Unit 3 Power Producing Devices

Boiler: A boiler is a closed vessel in which fluid (generally water) is heated. The fluid does not necessarily boil. The heated or vaporized fluid exits the boiler for use in various processes or heating applications, including water heating, central heating, boiler-based power generation, cooking, and sanitation.

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

1. According to Relative Passage of water and hot gases: 1. Water Tube Boiler: A boiler in which the water flows through some small tubes which are surrounded by hot combustion gases, e.g., Babcock and Wilcox, Stirling, Benson boilers, etc. 2. Fire-tube Boiler: The hot combustion gases pass through the boiler tubes, which are surrounded by water, e.g., Lancashire, Cochran, locomotive boilers, etc.

2. According to Water Circulation Arrangement: 1. Natural Circulation: Water circulates in the boiler due to density difference of hot and water, e.g., Babcock and Wilcox boilers, Lancashire boilers, Cochran, locomotive boilers, etc. 2. Forced Circulation: A water pump forces the water along its path, therefore, the steam generation rate increases, e.g., Benson, La Mont, Velox boilers, etc.

3. According to the Use: 1. Stationary Boiler: These boilers are used for power plants or processes steam in plants. 2. Portable Boiler: These are small units of mobile and are used for temporary uses at the sites. 3. Locomotive: These are specially designed boilers. They produce steam to drive railway engines. 4. Marine Boiler: These are used on ships.

4. According to Position of the Boilers: . Horizontal, inclined or vertical boilers

5. According to the Position of Furnace 1. Internally fired: The furnace is located inside the shell, e.g., Cochran, Lancashire boilers, etc. 2. Externally fired: The furnace is located outside the boiler shell, e.g., Babcock and Wilcox, Stirling boilers, etc.

6. According to Pressure of steam generated . Low-pressure boiler: a boiler which produces steam at a pressure of 15-20 bar is called a low-pressure boiler. This steam is used for process heating. . Medium-pressure boiler: It has a working pressure of steam from 20 bars to 80 bars and is used for power generation or combined use of power generation and process heating. . High-pressure boiler: It produces steam at a pressure of more than 80 bars. . Sub-critical boiler: If a boiler produces steam at a pressure which is less than the critical pressure, it is called as a subcritical boiler. . Supercritical boiler: These boilers provide steam at a pressure greater than the critical pressure. These boilers do not have an evaporator and the water directly flashes into steam, and thus they are called once through boilers.

7. According to charge in the furnace. . Pulverized fuel, . Supercharged fuel and . Fluidized bed combustion boilers.

 Fire Tube Boilers-

In firetube boilers, the combustion gases pass inside boiler tubes, and heat is transferred to water on the shell side. Scotch marine boilers are the most common type of industrial firetube boiler. The Scotch marine boiler is an industry workhorse due to low initial cost, and advantages in efficiency and durability. Scotch marine boilers are typically cylindrical shells with horizontal tubes configured such that the exhaust gases pass through these tubes, transferring energy to boiler water on the shell side. Scotch marine boilers contain relatively large amounts of water, which enables them to respond to load changes with relatively little change in pressure. However, since the boiler typically holds a large water mass, it requires more time to initiate steaming and more time to accommodate changes in steam pressure. Also, Scotch marine boilers generate steam on the shell side, which has a large surface area, limiting the amount of pressure they can generate. In general, Scotch marine boilers are not used where pressures above 300 psig are required. Today, the biggest firetube boilers are over 1,500 boiler horsepower. Firetube boilers are often characterized by their number of passes, referring to the number of times the combustion (or flue) gases flow the length of the pressure vessel as they transfer heat to the water. Each pass sends the flue gases through the tubes in the opposite direction. To make another pass, the gases turn 180 degrees and pass back through the shell. The turnaround zones can be either dryback or water- back. In dryback designs, the turnaround area is refractory lined. In water-back designs, this turnaround zone is water-cooled, eliminating the need for the refractory lining.

