Vapor Power Cycles Used in Steam Power Plants

This article discusses the different vapor power cycles used in steam power plants worldwide, such as the Carnot cycle, the Rankine cycle, the Regenerative cycle, and the Reheat cycle. How does each compare?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.



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.



Power Plant

Popular Cycles

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:



1.Steam is not fully condensed to water in the condenser, but to a water and steam mixture.

2.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.

3.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.





Rankine Cycle and Carnot Cycle



Continue on to the next page for a discussion of other vapor power cycles used in steam power plants.