tubes
• two pass
• three pass boilers
• dry back boilers.
According to the layout of tubes
• two pass
• three pass boilers
1- Fire tube boilers
Firetube boilers consist of a series of straight tubes that are housed inside a water-filled outer shell.
The tubes are arranged so that hot combustion gases flow through the tubes. As hot gases flow through the tubes, they heat the water that surrounds the tubes. The water is confined by the outer shell of the boiler. To avoid the need for a thick outer shell, firetube boilers are used for lower-pressure applications.
Generally, the heat input capacities for firetube boilers are limited to 50 MBtu/h or less,5 but in recent
years the size of firetube boilers has increased.
 |
| FIG 6 |
Firetube boilers are subdivided into three groups.
-Horizontal return tubular (HRT) boilers typically have horizontal, self-contained firetubes with a separate combustion chamber.
- Scotch, Scotch marine, orshell boilers have the firetubes and combustion chamber housed within the same shell.
- Firebox boilers have a water-jacketed firebox and employ, at most, three passes of combustion gases.
Advantages of Fire Tube Boiler
1. It is quite compact in construction.
2. Fluctuation of steam demand can be met easily.
3. It is also quite cheap.
Disadvantages of Fire Tube Boiler
1. As the water required for operation of the boiler is quite large, it requires long time for rising steam at desired pressure.
2. As the water and steam are in same vessel the very high pressure of steam is not
possible.
3. The steam received from fire tube boiler is not very dry.
2- Water tube Boilers
Water tube boilers are designed to circulate hot combustion gases around the outside of a large
number of water-filled tubes. The tubes extend between an upper header, called a steam drum, and one or more lower headers or drums. In older designs, the tubes are either straight or bent into simple shapes. Newer boilers have tubes with complex and diverse bends. Because the pressure is confined inside the tubes, watertube boilers can be fabricated in larger sizes and used for higher-pressure applications. Small watertube boilers, which have one and sometimes two burners, are generally fabricated and supplied as packaged units. Because of their size and weight, large watertube boilers are often fabricated in pieces and assembled in the field.
 |
| FIG 7 |
1. Horizontal straight tube boilers
(a) Longitudinal drum
(b) Cross-drum.
2. Bent tube boilers
(a) Two drum
2. Bent tube boilers
(a) Two drum
(b) Three drum
(c) Low head three drum
(d) Four drum.
3. Cyclone fired boilers
A boiler may be classified as either a steam boiler or hot water boiler.
3. Cyclone fired boilers
A boiler may be classified as either a steam boiler or hot water boiler.
Advantages of Water Tube Boiler
There are many advantages of water tube boiler due to which these types of boiler are
essentially used in large thermal power plant.
1. Larger heating surface can be achieved by using more numbers of water tubes.
2. Due to convectional flow, movement of water is much faster than that of fire tube
boiler, hence rate of heat transfer is high which results into higher efficiency.
3. Very high pressure in order of 140 kg/cm can be obtained smoothly.
Disadvantages of Water Tube Boiler
1. The main disadvantage of water tube boiler is that it is not compact in construction.
2. Its cost is not cheap.
3. Size is a difficulty for transportation and construction.
3- Cast iron boilers
Cast iron boilers are fabricated from a number of cast iron sections that are bolted together. The
design of each section includes integral water and combustion gas passages. When fully assembled, the interconnecting passages create chambers where heat is transferred from the hot combustion gases to the water. These boilers generally produce low-pressure steam (15 psig) or hot water (30 psig) and burn either oil or natural gas.
Cast iron boilers (Figure 8 ) are made in three general types: horizontal-sectional, vertical-sectional, and one-piece
 |
| FIG 8 |
These boilers come as complete package. It requires only the steam, water pipe work, fuel supply and
electrical connections to be made for it to become operational.
The applications of this boiler mainly include process steam for industrial, chemical or used as power generator with a steam turbine. The package boiler can be used as a peak-load boiler because of an addition to another power that kicks in while other supplies not succeed.
Package boilers are generally shell type with fire tube design so as to achieve high heat transfer rates
The packaged boiler is so called because it comes as a complete package. Once delivered to site, it
requires only the steam, water pipe work, fuel supply and electrical connections to be made for it to
become operational. Package boilers are generally of shell type with fire tube design so as to achieve
high heat transfer rates by both radiation and convection
 |
| FIG 9 |
1. Small combustion space and high heat release rate resulting in faster evaporation.
2. Large number of small diameter tubes leading to good convective heat transfer.
3. Forced or induced draft systems resulting in good combustion efficiency.
4. Number of passes resulting in better overall heat transfer.
5. Higher thermal efficiency levels compared with other boilers
5- Longitudinal drum boiler.