 Water Tube Boilers-

In watertube boilers, boiler water passes through the tubes while the exhaust gases remain in the shell side, passing over the tube surfaces. Since tubes can typically withstand higher internal pressure than the large chamber shell in a firetube, watertube boilers are used where high steam pressures are required. Watertube boilers are also capable of high efficiencies and can generate saturated or superheated steam. The ability of watertube boilers to generate superheated steam makes these boilers particularly attractive in applications that require dry, high- pressure, high-energy steam, including steam turbine power generation. The performance characteristics of watertube boilers make them highly favorable in process industries, including chemical manufacturing, pulp and paper manufacturing, and refining. Although firetube boilers account for the majority of boiler sales in terms of units, water-tube boilers account for the majority of boiler capacity.

 BOILER MOUNTINGS AND ACCESSORIES Boiler mountings and accessories both are different things, the mountings are used for the safety of the boiler whereas the accessories are used to increase the efficiency of the boiler with same amount of fuel. The different boiler mountings and accessories that are installed on the steam boiler are as follows:

Boiler mountings:

1. Water level indicator (Water level gauge) 2. Pressure gauge 3. Safety valves 4. Stop valve 5. Blow off cock (Blow off valve) 6. Feed check valve The function of various boiler mountings and accessories are: 1. Water Level Indicator It is fitted in front of the boiler and generally present two in number. It is used to indicate the water level inside the boiler. It shows the instantaneous level of water that is present inside the steam boiler which is necessary for its proper working. 2. Pressure gauge It is also present in front of the boiler. It is used to measure the pressure of the steam inside the boiler. The pressure gauges generally used are of Bourden type 3. Safety Valves Safety valves are attached to the steam boiler chest. It is used to prevent explosion due to excessive internal pressure. When the internal pressure inside the boiler exceeds its working pressures than the safety valves blow off the steam and maintains the internal pressure. Generally two safety valves are present on a boiler.

4. Stop Valve (steam stop valve) It is usually fitted on the highest part of the boiler with the help of a flange.The main function of the stop valve is to control the flow of steam from the boiler to the main steam pipe. To completely shut off the steam supply when required. 5. Blow Off Cock It is fitted at the bottom of the boiler drum. The functions of blow off cock is i. To empty the boiler whenever required. ii. To discharge the scale, mud and sediments which gets collected at the bottom of the boiler. 6. Feed Check Valve It is non-return valve and fitted to a screwed spindle to regulate the lift. It is fitted to the shell slightly below the normal water level of the boiler. A boiler must have its spindle lifted before the pump is started. It regulates the supply of water which is pumped into the boiler by feed pump. 7. Fusible Plug It is fitted to the crown plate of the furnace or firebox. Its function is to extinguish fire in the furnace when the water level in the boiler falls to an unsafe limit. This avoids the explosion that may takes place because of the overheating of the furnace plate.

 Boiler accessories: the boiler accessories are the integral parts of the boiler. They are used in the boiler to improve its efficiency.

Boiler accessories:

1. Air pre-heater 2. Super heater 3. Economiser

1. Air preheater It is used to recover heat from the exhaust gases. It is installed between the economiser and the chimney. 2. Super heater It is placed in the path of hot flue gases from the furnace. A super heater is an important accessory used in the boiler. Its main function is to increase the temperature of saturated steam without raising its pressure. 3. Economiser  It is used to heat the feed water by the utilization of heat from the hot fuel gases before it leaves the chimney.  A economiser improves the economy of the steam boilers.

TURBINE: A turbine is a device that harnesses the kinetic energy of some fluid - such as water, steam, air, or combustion gases - and turns this into the rotational motion of the device itself. These devices are generally used in electrical generation, engines, and propulsion systems and are classified as a type of engine. They are classified as such because engines are simply technologies that take an input and generate an output. A simple turbine is composed of a series of blades - currently steel is one of the most common materials used - and allows the fluid to enter the turbine, pushing the blades. These blades then spin and eject the fluid which now has less energy it did than when it entered the turbine. Some of the energy is captured by the turbine and used. Turbines are used in many different areas, and each type of turbine has a slightly different construction to perform its job properly. Turbines are used in wind power, hydropower, in heat engines, and for propulsion. Turbines are extremely important because of the fact that nearly all electricity is generated by them.

