Fig.: macroscopic kinetic energy is an organized form of energy and is much more useful

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Clarissa Garrett
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The vast majority of electrical generating plants are variations of vapor power plants in which water

1 Harnessing Energy Fig.: macroscopic kinetic energy is an organized form of energy and is much more useful Vapor Power Cycle The vast majority of electrical generating plants are variations of vapor power plants in which water is the working fluid. The basic components of a simplified fossil-fuel vapor power plant are shown schematically in the figure below. Fig.: Schematic Diagram of a Vapor Power Plant

2 Carnot Vapor Cycle Fig.: Carnot Vapor Cycle (schematic diagram on the left and T-s diagram on the right) There are four processes in the Carnot Vapor cycle, each changing the state of the working fluid. These states are identified by number in the diagram to the above left. Process 1-2 Process 2-3 Process 3-4 Process 4-1 The working fluid is compressed from low to high pressure. As the fluid is a liquid vapor mixture at this stage the compression requires relatively less input energy. The high pressure liquid enters a boiler where it is heated at constant pressure by an external heat source to become a dry saturated vapor. The dry saturated vapor expands through a turbine, generating power. This decreases the temperature and pressure of the vapor, and some condensation may occur. The wet vapor then enters a condenser where it is condensed at a constant pressure and temperature to become a saturated liquid. The pressure and temperature of the condenser is fixed by the temperature of the cooling coils as the fluid is undergoing a phase-change. Several impracticalities are associated with this cycle The isentropic compression process (1-2) involves the compression of a liquid vapor mixture to a saturated liquid. There are two difficulties with this process. First, it is not easy to control the condensation process so precisely as to end up with the desired quality at state 1. Second, it is not practical to design a compressor that will handle two phases. The isentropic expansion process (3-4) can be approximated by closely by a well designed turbine. However, the quality of the steam decreases during this process as shown on the T-s diagram. Thus the turbine will have to handle steam with low quality, i.e. steam with high moisture content. The impingement of liquid droplets on the turbine blades causes erosion and is a major source of wear.

3 Rankine Cycle Fig.: Rankine Cycle (schematic diagram on the left and T-s diagram on the right) Many of the impracticalities associated with the Carnot cycle can be eliminated by superheating the steam in the boiler and condensing it completely after expansion in the condenser. The cycle that results is the Rankine cycle. Similar to the Carnot cycle, there are four processes in the Rankine cycle, each changing the state of the working fluid. These states are identified by number in the diagram to the above left. Process 1-2 Process 2-3 Process 3-4 Process 4-1 The working fluid is pumped from low to high pressure. As the fluid is a saturated / sub cooled liquid at this stage the pumping requires very small input energy. The high pressure sub cooled liquid enters a boiler where it is heated at constant pressure by an external heat source to become superheated vapor. The superheated vapor expands through a turbine, generating power. This decreases the temperature and pressure of the vapor, and some condensation may occur. The wet vapor then enters a condenser where it is condensed at a constant pressure and temperature to become a saturated liquid. The pressure and temperature of the condenser is fixed by the temperature of the cooling coils as the fluid is undergoing a phase-change. Thermodynamically these four processes can be defined as Process 1 2: Isentropic Compression in a Pump Process 2 3: Constant Pressure Heat Addition / Isothermal Heat Addition in a Boiler Process 3 4: Isentropic Expansion in a Turbine Process 4 1: Constant Pressure Heat Rejection / Isothermal Heat Rejection in a Condenser In an ideal Rankine cycle the pump and turbine would be isentropic, i.e., the pump and turbine would generate no entropy and hence maximize the net work output. In a real Rankine cycle,

the compression by the pump and the expansion in the turbine are not isentropic. In other words, these processes are non-reversible and entropy is increased during the two processes.

4 the compression by the pump and the expansion in the turbine are not isentropic. In other words, these processes are non-reversible and entropy is increased during the two processes. This somewhat increases the power required by the pump and decreases the power generated by the turbine. Analyzing Vapor Power Systems Rankine Cycle Fig.: Principal Work and Heat transfers from the components of Rankine Cycle Note: As a sign convention, work done by the surroundings on the system and heat flowing from the surroundings into the system are taken as positive. Conversely, Work done by the system on the surroundings and heat flowing from the system into the surroundings are taken as negative. Pump The working fluid starts a cycle as the liquid condensate leaving the condenser at 1 is pumped from the condenser into the higher pressure boiler. Taking a control volume around the pump and assuming no heat transfer with the surroundings, steady flow energy equation gives W P = -m (h 2 h 1 ) Boiler The liquid leaving the pump at 2, called the boiler feedwater, is evaporated in the boiler and is heated beyond saturation producing superheated steam. Taking a control volume enclosing the boiler tubes and drums carrying the feedwater from state 2 to state 3, steady flow energy equation gives Q in = m (h 3 h 2 ) Turbine Vapor from the boiler at state 3, having an elevated temperature and pressure, expands through the turbine to produce work and then is discharged to the condenser at state 4 with relatively low pressure. Neglecting heat transfer with the surroundings, steady flow energy equation gives - W T = m (h 4 h 3 ) or, W T = m (h 3 h 4 )