The feedwater is feed in drum. The drum is placed above the heat source. The cooler water goes to the inclined tubes and the water is heating eventually in the hot tubes. As the water boils its density
decreases and there is circulation of hot water and steam. Steam is separated from water in steam
drum and taken out.
6- Cross drum boiler.
The drum in this type is placed in cross to the heat source. The temperature obtained in this type of
arrangement is more uniform. When the steam loads are high the upper tubes can become dry which
cause them to fail. The layout of tubes is made in such a way that large numbers of tubes are made
available.
7- Stirling boiler
A Stirling boiler has near vertical, almost straight water tubes that zig-zag between a number of steam
and water drum. Usually there are three banks of tubes in a four drum layout.
The feedwater enters the left upper drum, from where it falls to lower water drum. Water in pipes and
two drums is heated, the steam produced rise in upper drum from where steam is separated and taken
off.
8- COCHRAN BOILER
Cochran Boiler used to start with the burning of fuel in the grate, the burning fuel emits the hot gases this gas is used to pass through the combustion chamber from this chamber is used to sent into fire tubes. These fire tubes are arranged in the horizontal axis in the boiler. These fire tubes are surrounded by the water. When this hot gas is passed through tubes water surrounded by it gets heated by the exchanging heat process and steam is produced by it, this steam is collected at the top of the boiler and used for output work with the help of stop valve, and the gases used to discharge to the atmosphere through the help of chimney.
decreases and there is circulation of hot water and steam. Steam is separated from water in steam
drum and taken out.
 |
| FIG 10 |
The drum in this type is placed in cross to the heat source. The temperature obtained in this type of
arrangement is more uniform. When the steam loads are high the upper tubes can become dry which
cause them to fail. The layout of tubes is made in such a way that large numbers of tubes are made
available.
 |
| FIG 11 |
7- Stirling boiler
A Stirling boiler has near vertical, almost straight water tubes that zig-zag between a number of steam
and water drum. Usually there are three banks of tubes in a four drum layout.
The feedwater enters the left upper drum, from where it falls to lower water drum. Water in pipes and
two drums is heated, the steam produced rise in upper drum from where steam is separated and taken
off.
 |
| FIG 12 |
8- COCHRAN BOILER
Cochran Boiler used to start with the burning of fuel in the grate, the burning fuel emits the hot gases this gas is used to pass through the combustion chamber from this chamber is used to sent into fire tubes. These fire tubes are arranged in the horizontal axis in the boiler. These fire tubes are surrounded by the water. When this hot gas is passed through tubes water surrounded by it gets heated by the exchanging heat process and steam is produced by it, this steam is collected at the top of the boiler and used for output work with the help of stop valve, and the gases used to discharge to the atmosphere through the help of chimney.
 |
| FIG 13 |
8- Lancashire boiler.
The Lancashire boiler is similar to the Cornish, but has two large flues containing the fires. Pressure
range of the boiler is about 0.7 MPa to 2 MPa and efficiency is 65 to 70%. Fuel in these boilers is added into the grate which heats the gases.
Hot gases enter the front section of the boiler and leave the boiler from back and then enter the bottom flue and start moving to front section of boiler. At front section hot gases leave the bottom flue and enter in side flue and move again towards the back of the boiler and enter the main outlet. 85% of heat is transferred when hot gases are in fire tube while 15% is transferred when they are in bottom and side flue.
 |
| FIG 14 |
9- Locomotive boiler
A locomotive boiler has three main components:
1. Double-walled firebox;
2. Horizontal, cylindrical "boiler barrel" containing a large number of small flue-tubes; and
3. Smokebox with chimney, for the exhaust gases.
Fuel is burned to produce the hot gases. Fuel is feed through fire hole. Hot gases are diverted to fire
tube with the help of fire brick arch. Steam is collected in the steam drum which is placed at the top of the shell.
The wet steam goes through inlet headers of super heater and after passing through tubes, it returns to
the outlet header of super heater and is taken out for steam engine.
Locomotive-type boilers are also used in traction engines, steam rollers, portable engines etc.