 Types of Turbines Turbines are also divided by their principle of operation and can be: 1. An Impulse turbine, which is driven by a high-velocity jet (or multiple jets) of water. Example: Pelton, Turgo, and Crossflow turbine.

2. A Reaction turbine. The rotor of a reaction turbine is fully immersed in water and is enclosed in a pressure casing. The runner blades are profiled so that pressure differences across them impose lift forces, just as on aircraft wings, which cause the runner to rotate faster than is possible with a jet. Example: propeller turbine (with Kaplan variant) and Francis turbine 3. A Gravity turbine is driven simply by the weight of water entering the top of the turbine and falling to the bottom, where it is released – for example, an overshot waterwheel. These are inherently slow-running machines. Example: reverse Archimedes Screw and overshot waterwheel

 Impulse Turbines:

 The Pelton Turbine consists of a wheel with a series of split buckets set around its rim; a high velocity jet of water is directed tangentially at the wheel. The jet hits each bucket and is split in half, so that each half is turned and deflected back almost through 180º. Nearly all the energy of the water goes into propelling the bucket and the deflected water falls into a discharge channel below. The Turgo turbine is similar to the Pelton but the jet strikes the plane of the runner at an angle (typically 20° to 25°) so that the water enters the runner on one side and exits on the other. Therefore the flow rate is not limited by the discharged fluid interfering with the incoming jet (as is the case with Pelton turbines). As a consequence, a Turgo turbine can have a smaller diameter runner and rotate faster than a Pelton for an equivalent flow rate. The Crossflow turbine has a drum-like rotor with a solid disk at each end and gutter-shaped “slats” joining the two disks. A jet of water enters the top of the rotor through the curved blades, emerging on the far side of the rotor by passing through the blades a 2nd time. The shape of the blades is such that on each passage through the periphery of the rotor the water transfers some of its momentum, before falling away with little residual energy.

Reaction Turbines:

Reaction turbines exploit the oncoming flow of water to generate hydrodynamic lift forces to propel the runner blades. They are distinguished from the impulse type by having a runner that always functions within a completely water-filled casing. All reaction turbines have a diffuser known as a ‘draft tube’ below the runner through which the water discharges. The draft tube slows the discharged water and so creates suction below the runner which increases the effective head. Propeller-type turbines are similar in principle to the propeller of a ship, but operating in reversed mode. A set of inlet guide vanes admits the flow to the propeller and these are often adjustable so as to allow the flow passing through the machine to be varied. In some cases the blades of the runner can also be adjusted, in which case the turbine is called a Kaplan. The mechanics for adjusting turbine blades and guide vanes can be costly and tend to be more affordable for large systems, but can greatly improve efficiency over a wide range of flows. The Francis turbine is essentially a modified form of propeller turbine in which water flows radially inwards into the runner and is turned to emerge axially. For medium-head schemes, the runner is most commonly mounted in a spiral casing with internal adjustable guide vanes. Since the cross-flow turbine is now a less costly (though less efficient) alternative to the spiral-case Francis, it is rare for these turbines to be used on sites of less than 100 kW output.

Gravity Turbines The Archimedes Screw has been used as a pump for centuries, but has only recently been used in reverse as a turbine. It’s principle of operation is the same as the overshot waterwheel, but the clever shape of the helix allows the turbine to rotate faster than the equivalent waterwheel and with high efficiency of power conversion (over 80%). However they are still slow-running machines, which require a multi-stage gearbox to drive a standard generator. A key advantage of the Screw is that it avoids the need for a fine screen and automatic screen cleaner because most debris can pass safely through the turbine. The Archimedian screw is proven to be a ‘fish-friendly’ turbine.  Turbine Types:

 Internal Combustion Engine: An engine which generates motive power by the burning of petrol, oil, or other fuel with air inside the engine, the hot gases produced being used to drive a piston or do other work as they expand.

