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Vapor Power Cycles
Vapor power cycles, as the name suggests, use vapor in one phase of the cycle for power generation or for moving the prime mover in steam power plants or in steam powered ships. Water is used as a working fluid in steam power plants because of its abundant supply, very low cost, and suitability. In this article we discuss the various types of cycles that have been used in steam power plants over the years and their modifications.
In countries where there are vast reserves of coal and oil, steam power plants are very popular because they can be set up and started in a very short time as compared with other alternatives, like nuclear power plants and hydro-electric plants.
The various vapor power cycles are the Rankine Cycle, the Regenerative Cycle, the Reheat Cycle, and the Carnot Cycle among others. The Carnot cycle is the most efficient cycle theoretically, but practically, the Rankine Cycle is best suited and more popular.
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Steam Power Plants
Steam power plants are used worldwide for the generation of electricity and for propulsion. The heat energy from an energy source like the combustion of coal, or from nuclear fission is utilized to heat the water which changes phase and becomes steam. This steam is superheated to avoid any possibility of water carriage and to increase the enthalpy. This steam is passed to a steam turbine where it does work and is expanded. The steam is finally cooled in a steam condenser, a change of phase occurs, and it enters the hot well as hot water.
This process is called as a cycle because the working fluid, i.e. water, starts from the hot well and then enters the boiler. After doing its work, it comes back to the hot well.
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There are different types of vapor power cycles, most of them adapted from the Rankine cycle, which is the theoretical cycle for a steam power plant. The Carnot cycle, being the most efficient cycle and defining the Carnot limit, is not the theoretical cycle for the steam power plant because of the following reasons:
- Steam is not fully condensed to water in the condenser, but to a water and steam mixture.
- It is very difficult to manufacture and maintain a pump that can handle both steam and water and to convert it to water at the outlet.
- Super heating is problematic in the Carnot cycle and in practical working, some degree of super heating is required to protect the turbine.
The Rankine Cycle and its adaptations is very popular and is the theoretical cycle for the steam power plants as the condensation of steam is complete and to water, which can be handled by the pumping system and is easier to maintain.
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Continue on to the next page for a discussion of other vapor power cycles used in steam power plants.
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The regenerative cycles are those that use system steam to heat the feed water entering the boiler in the steam power plant. The reheat cycle is one in which the steam is reheated between the stages to reduce the moisture content and avoid mechanical damage in a steam power plant.
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Regenerative Feed Heating Cycles
The efficiency of the Rankine cycle is less than that of the Carnot cycle because irreversible mixing of cold condensate with hot water reduces thermal efficiency. To cater to this, regenerative heating is used in which the expanding steam from the turbine is used to heat the feed water. In the regenerative feed heating cycle, the objective is to heat the feed water with the steam expanding in the turbine so that the feed water is supplied to the boiler at a higher temperature than that of the condenser.
This system is also advantageous as the cold water entry into the boiler caused thermal shocks and damages. Thus regenerative feed heating cycle is a safer and more efficient cycle.
However the ideal regenerative feed heating cycle is not practically achievable because of the following reasons:
- Water cannot be passed through the turbine casing because of the risk of thermal shocks.
- If feed water for heating is passed through the turbine casing then it would reduce the super heating of the steam and may cause the steam to become unsaturated and carrying water drops which is mechanically not safe for the turbine.
Thus, in the practical regenerative cycle, the steam is taken out from a few points and fed to the heaters to heat the feed water.
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When the steam is expanded in the turbine, it becomes unsaturated and if the water content exceeds above 10%, it can cause extreme damage to the turbines. This presence of water can cause corrosion and erosion problems and lead to mechanical damages. Needless to say, the nozzle efficiency, blade efficiency, and the thermal efficiency also suffers.
Thus, to resolve all these problems, the steam is reheated and the thermal efficiency of the plant is increased. In the reheat cycle, a part or whole of the steam is reheated using superheated steam until it is near its initial temperature and then the steam is re-entered into the turbine and then expanded to the condenser pressure to do work. Generally this cycle is a combination of the reheating and the regeneration.
The other types of cycles which are in use are the Binary Vapor cycle, Nuclear Power cycle, etc.
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Continue on to the next page for ideas on how to increase the thermal efficiency of steam power plants.
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The Carnot cycle is the most efficient cycle theoretically and has set the Carnot limit for a steam power plant. Studying the cycle, we find the ways to increase the plant efficiency. The working fluid must have some desirable properties which are described in detail.
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How to Increase the Thermal Efficiency of Steam Power Plants
As we know, the Carnot cycle limits the efficiency of a steam power plant – this is called the Carnot limit. By studying the Carnot cycle, we learn that the plant efficiency is increased by heat addition at elevated temperatures and heat rejection at lowest temperatures. Thus by modifying a cycle in such a way that the heat addition is done at the highest temperature that is possible and the working fluid is so utilized that the temperature at which the heat is rejected to the cycle is the lowest, we can increase the efficiency of the cycle. The various methods based on the above two principle to increase the efficiency of steam power plants are as follows:
- The super heating temperature of the steam should be increased to maximum.
- By increasing the the working steam pressure, because temperature of the steam is dependent on the the pressure of the steam.
- Reduce the pressure at which the steam is exhausted from the system as again the exhausting temperature is dependent on the pressure of the steam exhausted.
- Using regenerative feed heating – as the feed is heated again, the super heat will increase and mechanical damage will be avoided.
- Reheating of steam and by extraction of water from the steam.
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Qualities Required from an Efficient Working Fluid
There are several compounds that can be used as the working fluid in the vapor power cycles and they are steam, Mercury vapor, sulfur dioxide, and some hydrocarbons. However they must have some essential characteristic before they can be used as a working fluid in a power plant.
- Low cost
- Large quantity available
- Freezing temperature lower the normal ambient temperature.
- Non Toxic
- Non Corrosive to the components
- Should decrease in volume upon condensation so that the handling pump can be small
- Chemically stable at maximum temperature
From all the above mentioned working fluids, water is widely used because it satisfies the maximum requirements and, more importantly, it is available in abundance and is very cheap.
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Among the various types of vapor power cycles is the Carnot cycle, which is theoretically the most efficient cycle and sets the limit for the efficiency of any vapor cycle. This limit is known as the Carnot limit. The Rankine cycle and its modifications are used widely and are theoretically the cycles best suited to steam power plants. By studying these cycles, we know practically what all must be done to increase the efficiency and cost effectiveness. It is hoped that this brief discussion will be helpful to all engineers.
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- Steam Power Plant: http://www.techsolsystems.com/images/steamplant.png
- Carnot Cycle: http://www.engr.usask.ca/classes/ME/227/rankine/cycles/carnot/carnot.gif
- Rankine Cycle: http://www.barber-nichols.com/images/rankine_cycle.jpg
- Regenerative Feed Cycle: http://nptel.iitm.ac.in/courses/Webcourse-contents/IIT-KANPUR/Basic_Thermodynamics/lecture28/images/28.3a.gif
- Reheat Cycle: http://www.ent.ohiou.edu/~thermo/Intro/Chapt.1_6/steamplant/supercrit2.gif