On the basis of construction these can be classified wet back boilers and dry back boilers.
10-Cornish boiler
These are the earliest form of high pressure fire tube boiler. These consist of long horizontal cylinder
with single large flue containing fire. Fuel is added in the grate area where it burn to produce hot gases.
The hot gases transfer the heat to the water.Water takes heat and after some time it starts 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 throws these gases out of the boiler into the atmosphere. Maximum heat transfer is taken
place at fire tube and shell section then taken place at side flue and at last at bottom flue.
For efficiency, the boiler was commonly encased beneath by a brick-built chamber.
A locomotive boiler has three main components:
1. Double-walled firebox;
2. Horizontal, cylindrical "boiler barrel" containing a large number of small flue-tubes; and
3. Smokebox with chimney, for the exhaust gases.
Fuel is burned to produce the hot gases. Fuel is feed through fire hole. Hot gases are diverted to fire
tube with the help of fire brick arch. Steam is collected in the steam drum which is placed at the top of the shell.
The wet steam goes through inlet headers of super heater and after passing through tubes, it returns to
the outlet header of super heater and is taken out for steam engine.
Locomotive-type boilers are also used in traction engines, steam rollers, portable engines etc.
On the basis of construction these can be classified wet back boilers and dry back boilers.
 |
| FIG 15 |
These are the earliest form of high pressure fire tube boiler. These consist of long horizontal cylinder
with single large flue containing fire. Fuel is added in the grate area where it burn to produce hot gases.
The hot gases transfer the heat to the water.Water takes heat and after some time it starts 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 throws these gases out of the boiler into the atmosphere. Maximum heat transfer is taken
place at fire tube and shell section then taken place at side flue and at last at bottom flue.
For efficiency, the boiler was commonly encased beneath by a brick-built chamber.
 |
| FIG 16 |
11- Stoker Fired Boiler
The stoker fired boiler is most capable in common range and cogeneration systems in sugar factories. These boilers have membrane design and operate completely automatic. These Boilers have intrinsic features for the operation of trouble-free and high-thermal eiciency. These are classified based on the technique of supply fuel to the boiler & with the grate type. The stoker-fired boilers are classified into two types namely chain-gate or traveling grate stoker and spreader stoker.
 |
| FIG 17 |
12-Natural circulation boiler
Boilers in which motion of the working fluid in the evaporator is caused by thermosiphon effect on heating the tubes are called “natural circulation boilers. circulation of water in natural circulation boilers depends on the difference between the density of a descending body of relatively cool and steam-free water and an ascending mixture of hot water and steam. The difference in density occurs because the water expands as it is heated, and thus, becomes less dense. All natural circulation boilers are drum-type boilers.
 |
| FIG 18 |
the density difference between the saturated liquid and saturated vapor starts diminishing at 18 MPa or higher fluid pressure, thus it is difficult to maintain natural circulation of fluid flow in boiler tubes. In such cases fluid flow is ensured with the help of forced/assisted circulation using pumps. The forced/assisted circulation principle applies equally in both supercritical and subcritical ranges.
 |
| FIG 19 |
Babcock and Wilcox boiler is a natural circulation, externally fired medium pressure, stationary horizontal water tube boiler in which water is flows in the inclined tubes. The furnace is located outside of the drum. • This boiler is invented by Stephen Wilcox and George Herman Babcock in the year 1967. So this is named as Babcock and Wilcox.
 |
| FIG 20 |
15- Benson Boiler
Benson boiler is also known as super critical steam generator which is developed by Mark Benson in the year 1922. This boiler can generate high pressure steam, which is further used in production of electricity and other industrial processes. It is a water tube boiler.
In the early stage water tube boiler used to generate steam at the pressure up to 10MPa, which is known as sub critical boiler.
