![]() Run the initial condition I10 230 MW_oil_auto: In order to calculate the overall plant thermal efficiency, we need to adjust the formulas above to incorporate heat added in the reheater sections of the boiler: A schematic of practical cycle is shown below: Regenerative cycle The Plant Thermal Efficiency As a result, the average temperature at which heat is supplied is increased. ![]() In the regenerative cycle, feedwater enters the boiler at some point between 2 and 2′. Relationship between Carnot cycle and Rankine cycle The relationship between Carnot cycle and Rankine cycle is shown below. In other words, the average temperature at which heat is supplied in the Rankine cycle is lower than in the Carnot cycle 1′-2′-3-4-1′, and consequently the efficiency of the Rankine cycle is less than that of the corresponding Carnot cycle. During the process between states 2 and 2′ the feedwater is heated and the average temperature is much lower during this process than during the vaporization process 2′-3. Rankine Cycle with reheat thermal efficiency can be expressed as:Īnother variation from the Rankine cycle is the regenerative cycle, which involves the use of feedwater heaters. In this cycle, the steam is expanded to some intermediate pressure in the turbine and is then reheated in the boiler, after which it expands in the low-pressure turbine to the exhaust pressure. To improve the efficiency, the reheat cycle has been developed which is shown schematically below: Rankine Cycle with Reheat If metals could be found that would allow us to reach higher temperatures, the Rankine cycle could be more efficient. The Reheat CycleĪbove we noted that the efficiency of the Rankine cycle is increased by superheating the steam. The result of superheating is that for a given power output, the plant using superheated steam will be of smaller size than that using dry saturated steam. Superheating the steam is done by increasing the time the steam is exposed to the flue gases. In a similar way, if the steam is superheated in the boiler, it is evident that the work is increased by area 3-3′-4′-4-3 (see diagram below): Effect of superheating If the exhaust pressure drops from P 4 to P 4‘ with the corresponding decrease in temperature at which heat is rejected in the condenser the net work is increased by area 1-4-4′-1′-2’-2-1 (see diagram below) Effect of exhaust pressure Improvements to the Rankine Cycle Efficiency Effect of Pressure and Temperature on the Rankine Cycle Referring to the diagram above and using the enthalpy values in the Rankine cycle, we can write: ![]() ![]() Rankine Cycle Efficiency then can be expressed as: In addition, feed pumps consume energy thus reducing the net work output. The Rankine Cycle EfficiencyĪs noted above, some heat is always lost from the steam to cooling water. This thermodynamic cycle is known as the Rankine Cycle. The pressure of the condensate leaving the condenser is increased in the pump thereby enabling the condensate to flow into the boiler. The exhaust steam leaves the turbine and enters the condenser where heat is transferred from the steam to cooling water. The steam expands in the turbine and does work which enables the turbine to drive the electric generator. High-pressure steam leaves the boiler and enters the turbine. A schematic diagram of a simple steam power plant is shown below: Schematic diagram of a steam power plant
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