I am having trouble understanding a steam turbine

[curiouschris]

I know the basic theory to how a steam turbine works. but I have a gap in my understanding of how the thermal dynamics work.

I'll outline a scenario and detail the bit I don't understand.

simple steam turbine

1/ heat source
2/ boiler
3/ steam jet
4/ impeller

The heat source heats the water in the boiler.
Pressure builds in the boiler
The heated water exits the jet, as the pressure drops the exiting heated water turns to steam.
The steam strikes the impeller which spins.
the spinning impeller is used to do work.

My problem is where does the energy to do the work come from?

The steam strikes the impeller at speed thus as the steam strikes the impeller some of the velocity is transferred to the impeller.

My question is what happens to the heat energy?

When the water exits the jet and changes to steam does the steam lose thermal energy which is converted to kinetic energy?

When the steam strikes the impeller does it lose further heat energy or is only the kinetic energy transferred?

How does one calculate the different thermal energies at different points of the process?

That last one I know would be rather complex, I would just like a general idea it doesn't have to be accurate.

Please ignore thermal losses from thermal conductivity and radiation assume everything is made from a super insulator of some sort.

CC

 

[schip666!]

Adding heat to water (or pretty much anything) makes it's molecules move around faster -- heat increases their kinetic energy (in fact heat _is_ kinetic energy in a very basic sense) . If they go fast enough they "boil" off of the liquid and become a gas -- this is called a phase-change. The gas takes up a lot more space than the same amount of liquid, I think I remember about 1700x as much space, and so the pressure above the boiling liquid increases dramatically. If you allow this pressure to vent, in your jet for instance, it can push something -- the active bouncing molecules of water hit your impeller blade and transfer their kinetic energy to it. In doing so they lose their new-found energy and condense back to good old liquid water. And the cycle can repeat.

So the work comes from the energy of the burning fuel you put under the boiler. I just answered (or made an attempt thereof) a question about fire, so scroll around this list to get an idea of what I think explains where the energy actually comes from...

For calculating the energies, I'd first try the wiki article on Steam Engines and then jump over to the Thermal Efficiency link. Hopefully that will lead you to more specific information.

 

[curiouschris]

Sorry for the delayed response.

Thanks for that.

I was under the misunderstanding that heat was a vectorless vibration, rather than a vector force. And thus the force derived from steam was from the expansion from the fluid state into the gas state (phase change). I had assumed the water molecules that made up steam still contained the same energy level and therefore vibrated at the same level as when heated and in the fluid state.

From what you say the molecules actually have a vector, thus the vibration described in other texts is actually the apparently random movement of molecules as they interact with other molecules, in the gas state this would be them literally bouncing off each other.

CC

 

[schip666!]

I think heat, as a macroscopic average, is vectorless. But the kinetic energy of each molecule (that is averaged) is a vector quantity. They bounce off each other, the walls of the container, the plunger or whatever of the pressure sensor, and the blades of your impeller.

You can go with the macroscopic description, "expansion from the fluid state into the gas state", or poke deeper to expand on what "expansion" means. The molecules in steam have to have a certain minimum kinetic energy to escape the liquid state, but there is a large range of actual energies. For a fun thought experiment read up on "Maxwell's Demon"...

 

[pmacias]

Thank you for reaching out with your question about steam turbines. I can provide some insight into the thermal dynamics at play in this process.

First, it is important to understand that a steam turbine works on the principle of converting heat energy into mechanical energy. The heat source, such as a coal-fired power plant or a nuclear reactor, provides the initial energy to heat the water in the boiler. This hot water is then converted into steam, which is directed through a nozzle or jet, creating a high-speed jet of steam.

As the steam exits the jet and enters the impeller, it loses some of its velocity. This decrease in velocity is due to the conversion of kinetic energy (energy of motion) into mechanical energy (energy of movement). The impeller, which is connected to a shaft, begins to rotate due to the force of the steam pushing against its blades. This rotation of the impeller is what ultimately drives the generator, producing electricity.

Now, to answer your question about the transfer of thermal energy. When the steam exits the jet and enters the impeller, it does lose some thermal energy due to the decrease in temperature and pressure. This is because some of the heat energy is converted into kinetic energy, as described above. However, this does not mean that the steam loses all of its heat energy. Some of the heat energy is still present in the steam, but it is now in the form of kinetic energy, which is used to spin the impeller.

To calculate the different thermal energies at different points of the process, one would need to consider factors such as the initial temperature and pressure of the steam, the rate of heat transfer, and the efficiency of the turbine. This can be a complex calculation and would require knowledge of thermodynamics and fluid mechanics.

In summary, a steam turbine works by converting heat energy into mechanical energy through the expansion of steam. The steam loses some of its thermal energy as it is converted into kinetic energy, but this does not mean that all of the heat energy is lost. I hope this helps to clarify your understanding of steam turbines. Let me know if you have any further questions.