It works on the principle that the pressure of the water is increased to the supercritical pressure (i.e. above critical pressure of 225 bar). When the pressure of water is increased to the super critical level, the latent heat of water becomes Zero and due to this, it directly changes into steam without boiling. And this prevents the formation of bubbles at tube surface.
 |
| FIG 21 |
16- Once-through boiler
This type of boiler does not have a drum. Simply put, a once-through boiler is merely a length of tube through which water is pumped, heat is applied, and the water is converted into steam. In actual practice, the single tube is replaced by numerous small tubes arranged to provide effective heat transfer, similar to the arrangement in a drumtype boiler. The fundamental difference lies in the heat-absorbing circuit. Feedwater in this type of boiler enters the bottom of each tube and discharges as steam from the top of the tube. The working fluid passes through each tube only once and water is continuously converted to steam. As a result there is no distinct boundary between the economizing, evaporating, and superheating zones. The circulation ratio of this type boiler is unity . These boilers can be operated either at subcritical or at supercritical pressures.
 |
| FIG 22 |
Loeffler Boiler is a forced circulation, high pressure, and water tube boiler with internally fired furnace. In this boiler, the 2/3 of superheated steam is used to evaporate the water in the evaporating drum and remaining 1/3 of the steam from the superheater is used by the turbine. A steam circulating pump is used to circulate the steam into the boiler.
 |
| FIG 23 |
Velox boiler is a forced circulation water tube boiler. It is mostly used in gas turbine. In this boiler, the velocity of flue gases is greater than the velocity of sound, which causes more heat transfer from gas to the water, which increases the steam generation rate. Due to this, it is most important boiler.
When the velocity of the gas is greater than the speed of sound, its heat transfer rate is also
increases. So more heat is transferred from gas to water as compare when the heat transfer at the subsonic speed. This is the basic principle of it. This boiler can increase the heat transfer rate or can say steam generation rate without increasing boiler size. This is why; Velox boiler is most successful boiler in the gas turbine industries.
 |
| FIG 24 |
19- Wet Back Boilers
In wet back boilers as the name suggests the reversal chamber (the posterior portion of the combustion chamber through which the flue gases travel from the first pass (furnace) to the second-pass tubes) is completely surrounded by water. The combustion reversal chamber is surrounded by water and therefore the heat in the flue gases is optimally utilized. Radiation losses are reduced as none of the parts of the combustion chamber are open to atmosphere instead they are surrounded by water. That means fewer losses, and lesser fuel bills. Most efficient modern boilers supplied are wetback type.
 |
| FIG 25 |
The reversal chamber in dry back boilers is not completely surrounded by water. The posterior part is
exposed to the atmosphere. This leads to the increased radiation losses, as the radiant heat is lost to
the atmosphere instead of going to the water as in wet back boilers. Earlier generation boilers used to
be dry back.
Thus wet back boilers ensure lesser radiation losses and hence save fuel.
The layout of the tubes involves the number of passes the tube will make to pass the heat from the
boiler furnace before being discharged. These can be two- pass and three pass boiler.
 |
| FIG 26 |
In two pass the combustion gases travels two times in the boiler.
Combustion gases should be cooled before entering the reversal chamber. Excess temperature causes
overheating and cracking of the tube. The heat transfer rate is maximum at the first pass, this rate
decreases with the increasing passes.
 |
| FIG 27 |
22- Three pass boilers
A three pass design provides three opportunities for heat transfer. The stack temperature of 3 pass will be lower than that of 2 pass boiler, of the same design and operating pressure. Efficiency is more than two pass boiler.
Each pass in boiler should be designed with cross sectional area to achieve optimal flue gas velocity,
which in turn maximizes heat transfer while also minimizing performance robbing sooth build up within the tubes.
No. Of pass Area of tube m² Temperature C⁰ Heat transfer
1st 11 1600 65%
2nd 43 400 25%
3rd 46 350 10%
 |
| FIG 28 |
Coal, fuel oil, and natural gas are the main types of fossil fuel. These fuels can generate a substantial quantity of heat by reacting with oxygen. These fuels consist of a large number of complex compounds comprised of five principal elements: carbon (C), hydrogen (H), oxygen (O), sulfur (S), and nitrogen (N). Any of these three fuels, i.e., coal, fuel oil, and natural gas, can be used in steam power stations, but coal plays a large role in power generation because of the enormous reserves of coal in many countries around the world,. Most of the large steam generating stations in these countries are coal based, and coal is expected to remain a dominant fuel for this purpose for many years. Coal-fired power plants at present account for about 41% of power produced globally.