5 Condenser In the condenser there is heat transfer from the vapor to cooling water flowing in a separate stream. The vapor condenses and the temperature of the cooling water increases. Taking a control volume enclosing the condenser tubes carrying the steam from state 4 to state 1, steady flow energy equation gives - Q out = m (h 1 h 4 ) or, Q out = m (h 4 h 1 ) Thermal Efficiency The thermal efficiency gauges the extent to which the energy input to the working fluid passing through the boiler is converted to the net work output. Using the quantities and expressions introduced above, the thermal efficiency of the Rankine cycle can be expressed as η = (W T - W P ) / Q in = [(h 3 h 4 ) (h 2 h 1 )] / (h 3 h 2 ) = 1 [(h 4 h 1 ) / (h 3 h 2 )] Back Work Ratio (BWR) A parameter used to describe power plant performance is the back work ratio or bwr. It is defined as the ratio of the pump work input to the work developed by the turbine. Using the quantities and expressions introduced above, the back work ratio for the Rankine cycle can be expressed as BWR = W P / W T = [(h 2 h 1 ) / (h 3 h 4 )] Class Problem 01 Steam is the working fluid in an ideal Rankine cycle. Saturated vapor enters the turbine at 8.0 MPa and saturated liquid exits the condenser at a pressure of MPa. The net power output of the cycle is 100 MW. Determine for the cycle (a) Thermal Efficiency (b) Back Work Ratio (c) Mass Flow Rate of the Steam (in kg/h) (d) Rate of Heat Transfer into the Working Fluid as it passes through the Boiler (in MW) (e) Rate of Heat Transfer from the Condensing Steam as it passes through the Condenser (in MW) Class Problem 02 Steam is the working fluid in a Rankine cycle. Superheated steam enters turbine at 8.0 MPa and 480 o C, and expands to the condenser pressure of MPa. The net power output is 100 MW. If the turbine and pump are isentropic, determine for the cycle (a) Thermal Efficiency (b) Back Work Ratio (c) Mass Flow Rate of the Steam (in kg/h) (d) Rate of Heat Transfer into the Working Fluid as it passes through the Boiler (in MW) (e) Rate of Heat Transfer from the Condensing Steam as it passes through the Condenser (in MW)

Steam Generator A steam generator or boiler is usually a closed vessel made of steel. Its function is to transfer the heat produced by the combustion of fuel to water and ultimately to generate steam.

6 Steam Generator A steam generator or boiler is usually a closed vessel made of steel. Its function is to transfer the heat produced by the combustion of fuel to water and ultimately to generate steam. The steam produced may be supplied (a) To an external combustion engine, i.e. steam engines and turbines. (b) At low pressures for industrial process work. (c) For producing hot water used for heating installations. Selection of a Steam Generator The selection of type and size of a steam boiler depends upon the following factors power required and the working pressure steam generation rate quality of fuel and water available geographical position of power house probable load factor Classification In general, steam generators can be classified into two categories Fire Tube Boiler Water Tube Boiler Fire Tube Boilers A fire tube boiler is a type of boiler in which hot gases / flue gases (products of combustion) from a fire (heat source) pass through one or more tubes running through a sealed container of water. The heat energy from the gases passes through the sides of the tubes by thermal conduction, heating the water and ultimately creating steam. A fire-tube boiler is sometimes called a "smoke-tube boiler" or "shell boiler" or sometimes just "fire pipe". Types of Fire Tube Boiler: Cornish boiler, Lancashire boiler, Scotch marine boiler, Locomotive boiler etc. Fig.: Schematic Diagram of a Fire Tube Boiler

Water Tube Boiler A water tube boiler is a type of boiler in which water circulates in tubes heated externally by the hot gases / flue gases. Water tube boilers are used for high-pressure boilers.

7 Water Tube Boiler A water tube boiler is a type of boiler in which water circulates in tubes heated externally by the hot gases / flue gases. Water tube boilers are used for high-pressure boilers. Fuel is burned inside the furnace, creating hot gas which heats up water in the steam generating tubes. Types of water tube boiler: Babcock & Wilcox boiler, Stirling boiler, Yarrow boiler etc. Fig.: Schematic Diagram of a Water Tube Boiler

8 Comparison between Water Tube and Fire Tube Boilers Fire Tube Boiler The hot gases from the furnace pass through the tubes which are surrounded by water in the shell It cannot handle high pressure Water Tube Boiler The water circulates inside the tubes which are surrounded by hot gases from the furnace It is a high pressure boiler The rate of generation of steam is relatively low The rate of generation of steam is high Overall efficiency is up to 75% Overall efficiency is up to 90% It is not preferable for fluctuating loads for a longer time period It is preferred for widely fluctuating loads The operating cost is less The bursting chances are less but bursting produces greater risk to the damage of the property It is generally used for supplying steam on a small scale and is not suitable for large power plants The operating cost is high The bursting chances are higher but bursting doesn t produce any destruction to the whole boiler It is used for large power plants Mountings & Accessories Mountings: These are the fittings, which are mounted on the boiler for its proper and safe functioning. Such as, water level indicator, pressure gage, safety valves, fusible plug etc. Accessories: These are the devices which are used as integral parts of a boiler, and help in running efficiently. Such as, superheater tubes, economizer, air preheater etc. Pressure Gage Fig.: A Bourdon Tube Pressure Gage

9 Safety Valve Fig.: A Spring Loaded Safety Valve Water Level Indicator Fig.: A Water Level Indicator

10 Fusible Plug Fig.: A Fusible Plug Economizer Fig.: An Economizer

11 Air Preheater Fig.: An Air Preheater

Chapter 10 VAPOR AND COMBINED POWER CYCLES

Thermodynamics: An Engineering Approach Seventh Edition Yunus A. Cengel, Michael A. Boles McGraw-Hill, 2011 Chapter 10 VAPOR AND COMBINED POWER CYCLES Copyright The McGraw-Hill Companies, Inc. Permission