Types of Internal Combustion Engines: Internal combustion engines can be classified into a large number of types based on several criteria. The classification of IC engines is given below: 1. Based on the fuel used i Diesel Engine ii Petrol Engine (or Gasoline Engine) 2. Based on the type of cycle i Otto Cycle Engine ii Diesel Cycle Engine iii Dual Cycle Engine 3. Based on the number of strokes per cycle i. Two-stroke Engine ii. Four-stroke Engine 4. Based on the number of cylinders i. Single Cylinder Engine ii. Multi cylinder Engine iii. Twin Cylinder Engine iv. Three Cylinder Engine v. Four Cylinder Engine vi. Six Cylinder Engine vii. Eight Cylinder Engine viii. Twelve Cylinder Engine ix. Sixteen Cylinder Engine 5. Based on the type of ignition i Spark Ignition Engine (S.I. Engine) ii Compression Ignition Engine (C.I. Engine) 6. Based on the lubrication system used i Dry sump lubricated engine ii Wet sump lubricated Engine 7. Based on the cooling system used i Air-cooled Engine ii Water-cooled Engine 8. Based on the arrangement of valves i L-head Engine ii I-head Engine iii T-head Engine iv F-head Engine 9. Based on the position of cylinders i Horizontal Engine ii Vertical Engine iii Radial Engine iv Opposed Piston Engine v Opposed Cylinder Engine vi V Engine vii W Engine viii Inline Engine 10. Based on the pressure boost given to the inlet air or air-fuel mixture i Naturally aspired Engine ii Supercharged Engine iii Turbocharged Engine iv Crankcase compressed Engine 11. Based on application i Automobile Engine ii Aircraft Engine iii Locomotive Engine iv Marine Engine v Stationary Engine

Two-Stroke Engines:

A two-stroke engine performs all the same steps, but in just two piston strokes. The simplest two-stroke engines do this by using the crankcase and the underside of the moving piston as a fresh charge pump. Such engines carry the official name “crankcase-scavenged two-strokes.”As the two-stroke’s piston rises on compression, its underside pulls a partial vacuum in the crankcase. An intake port of some kind (cylinder wall port, reed valve or rotary disc valve) opens, allowing air to rush into the crankcase through a carburetor. As the piston nears Top Dead Center, a spark fires the compressed mixture. As in a four-stroke, the mixture burns and its chemical energy becomes heat energy, raising the pressure of the burned mixture to hundreds of psi. This pressure drives the piston down the bore, rotating the crankshaft. As the piston continues down the bore, it begins to expose an exhaust port in the cylinder wall. As spent combustion gas rushes out through this port, the descending piston is simultaneously compressing the fuel-air mixture trapped beneath it in the crankcase. As the piston descends more, it begins to expose two or more fresh-charge ports, which are connected to the crankcase by short ducts. As pressure in the cylinder is now low and pressure in the crankcase higher, fresh charge from the crankcase rushes into the cylinder through the fresh-charge (or “transfer”) ports. These ports are shaped and aimed to minimize direct loss of fresh charge to the exhaust port. Even in the best designs, there is some loss, but simplicity has its price! This process of filling the cylinder while also pushing leftover exhaust gas out the exhaust port is called “scavenging.” While the piston is near Bottom Dead Center, mixture continues to move from the crankcase, up through the transfer ports, and into the cylinder. As the piston rises, it first covers the transfer ports, leaving only the exhaust port still open. If there were no way to stop it, much of the fresh charge would now be pumped out the exhaust. The world’s most efficient piston engines are in fact the giant, slow-turning marine diesels that carry the world’s international shipping trade—they are twice as efficient as the usual four-stroke spark-ignition engines found in cars and motorcycles. Four Stroke Engine

A four-cycle engine works with 4 basic steps to a successful rotation of the crankshaft: the intake, compression, power and exhaust stroke. Each engine cylinder has four openings for the intake, exhaust, spark plug and fuel injection. The piston is driven by the engine's crankshaft whereas the intake and exhaust valves are driven by the camshaft. The crankshaft and camshaft are connected by a timing belt/chain to maintain synchronization between them. The various processes comprising the cycles of a four-stroke engine are explained below:

 Intake Stroke: The intake stroke is where the intake valves are open and the air is drawn into the cylinder. The fuel injector sprays the fuel into the cylinder to achieve the perfect air-fuel ratio. The downward movement of the piston causes the air and fuel to be sucked into the cylinder.

