Feedwater Heating System – Heat Regeneration

In general, the feedwater heating system consists of:

  • low-pressure feedwater heaters
  • deaerator
  • high-pressure feedwater heaters

Low-pressure feedwater heaters

Condenser - LP Heaters - DeaeratorThe 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 turbine. These non-return valves prevent the reverse steam or water flow in case of turbine trip, which causes 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

Deaerator

A schematic diagram of a typical tray-type deaerator. Source: wikipedia.org License: CC BY-SA 3.0

In the deaerator, the condensate is heated to saturated conditions usually by the steam extracted from the steam turbine. The extraction steam are 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 to 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 transient. The deaerator and the storage tank is 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

Feedwater Pumps - HP HeatersThe 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.

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 economiser that are heat exchangers intended to reduce energy consumption, especially in case of preheating of a fluid. On the other hand, the process of draining steam from the turbine at 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 and very important:

  • Heat regeneration increases the thermal efficiency, since more of the heat flow into the cycle occurs at 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 very high specific volume, which requires large blades of the last stage.
  • Heat regeneration causes an increases in working steam quality, since the drains are situated at the periphery of turbine casing, where is higher concentration of water droplets. Improved turbine drainage implies less problems with erosion of blades.
Steam turbine of typical 3000MWth PWR

Schema of a steam turbine of a typical 3000MWth PWR.

Nuclear and Reactor Physics:

  1. J. R. Lamarsh, Introduction to Nuclear Reactor Theory, 2nd ed., Addison-Wesley, Reading, MA (1983).
  2. J. R. Lamarsh, A. J. Baratta, Introduction to Nuclear Engineering, 3d ed., Prentice-Hall, 2001, ISBN: 0-201-82498-1.
  3. W. M. Stacey, Nuclear Reactor Physics, John Wiley & Sons, 2001, ISBN: 0- 471-39127-1.
  4. Glasstone, Sesonske. Nuclear Reactor Engineering: Reactor Systems Engineering, Springer; 4th edition, 1994, ISBN: 978-0412985317
  5. W.S.C. Williams. Nuclear and Particle Physics. Clarendon Press; 1 edition, 1991, ISBN: 978-0198520467
  6. Kenneth S. Krane. Introductory Nuclear Physics, 3rd Edition, Wiley, 1987, ISBN 978-0471805533
  7. G.R.Keepin. Physics of Nuclear Kinetics. Addison-Wesley Pub. Co; 1st edition, 1965
  8. Robert Reed Burn, Introduction to Nuclear Reactor Operation, 1988.
  9. U.S. Department of Energy, Nuclear Physics and Reactor Theory. DOE Fundamentals Handbook, Volume 1 and 2. January 1993.

Advanced Reactor Physics:

  1. K. O. Ott, W. A. Bezella, Introductory Nuclear Reactor Statics, American Nuclear Society, Revised edition (1989), 1989, ISBN: 0-894-48033-2.
  2. K. O. Ott, R. J. Neuhold, Introductory Nuclear Reactor Dynamics, American Nuclear Society, 1985, ISBN: 0-894-48029-4.
  3. D. L. Hetrick, Dynamics of Nuclear Reactors, American Nuclear Society, 1993, ISBN: 0-894-48453-2. 
  4. E. E. Lewis, W. F. Miller, Computational Methods of Neutron Transport, American Nuclear Society, 1993, ISBN: 0-894-48452-4.

Other References:

Diesel Engine – Car Recycling