Monday, 26 September 2016

WHAT IS RANKINE CYCLE IN A THERMAL POWER PLANT

WHAT IS RANKINE CYCLE IN A THERMAL POWER PLANT

We were discussing “Thermodynamic cycle” as well as we have also classified the thermodynamic cycles in our recent post Classification of thermodynamic cycles”. We have also seen the importance of a thermodynamic cycle there, now it’s time to go ahead to discuss another topic in the category of thermal engineering.

Today we will see here the basic concept of “Power plant cycle or rankine cycle” with the help of this post. We will see here the various components of a thermal power plant and also we will come to know the calculation of thermal efficiency of a thermal power plant after studying this post.

So let us see here the brief introduction to thermal power plant

 
Before studying the thermal power plant cycle, we must have idea that why we need to understand the thermal power plant cycle. When we think to design a thermal power unit, first important thing is to consider the design of power plant by thermodynamic point of view and such a design of a thermal power plant will be termed as thermodynamic design of the thermal power plant.

Thermodynamic design of a thermal power plant will tell us that which thermodynamic cycle will be the best cycle for a particular power generation unit. We will have idea about the effect on performance of a thermal power unit with respect to changes in operating parameters of that particular power plant. A thermal power plant will be operated on Rankine cycle.

Let us see the arrangement of a thermal power plant cycle in following figure, we will have an idea that there will be four components in a thermal power plant i.e. boiler, turbine, condenser and feed pump. T-S diagram is also displayed here for the thermal power plant cycle and this cycle is also termed as Rankine cycle.
Thermal power plant cycle and T-S diagram

Let us start to define the various processes here in order to understand the function of each component of a thermal power plant cycle.

Process: 1-2

Steam will enter in to the turbine at high pressure and temperature at point 1 as shown in figure and steam will be expended here and due to expansion of steam from high pressure and temperature to low pressure and temperature, we will have work output from turbine WT. Therefore, turbine receives steam at high pressure and temperature at point 1 and rejects steam at low pressure and temperature at point 2.
Work energy output from cycle = WT

This is a very important component of a thermal power plant cycle as work will be produced by this component i.e. turbine only and that is our main objective to secure work energy from thermal power cycle.

Hence, during process 1-2, steam will be expanded from high pressure and temperature to low pressure and temperature. Pressure falls from p1 to p2 as shown in T-S diagram of thermal power cycle or Rankine cycle. 

We can also see from T-S diagram of thermal power cycle that steam is dry saturated steam at point 1 at high pressure and temperature and at point 2 it will be wet steam at relatively low pressure and temperature.

Process: 2-3

As we have seen above that steam will be in the state of wet steam at point 2 and now this wet steam will enter in to the condenser and will be condensed here. Condenser is basically one type of heat exchanger where cooling water will flow continuously and hence latent heat of wet steam will be taken away from wet steam. 

Hence, wet steam enters to the condenser at point 2 and will leave condenser at point 3 and will be in the state of saturated liquid state. Process 2-3 will be steady flow process and hence pressure will be constant throughout this process. Temperature will also be constant throughout the process 2-3.

Now so what’s going on here during the process 2-3, there will be only change in phase of the working fluid i.e. wet steam will be converted in to saturated liquid state. Working fluid is rejecting heat energy Q2 to the condenser during this process. Pressure and temperature will be constant throughout the process 2-3 and it is displayed in T-S diagram of thermal power cycle or Rankine cycle.
Heat rejected by the system to surrounding = Q2

Process: 3-4

Saturated liquid leaving the condenser will enter to the feed pump at lower pressure and temperature at point 3. Feed pump is basically a type of centrifugal pump. This centrifugal pump will be used here for feeding the working fluid to the boiler and hence this centrifugal pump will be termed as feed pump.

 
Feed pump will receive saturated liquid water at lower pressure and temperature at point 3 and will deliver the liquid water to the boiler at higher pressure. As we know that liquid water is one type of incompressible liquid and hence during pumping of liquid water by feed pump, temperature of liquid water will also increased but rise of temperature will be less.

So what happened during the process 3-4? 

Pressure of working fluid i.e. liquid water will be increased and it will be same pressure as the pressure of working fluid at the inlet of turbine. Temperature will also be increased during this process but rise in temperature will be small.

Work will be done here on the system in order to pump the liquid water at higher pressure. WP is the amount of work, as displayed in above figure, done over the system.

Work energy input to the system = WP

Now this high pressure liquid water will enter to the boiler at point 4 and it is shown in the arrangement of components of power plant cycle in above figure. Pressure at point 4 and at point 1 will be same and it is displayed in T-S diagram of thermal power cycle or Rankine cycle.

Process: 4-1

High pressure liquid water will enter to the boiler at point 4 at high pressure and heat energy will be supplied here to the working fluid i.e. water and therefore water will be heated and will be converted in to steam.

Addition of heat energy to the working fluid i.e. water will be done at constant pressure because boiler is also one steady flow device and hence pressure will be constant throughout the process 4-1.

Heat addition to the system from surrounding = Q1

 
So, working fluid will change its phase from liquid to steam during this process and steam will be available at high pressure and temperature to enter to the turbine at point 1 as displayed in above figure.

Thermal efficiency of the power plant

Let us determine the thermal efficiency of the power plant

Thermal efficiency of the power plant = Net work / Heat energy supplied
η= WN /Q1
Net work, WN = WT-WP
η= (WT-WP)/Q1
For unit mass of working fluid, we will have 

Do you have any suggestions? Please write in comment box.
We will see another topic i.e. "Entropy as the property of the system" in our next post in the category of thermal engineering.

Reference:

Engineering thermodynamics by P. K. Nag
Engineering thermodynamics by Prof. S. K. Som
Image Courtesy: Google
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