The four basic components of Rankine cycle are shown in figure 1 each component in the cycle is regarded as control volume, operating at steady state.
Pump: The liquid condensate leaving the condenser at the state 1 is pumped to the operating pressure of the boiler. The pump operation is considered isentropic.
Boiler: The heat is supplied in the working fluid (feed water) in the boiler and thus vapour is generated. The vapour leaving the boiler is wther saturated at the state 3 or superheated at the state 311, depending upon the amount of heat supplied in the boiler.
Steam power plant that operates on the Rankine cycle
Figure1: Simple steam power plant that operates on the Rankine cycle
Turbine: The …show more content…
Figure 2: Rankine cycle
As mentioned earlier, the Rankine cycle also includes the possibility of superheating the vapor, as cycle 1–2–3–4–1.
If changes of kinetic and potential energy are neglected, heat transfer and work may be represented by various areas on the T–s diagram. The heat transferred to the working fluid is represented by area a–2–2–3–b–a and the heat transferred from the working fluid by area a–1–4–b–a. From the first law we conclude that the area representing the work is the difference between these two areas—area 1–2–2–3–4–1. The thermal efficiency is defined by the relation
For analyzing the Rankine cycle, it is helpful to think of efficiency as depending on the average temperature at which heat is supplied and the average temperature at which heat is rejected. Any changes that increase the average temperature at which heat is supplied or decrease the average temperature heat is rejected will increase the Rankine-cycle efficiency.
In analyzing the ideal cycles in this chapter, the changes in kinetic and potential energies from one point in the cycle to another are neglected. In general, this is a reasonable assumption for the actual …show more content…
Why not select the Carnot cycle 1–2–3–4–1? At least two reasons can be given. The first reason concerns the pumping process. State 1 is a mixture of liquid and vapor. Great difficulties are encountered in building a pump that will handle the mixture of liquid and vapor at 1 and deliver saturated liquid at 2. It is much easier to condense the vapor completely and handle only liquid in the pump: The Rankine cycle is based on this fact. The second reason concerns superheating the vapor. In the Rankine cycle the vapor is superheated at constant pressure, process 3–3. In the Carnot cycle all the heat transfer is at constant temperature, and therefore the vapor is superheated in process 3–3. Note, however, that during this process the pressure is dropping, which means that the heat must be transferred to the vapor as it undergoes an expansion process in which work is done. This heat transfer is also very difficult to achieve in practice. Thus, the Rankine cycle is the ideal cycle that can be approximated in practice. In the following sections, we will consider some variations on the Rankine cycle that enable it to approach more closely the efficiency of the Carnot