I think i understand the mechanics in a steam locomotive:

  • Fill boiler with water
  • Close off boiler
  • Heat boiler until pressure is sufficient
  • Keep boiler hot and use steam pressure until no more water left
  • repeat

Boiler is heated and energy is given to water in form of heat. Water gets hot and vaporizes. Water vapor tries to use a lot more space, this expansion is used in pistons to create mechanical force. Since the boiler is closed off while in use, the water/steam pressure has nowhere to go but to the pistons. This seems clear.

Now, in power plants (all types that heat water: nuclear, coal, gas and so on) water is circulated. This means that they do not shut down the plant to refill it with water. Here's what i have trouble understanding: if you have to use X amount of energy to pump in the water to a cold boiler to keep the pressure up, you would also have the same amount on energy available to extract on the other end. But if you heat it, you add energy to the water while it is in the boiler, you increase the pressure. Logic would say you have more energy now available on the other end. But wouldn't the pressure try to press out form the pump side also? The pump would have to fight more to keep the water in and waste more energy. So i think the pressure is not increased in the boiler. How is the heat energy then extracted by the turbines? Is the water converted to steam and the speed of the steam is increased using heat while keeping the pressure the same and then the energy is extracted from the very fast speed of the flowing steam(kinetic energy essentially)? Much like a jet engine, the last compressor stage is the highest pressure point and downstream only speed is added while pressure is lowered? If so then it would seem the pump would have to maintain pressure while the steam slowly leaves via turbines. Or is there some other mechanism that eludes me.

If somebody says that turbines work by using the pressure difference then please explain where does the energy come from if the pressure is generated by the high pressure feedwater pump before the boiler. All you could extract would be what the pump puts in and boiler should increase pressure both ways making it harder for the pump to put anything in at all.

Thank you.

  • $\begingroup$ tl;dr: Work is proportional to pressure differential and volume of the medium pumped, not mass. Same kilogram of water evaporated into superheated steam can produce far more energy in the turbine, than the pump propelled by that turbine takes to inject a kilogram of cool liquid water back in, because 1kg of steam is much larger volume, so it spreads the pressure over much larger surface of turbine blades. $\endgroup$ – SF. Jun 13 '19 at 7:40

Google "Rankine cycle". The boiler operates at an elevated pressure (sometimes several atmospheres). But because steam at a given pressure is many times bigger than liquid water at the same pressure, the energy required to pump water into the boiler is considerably less than the energy obtained from using that water (as steam) to power the extracting mechanism (piston engine, turbine, whatever).


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