In general, the feedwater heating system consists of:
low-pressure feedwater heaters
deaerator
high-pressure feedwater heaters
Low-pressure feedwater heaters
The condensate from condensate pumps then passes through several stages of low-pressure feedwater heaters, in which the temperature of the condensate is increased by heat transfer from steam extracted from the low-pressure turbines. There are usually three or four stages of low-pressure feedwater heaters connected in the cascade. The condensate exits the low-pressure feedwater heaters at approximately p = 1 MPa, t = 150°C, and enters the deaerator. The main condensate system also contains a mechanical condensate purification system for removing impurities. The feedwater heaters are self-regulating. It means that the greater the flow of feedwater, the greater the rate of heat absorption from the steam and the greater the flow of extraction steam.
There are non-return valves in the extraction steam lines between the feedwater heaters and the turbine. These non-return valves prevent the reverse steam or water flow in case of turbine trip, which causes a rapid decrease in the pressure inside the turbine. Any water entering the turbine in this way could cause severe damage to the turbine blading.
Deaerator
A schematic diagram of a typical tray-type deaerator. Source: wikipedia.org License: CC BY-SA 3.0
The condensate is heated to saturated conditions in the deaerator, usually by the steam extracted from the steam turbine. The extraction steam is mixed in the deaerator by a system of spray nozzles and cascading trays between which the steam percolates. Any dissolved gases in the condensate are released in this process and removed from the deaerator by venting to the atmosphere or the main condenser. Directly below the deaerator is the feedwater storage tank, in which a large quantity of feedwater is stored at near saturation conditions. In the turbine trip event, this feedwater can be supplied to steam generators to maintain the required water inventory during a transient. The deaerator and the storage tank are usually located at a high elevation in the turbine hall to ensure an adequate net positive suction head (NPSH) at the inlet to the feedwater pumps. NPSH is used to measure how close a fluid is to saturated conditions. Lowering the pressure at the suction side can induce cavitation. This arrangement minimizes the risk of cavitation in the pump.
High-pressure feedwater heaters
The water discharge from the feedwater pumps flows through the high-pressure feedwater heaters, enters the containment, and then flows into the steam generators.
The high-pressure feedwater heaters are heated by extraction steam from the high-pressure turbine, HP Turbine. Drains from the high-pressure feedwater heaters are usually routed to the deaerator.
The feedwater (water 230°C; 446°F; 6,5MPa) is pumped into the steam generator through the feedwater inlet.[/lgc_column]
Heat Regeneration
The process of heat regeneration significantly increases the thermal efficiency of steam turbine by reducing the amount of fuel that must be added in the boiler. This process is known as heat regeneration, and a variety of heat regenerators can be used for this purpose. Sometimes engineers use the term economizer, which is a heat exchanger intended to reduce energy consumption, especially in preheating a fluid. On the other hand, the process of draining steam from the turbine at a certain point of its expansion and using this steam for heating the feedwater supplied to the boiler is known as bleeding, and it must be noted, a small amount of work, WT, is lost by the turbine.
As can be seen in the article “Steam Generator,” the feedwater (secondary circuit) at the inlet of the steam generator may have about ~230°C (446°F) and then is heated to the boiling point of that fluid (280°C; 536°F; 6,5MPa) and evaporated. But the condensate at the condenser outlet may have about 40°C, so the heat regeneration in typical PWR is significant:
Heat regeneration increases the thermal efficiency since more of the heat flow into the cycle occurs at a higher temperature.
Heat regeneration causes a decrease in the mass flow rate, especially through low-pressure stages of the steam turbine; hence the LP Isentropic Turbine Efficiency increases. Note that at the last stage of expansion, the steam has a very high specific volume, which requires large blades of the last stage.
Heat regeneration causes an increase in working steam quality since the drains are situated at the periphery of the turbine casing, where is a higher concentration of water droplets. Improved turbine drainage implies fewer problems with erosion of blades.
Schema of a steam turbine of a typical 3000MWth PWR.
References:
Nuclear and Reactor Physics:
J. R. Lamarsh, Introduction to Nuclear Reactor Theory, 2nd ed., Addison-Wesley, Reading, MA (1983).
J. R. Lamarsh, A. J. Baratta, Introduction to Nuclear Engineering, 3d ed., Prentice-Hall, 2001, ISBN: 0-201-82498-1.
W. M. Stacey, Nuclear Reactor Physics, John Wiley & Sons, 2001, ISBN: 0- 471-39127-1.
The condensate from condensate pumps then passes through several stages of low-pressure feedwater heaters, in which the temperature of the condensate is increased by heat transfer from steam extracted from the low-pressure turbines. There are usually three or four stages of low-pressure feedwater heaters connected in the cascade. The condensate exits the low-pressure feedwater heaters at approximately p = 1 MPa, t = 150°C, and enters the deaerator. The main condensate system also contains a mechanical condensate purification system for removing impurities. The feedwater heaters are self-regulating. It means that the greater the flow of feedwater, the greater the rate of heat absorption from the steam and the greater the flow of extraction steam.
The water discharge from the feedwater pumps flows through the high-pressure feedwater heaters, enters the containment, and then flows into the steam generators.