 Compression Stroke: The next is the compression cycle where both the intake and exhaust valves are closed. The upward movement of the piston causes the air- fuel mixture to be compressed upwards towards the spark plug. The compression makes the air-fuel combination volatile for easier ignition.

 Combustion/Power Stroke: During the power/combustion stroke, both the intake and exhaust valves are still closed. The spark plug produces a spark to ignite the compressed air-fuel mixture. The resulting energy of the combustion forcefully pushes the piston downward.

 Exhaust Stroke: The last cycle is the exhaust stroke, when the exhaust valves open and the exhaust gases are forced up by the returning piston.

Thermal Power Plant:

A thermal power station is a power station in which heat energy is converted to electric power. In most of the places in the world the turbine is steam-driven. Water is heated, turns into steam and spins a steam turbine which drives an electrical generator. After it passes through the turbine, the steam is condensed in a condenser and recycled to where it was heated; this is known as a Rankine cycle. The greatest variation in the design of thermal power stations is due to the different heat sources; fossil fuel dominates here, although nuclear heat energy; solar heat energy, biofuels and waste incineration are also used. Some prefer to use the term energy center because such facilities convert forms of heat energy into electrical energy. Certain thermal power stations are also designed to produce heat energy for industrial purposes, or district heating, or desalination of water, in addition to generating electrical power Working- The fuel is transported from mines via trains to the fuel storage facility in a power plant. The fuel transported to the plant is generally bigger in particle size and before it is fed to the boiler furnace it is broken down into smaller pieces using crushers. The fuel is then fed to the boiler generating a large amount of combustion heat. On the other hand treated water free from impurities and air is fed to the boiler drum where the combustion heat from the fuel is transferred to water to convert it into high pressure and temperature steam. Generally, flue gases from the boiler exhaust are at high temperature and if this heat is not utilized will lead to a large number of losses resulting in reduced boiler efficiency. So generally, this waste heat is recovered by heating either air required for combustion or preheat water before sending it into a boiler. Flue gases are then allowed to pass through a dust collector or a bag filter to arrest dust particles so as to prevent air pollution before sending it to the atmosphere through a chimney.

Advantages: i. Thermal power station has less initial cost as compared to hydro-electric generating station. ii. It requires less space as compared to the hydro-electric power station. iii. The fuel cost is less as compared to gas. iv. Huge amount of power can be generated by TPS. v. The cost of generation is less as compared to diesel power station.

Disadvantages i. The running cost of thermal power station is more as compared to hydro power station. ii. It pollutes the atmosphere due to production of large amount of smoke and fumes. iii. Maintenance cost is more. iv. Skilled persons are required for erecting and maintaining the power station. v. Land requirement is more for storage of coal and ash.

 Hydro Power Plant:

The Three Gorges Dam in China; the hydroelectric dam is the world's largest power station

Hydropower or water power is power derived from the energy of falling or fast-running water, which may be harnessed for useful purposes. Since ancient times, hydropower from many kinds of watermills has been used as a renewable energy source for irrigation and the operation of various mechanical devices, such as gristmills, sawmills, textile mills, trip hammers, dock cranes, domestic lifts, and ore mills. A trompe, which produces compressed air from falling water, is sometimes used to power other machinery at a distance. Working- Hydropower plants capture the energy of falling water to generate electricity. A turbine converts the kinetic energy of falling water into mechanical energy. Then a generator converts the mechanical energy from the turbine into electrical energy.