Coal-based steam-generating stations also dominate in those countries where natural gas and fuel oil are abundantly available, since the cost of natural gas and fuel oil is exorbitantly high compared to the cost of coal. Additionally, natural gas and fuel oil are more economical in other industries. In modern, large-capacity, coal-fired steam generators coal is burnt in suspension.
 |
| FIG 29 |
Fluidized-bed combustion ensures burning of solid fuel in suspension, in a hot inert solid-bed material of sand, limestone, refractory, or ash, with high heat transfer to the furnace and low combustion temperatures (1075-1225 K). The combustor-bed material consists of only 35% coal. Fluidized-bed combustion is comprised of a mixture of particles suspended in an upwardly flowing gas stream, the combination of which exhibits fluid-like properties. Fluidized-bed combustors are capable of firing a wide range of solid fuels with varying heating value, ash content, and moisture content. In this type of boiler, pollutants in products of combustion are reduced concurrently with combustion much of the ash and hence the particulate matter as well as sulfur is removed during the combustion process. Further, lower temperature combustion in the fluidized bed results in lower production of NOX and obviates any slagging problem.
 |
| FIG 30 |
25-Scotch Marine Boiler
Scotch Marine Boiler is a Fire Tube Boiler. It is also a scotch or tank type boiler. It is a common boiler in the marine industry. This boiler has some special advantage like its compactness and operation efficiency. Scotch Marine Boiler can use any type of water.
The general layout of the Scotch Marine Boiler is that of a squat horizontal cylinder. At the lower part of the boiler shell, one or more large cylindrical furnaces are present. Burned gases and smoke from the boiler furnace pass through the back side of this boiler. After that gases and smoke return through the small tubes and up and out through the boiler chimney.
 |
| FIG 31 |
A waste-heat recovery boiler (WHRB) or a heat-recovery steam generator (HRSG) is a heat exchanger that recovers heat from a gas stream and in turn produces steam that can be used in a process or to drive steam turbines. A common application for HRSGs is in a combined cycle power plant (CCPP), where hot exhaust gas from a gas turbine is fed to the HRSG to generate steam.
 |
| FIG 32 |
Lamont boiler is the first forced convection boiler which is introduced in the year 1925.
A LaMont boiler is a type of forced circulation water-tube boiler in which the boiler water is circulated through an external pump through long closely spaced tubes of small diameter. The mechanical pump is employed in order to have an adequate and positive circulation in steam and hot water boilers. A water circulating pump is used in the boiler to circulate water inside the
boiler. This pump is driven by the steam turbine which uses steam of boiler. It is used in power plant industries to generate electricity.
 |
| FIG 33 |
Compact wall-mounted electric boilers are used in residential hotwater (hydronic) heating systems (Figure 34 ). Heat is generated by electric heating elements immersed in water housed in a waterproof
cast-iron shell. Although small in size, these boilers are capable of generating as high as 90,000 Btu/h, enough heat for the average eight-room house.
 |
| FIG 34 |
29- Biomass boilers
Biomass boilers work by burning biological matter and outputting the resulting heat for use in heating systems. Wood pellets, chips, logs or other biological materials are fed – automatically, semi-automatically, or by hand – into a combustion chamber where they are ignited. The hot gas and air produced by this process travel through a flue, and are then passed through a heat exchanger, which transfers the heat to the water used in the property’s central heating system. The excess heat is also stored in a thermal tank (also called a buffer vessel).
 |
| FIG 35 |
The Nuclear Boiler System (NBS) produces steam from the nuclear fission process, and directs this steam to the main turbine. The NBS is comprised of (1) the reactor vessel, which serves as a housing for the nuclear fuel and associated component, (2) the recirculation system, (3) the control rod drive system, (4) the main steam system and (5) the reactor building portion of the feedwater system.
 |
| FIG 36 |
REQUIREMENTS OF A GOOD BOILER
A good boiler must possess the following qualities:
1. The boiler should be capable to generate steam at the required pressure and quantity as quickly as possible with minimum fuel consumption.
2. The initial cost, installation cost and the maintenance cost should be as low as possible.
3. The boiler should be light in weight, and should occupy small floor area.
4. The boiler must be able to meet the fluctuating demands without pressure fluctuations.
5. All the parts of the boiler should be easily approachable for cleaning and inspection.
6. The boiler should have a minimum of joints to avoid leaks which may occur due to expansion and contraction.
7. The boiler should be erected at site within a reasonable time and with minimum labor.
8. The water and flue gas velocities should be high for high heat transfer rates with minimum pressure drop through the system.
9. There should be no deposition of mud and foreign materials on the inside surface and soot deposition on the outer surface of the heat transferring parts.
10.The boiler should conform to the safety regulations as laid down in the Boiler Act.
