How Does Recirculation Work in Steam Engines?

[Xyius]

I am very fascinated by steam engines. When I took my thermodynamics course a few years back, I remember learning that the steam is recirculated after it does work. I am confused on how this happens.

After the steam is used to do work, it is expelled at a lower pressure than it started with. If this steam were to be pumped back into the boiler via a one way valve, shouldn't it need to be at a higher pressure to be able to open the valve and move back into the boiler? How can low pressure steam be pumped into a boiler of higher pressure?

Is there some external source that provides the extra energy to pump back into the boiler?

 

[montoyas7940]

Steam doesn't pump very easily.
The steam exhausts to a condenser where it is cooled and collected as a liquid before being pumped back to the boiler. Another advantage to this system is the condenser can be kept close to a vacuum which increases the temperature difference between the steam and condensate. Find a mollier diagram online and look at the difference in enthalpy for different saturation pressures. There are big gains when running the condenser at a vacuum.

 

[Xyius]

Steam doesn't pump very easily.
The steam exhausts to a condenser where it is cooled and collected as a liquid before being pumped back to the boiler. Another advantage to this system is the condenser can be kept close to a vacuum which increases the temperature difference between the steam and condensate. Find a mollier diagram online and look at the difference in enthalpy for different saturation pressures. There are big gains when running the condenser at a vacuum.

Hmmm, I am not sure I quite understand. How can the pressure in the condenser be at vacuum if steam is being pumped into it?

So liquid water is easier to pump back into the boiler? How come?

I apologize for my confusion but I really want to understand this!

 

[montoyas7940]

A condenser is just a heat exchanger. Cooling water is used the cool the shell externally or is run through tubes internally. As the steam flows in and is cooled it changes phase back to a liquid.

Not all condensers operate at a vacuum.
But in a vacuum condenser the vacuum is initially established with a pump before steam is introduced. Then as steam flows in and is cooled and condensed the collapse of volume due to the phase change maintains the vacuum.

Steam is compressable and low mass to volume. After having energy extracted by the engine the steam would also be very wet. So two phases of water would have to be managed. Water is high mass to volume and low compressablity. Only small volumes as compared to steam have to be injected into the boiler. And only one phase of water has to be pumped.

 

[montoyas7940]

Here is a good picture of a tube condenser from wikipedia.

http://en.wikipedia.org/wiki/Surface_condenser#mediaviewer/File:Surface_Condenser.png

 

[Xyius]

I see now! Thanks a lot!

 

[Xyius]

Oh one last question, (and I realize the answer might vary depending on design) would it be possible to run the water pump by the actual engine itself? Or would that violate conversation of energy? Does it take a great amount energy to pump the water back in the boiler?

 

[montoyas7940]

Sure, the engine could also run the pump. The energy comes from the heat input into the boiler. So as long as the engine is capable of driving the load plus a little extra to run the pump the boiler could be sized and fired to provide enough steam.

The water pumped to the boiler is called feedwater. The power required to drive feedwater pumps is a very small fraction of a boilers capacity.