Vapor Power Cycles

 

VAPOR POWER CYCLES

INTRODUCTION

            Vapor power cycles are external combustion systems in which the working fluid is alternatively vaporized and condensed. Hence it is the most employed working fluid. Due to its use as working substance in vapor power cycle, this cycle is often referred as steam power cycle. 

            The vapor is generated in a steam boiler which then enters the steam turbine, a condenser and a feed pump. The fuel is burnt, hot flue gases are used to produce steam in the steam generator. This steam so produced is expanded in a steam turbine to do work.

OBJECTIVE

              In a vapor power cycle, the main objective is to convert the energy present in the fuels into mechanical energy and then to electrical energy. A power cycle continuously converts heat energy into work, in which a working fluid performs a succession of processes.

The Carnot Vapor Cycle

A Carnot cycle with two isothermal and two isentropic processes can be thought of as a vapor power cycle.

However, in practice, it is almost impossible to design a vapor power plant, based on Carnot cycle.

Heat is transferred to water in the boiler from an external source to raise steam.

The high pressure, high temperature steam leaving the boiler expands in the turbine to produce shaft work.

The steam leaving the turbine condenses into water in the condenser, rejecting heat and then water is pumped back to the boiler.

The four Carnot Cycle Processes are:

•       Isothermal Heat addition from process 1-2

•       Isentropic expansion of steam in an expander from process 2-3

•       Isothermal heat rejection in the condenser from process 3-4

•       Isentropic compression of a mixture of vapor and liquid from process 4-1

Limitations of Carnot Vapor Cycle

          Theoretically the Carnot vapor cycle is most efficient; the following difficulties are associated with it during its operation.

1.    It s difficult to compress a wet vapor isentropically to the saturated state in the process 4-1

2.    It is difficult to control the quality of the condensate coming out of the condenser so that the state ‘4’ is exactly obtained

3.    Isentropic compression of a vapor requires more work due to its high specific volume thereby reducing the work ratio

4.    Isothermal heat addition after the saturated vapor line is very difficult to achieve as it involves heat addition at the same time expansion of steam.

 

Rankine Vapor Cycle

Many of the impracticalities associated with the Carnot cycle can be eliminated by superheating the steam in the boiler and condensing it completely in the condenser.

Rankine cycle is the ideal cycle for vapor power plants.

The ideal Rankine cycle does not involve any internal irreversibilities and consists of the following four processes:

    Process 1-2:

           Isentropic compression in a pump: Water enters the pump at state 1 as saturated liquid and is compressed isentropically to the operating pressure of the boiler. The water temperature increases somewhat during this isentropic compression process due to a slight decrease in the specific volume of water.

 

     Process 2-3: 

           Constant temperature heat addition in a boiler: Water enters the boiler as a compressed liquid at state 2 and leaves as a superheated vapor at state 3. The boiler is basically a large heat exchanger where the heat originating from the combustion gases, nuclear reactors or other sources is transferred to the water essentially at constant pressure.  The boiler, together with the section where the steam is superheated, is often called as the steam generator.

     Process 3-4:

    The superheated vapor at state 3 enters the turbine, where it expands isentropically and produces work by rotating the shaft connected to the electric-generator.                                                         The pressure and temperature of the steam drop during this process to the values at state 4, where steam enters the condenser.

     Process 4-1:

    Constant pressure heat rejection in a condenser:

    At the state 4 the steam is usually a saturated liquid-vapor mixture with a high quality.  Steam is condensed at constant pressure in the condenser, which is basically a large heat exchanger, by rejecting heat to a cooling medium such as lake, a river, or the atmosphere. Steam leaves the condenser as saturated liquid and enters the pump, completing the cycle.

The Ideal Reheat Rankine

           The ideal reheat Rankine cycle differs from the simple ideal Rankine cycle in that the expansion process takes place in two stages.

   In the first stage (HP turbine), steam is expanded isentropically to an intermediate pressure and sent back to the boiler where it is reheated at constant pressure usually to the inlet temperature of the first turbine stage.

Steam then expands isentropically in the second stage (LP turbine) to the condenser pressure.

The Ideal Regenerative Rankine cycle

            Reheating has the limited ability to improve the thermodynamic efficiency of Rankine cycle, but is quite useful in the reduction of moisture in the turbine. However, it is observed that the largest single loss of energy in a power plant occurs at the condenser in which the heat is rejected to the coolant.

All these methods involve large temperature differences and are inherently irreversible. If the amount of heat required for this purpose is supplied internally, the cycle thermal efficiency would approach to that of Carnot cycle. 

The device where the feed water is heated by regeneration is called regenerator or a feed water heater (FWH).

A feed water heater is basically a heat exchanger where heat is transferred from steam to the feed water either by mixing the two fluid streams (open feed water heaters) or without mixing them (closed feed water heaters).

Open feed water heaters

 

Closed feed water heaters

Conclusion

Among the various types of vapor power cycles is the Carnot cycle, which is theoretically the most efficient cycle.

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.