Advantages- 1. If electricity is not needed, the sluice gates can be shut, stopping electricity generation. The water can be saved for use another time when electricity demand is high. 2. Dams are designed to last many decades and so can contribute to the generation of electricity for many years / decades. 3. The lake that forms behind the dam can be used for water sports and leisure / pleasure activities. Often large dams become tourist attractions in their own right. 4. The lake's water can be used for irrigation purposes. 5. The build up of water in the lake means that energy can be stored until needed, when the water is released to produce electricity. 6. When in use, electricity produced by dam systems do not produce green house gases. They do not pollute the atmosphere.

Disadvantages- 1. Dams are extremely expensive to build and must be built to a very high standard. 2. The high cost of dam construction means that they must operate for many decades to become profitable. 3. The flooding of large areas of land means that the natural environment is destroyed. 4. People living in villages and towns that are in the valley to be flooded, must move out. This means that they lose their farms and businesses. In some countries, people are forcibly removed so that hydro-power schemes can go ahead. 5. The building of large dams can cause serious geological damage.

Wind Power Plant-

Wind power or wind energy is the use of air flow through wind turbines to provide the mechanical power to turn electric generators and traditionally to do other work, like milling or pumping. Wind power is a sustainable and renewable alternative to burning fossil fuels, and has a much smaller impact on the environment. Wind farms consist of many individual wind turbines, which are connected to the electric power transmission network. Onshore wind is an inexpensive source of electric power, competitive with or in many places cheaper than coal or gas plants. Onshore wind farms also have an impact on the landscape, as typically they need to be spread over more land than other power stations and need to be built in wild and rural areas, which can lead to "industrialization of the countryside and habitat loss. Offshore wind is steadier and stronger than on land and offshore farms have less visual impact, but construction and maintenance costs are considerably higher. Small onshore wind farms can feed some energy into the grid or provide electric power to isolated off-grid locations.

Working - Blades of the wind turbine work as an airfoil of different cross-sections all along the length. When fluid (air) moves over this airfoil it generates a lift force thus making the blade to rotate at its axis. The generator is also connected to the rotor shaft starts rotating and produces electricity. Advantages

i. Air as a fuel is free and inexhaustible. ii. It is clean source of energy and does note pollute the environment. iii. The cost of electricity is too low and wind turbine could be used over more than 20 years iv. Wind Energy is cheap, only the installation and maintenance cost is more. v. Wind energy is one of the fastest growing sectors all around the globe so it is generating a lot of employment in manufacturing, installation and maintenance.

Disadvantages i. It takes a lot of research and effort to decide the location where wind power plant has to be installed, due to fluctuating pattern of wind. ii. Its initial setup cost is too high as to setup a turbine you have to go through a survey to determine the wind speed of the location. It all adds up to the cost. iii. The greatest disadvantage to local bird population as they die due to collision with blades. iv. Noise pollution is the one of the major disadvantage. v. Wind power plant is only useful to the countries with coastal or hilly areas.

Solar Power Plant-

Solar power is the conversion of energyfrom sunlight into electricity, either directly using photovoltaics (PV), indirectly using concentrated solar power, or a combination. Concentrated solar power systems use lenses or mirrors and tracking systems to focus a large area of sunlight into a small beam. Photovoltaic cells convert light into an electric current using the photovoltaic effect. Photovoltaics were initially solely used as a source of electricity for small and medium-sized applications, from the calculator powered by a single solar cell to remote homes powered by an off-grid rooftop PV system. Advantages: 1. Solar power is pollution free and causes no greenhouse gases to be emitted after installation 2. Reduced dependence on foreign oil and fossil fuels 3. Renewable clean power that is available every day of the year, even cloudy days produce some power 4. Return on investment unlike paying for utility bills 5. Virtually no maintenance as solar panels last over 30 years 6. Excess power can be sold back to the power company if grid inter-tied 7. Use batteries to store extra power for use at night 8. Solar can be used to heat water, power homes and building, even power cars 9. Federal grants, tax incentives, and rebate programs are available to help with initial costs Disadvantages 1. High initial costs for material and installation 2. Needs lots of space as efficiency is not 100% yet 3. No solar power at night so there is a need for a large battery bank 4. Depending on geographical location the size of the solar panels vary for the same power generation 5. Cloudy days do not produce much energy 6. Lower production in the winter months