BOILER MOUNTINGS and ACCESSORIES BOILER MOUNTINGS
are the components generally mounted on the surface of the boiler to have safety during operation. These are the essential parts of the boiler, without which the boiler operation is not possible. The following are the important mountings of the boiler:
1. Safety valves
[a]. Dead weight safety valve
[b] Spring loaded safety valve
[c] Lever safety
2. Water level indicator
3. Pressure Gauge
4. Steam stop valve
5. Feed check valve
6. Blow off cock
7. Manhole
8. Fusible plug
are the components generally mounted on the surface of the boiler to have safety during operation. These are the essential parts of the boiler, without which the boiler operation is not possible. The following are the important mountings of the boiler:
1. Safety valves
[a]. Dead weight safety valve
[b] Spring loaded safety valve
[c] Lever safety
2. Water level indicator
3. Pressure Gauge
4. Steam stop valve
5. Feed check valve
6. Blow off cock
7. Manhole
8. Fusible plug
 |
| FIG 37 |
Boiler efficiency - combustion gross and net calorific value
Boiler efficiency may be indicated by
- Combustion Efficiency - indicates a burners ability to burn fuel measured by unburned fuel and excess air in the exhaust
- Thermal Efficiency - indicates the heat exchangers effectiveness to transfer heat from the combustion process to the water or steam in the boiler, exclusive radiation and convection losses
- Fuel to Fluid Efficiency - indicates the overall efficiency of the boiler inclusive thermal efficiency of the heat exchanger, radiation and convection losses - output divided by input.
Boiler Efficiency is in general indicated by either Thermal Efficiency or Fuel to Fluid Efficiency depending the context.
Boiler Efficiency related to the boilers energy output to the boilers energy input can be expressed as:
m = mass of water actually evaporated
hfg = latent heat of steam at boiler pressure in kJ
h1 = specific enthalpy of feed water at the generation temp[erature , kJ
x = dryness fraction
me = equivalent evaporation from and at 100 C.
H1 = Heat required to produce 1 kg of steam.
me = equivalent evaporation from and at 100 C.
H1 = Heat required to produce 1 kg of steam.
Heat Exported from the Boiler to the Fluid
If a fluid like water is used to transfer heat from the boiler - the heat transfer can be expressed as:
q = (m / t) cp dT
where
q = heat transfer (kJ/s, kW)
m / t = mass flow (kg/s)
m = mass (kg)
t = time (s)
cp = specific heat (kJ/kg C)
dT = temperature difference between inlet and outlet of the boiler (oC)
q = (m / t) he
where
m = mass flow of evaporated water (kg)
t = time (s)
he = evaporation energy in the steam at the saturation pressure the boiler is running (kJ/kg)
The energy provided by a fuel may be expressed in two ways - 'Gross' or 'Net' Calorific Value.
Gross Calorific Value
This is the theoretical total of the energy in the fuel. The gross calorific value of the fuel includes the energy used for evaporating the water in the combustion process. The flue gases from boilers are in general not condensed. The actual amount of heat available to the boiler plant is therefore reduced.
- Gross Calorific Values of some common Fuels
An accurate control of the air supply is essential to boilers efficiency.
- to much air cools the furnace and carries away useful heat
- too little air and the combustion will be incomplete. Unburned fuel will be carried over and smoke produced
Net calorific value
Net calorific value excludes the energy in the water vapor discharged to the stack in the combustion process. The combustion process can be expressed as:
[C + H (fuel)] + [O2 + N2 (Air)] -> (Combustion Process) -> [CO2 + H2O + N2 (Heat)]
where
C = Carbon
H = Hydrogen
O = Oxygen
N = Nitrogen
In general it is possible to use the approximation:
net calorific value = gross calorific value - 10%
BOILER FUEL CONSUMPTION
FC = Fuel consumption
SP = steam produced
hs = enthalpy of steam @ 100 PSIG
hw = enthalpy of feedwater @ saturation temperature
BE = Boiler efficiency
VHI = Fuel heating value
FC = [ SP * ( hs – hw ) / ( BE * VHI ) ]
Where,FC = Fuel consumption
SP = steam produced
hs = enthalpy of steam @ 100 PSIG
hw = enthalpy of feedwater @ saturation temperature
BE = Boiler efficiency
VHI = Fuel heating value
Btu Content of heating fuel
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