The cornerstone of modern steam power plants is the thermodynamic cycle proposed by W. J. M. Rankine, a Scottish engineer. The main components of a Rankine cycle steam plant are:
• A boiler that generates steam, usually at a high pressure and temperature
• A turbine that expands the steam to a low pressure and temperature, thereby producing work
• A generator driven by the steam turbine
• A condenser that cools the steam to a liquid so that it can be pumped back into the boiler
• A feed pump
• Feed heaters, which preheat the water before it enters the boiler
• A reheater, which is part of the boiler, reheats the steam after it has been partially expanded
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Figure 2.8, a temperature-entropy diagram, illustrate the state changes for the Rankine cycle. Such a cycle avoids transporting and compressing two-phase fluid by trying to condense all fluid exiting from the turbine into saturated liquid before it is compressed by a pump. The actual power plant introduces regenerative and reheat modifications in the basic Rankine cycle.
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Figure 2.9 Rankine cycle with superheating. |
It is evident in the T-s diagram of Figure 2.8 that the ideal Rankine cycle is less efficient than a Carnot cycle for the same maximum and minimum temperatures. The Rankine cycle work is represented by the area 2-a-3-4-1-2, which is less than the Carnot cycle work represented by the area 2-b-3-4-1-2. Figure 2.9 illustrates a typical superheated Rankine cycle, the following path describing the cycle:
• 1-2: Pump (q = 0); isentropic compression (pump)
Wpump h2 h1 V ip2 Pi)
• 2-3: Boiler (W = 0); isobaric heat supply (boiler)
Qin h3 h2
• 3-4: Turbine (q = 0); isentropic expansion (steam turbine)
Wout = h3 h4
• 4-1: Condenser (W = 0); isobaric heat rejection (condenser)
Qout = h4 h1
By increasing the steam temperature, point 3 is shifted up and the dry saturated steam from the boiler is passed through a second bank of smaller-bore tubes within the boiler until the steam reaches the required temperature. Increasing the steam temperature increases the cycle efficiency and reduces the moisture content at the turbine exhaust end; sometimes two stages of reheating are connected in tandem. Three ways to improve the thermal efficiency are depicted in Figure 2.10.
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Figure 2.10 Thermal efficiency improvement with modified Rankine cycle: (a) lowering the condensing pressure (lower condensing temperature, lower TL); (b) superheating the steam to higher temperature; (c) increasing the boiler pressure (increase boiler temperature, increase TH). |
The optimal way to increase the boiler pressure but not increase the moisture content in the exiting vapor is to reheat the vapor after it exits from a first-stage turbine and to redirect this reheated vapor into a second turbine. Regeneration helps to improve the Rankine cycle efficiency by preheating the feedwater into the boiler. Regeneration can be achieved by open or closed feedwater heaters. In open feedwater heaters, a fraction of the steam exiting a high-pressure turbine is mixed with the feedwater at the same pressure. In a closed system, the steam bled from the turbine is not mixed directly with the feedwater. Therefore, the two streams can be at different pressures, but such details are outside the scope of this book.
Fluids other than water and steam may be used in the Rankine cycle if they are considered more appropriate for a particular process. For example, ocean thermal conversion plants depend for their operation on the temperature difference between the hotter surface water (say up to 27°C) and the colder deep water (down to 4°C). The working fluid for these plants is ammonia because of its low boiling point, but the cycle used is the basic Rankine cycle.
