HELP! Condensation and pressure difference

[pm1]

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In order for a gas to be condensed in to a liquid pressure needs to increase, right?

But I am a little thrown off by a TBR passage that describes an engine with a condenser saying that the gas expands and cool, and it goes from vapor --> liquid.

I get the part where air cools while it expands, but it doesn't make sense to me that a vapor would be condensed into liquid under a lower pressure.

According to TBR's explanation:

This results in the pressure in the condenser being lower than the pressure in the piston. Because of the pressure differential between the piston chamber and the condenser, steam flows from the piston through the exhaust port and into the condenser (from a region of higher pressure to a region of lower pressure).

Can someone please explain how does this work? 🙂
(I can add more details of the passage if needed. Thanks)

 

[circulus vitios]

The condenser removes heat from the gaseous vapor. The phase progression from lowest energy to highest energy is: solid, liquid, gas. If enough heat is removed from a gaseous vapor, it's going to change to a liquid.

 

[pm1]

The condenser removes heat from the gaseous vapor. The phase progression from lowest energy to highest energy is: solid, liquid, gas. If enough heat is removed from a gaseous vapor, it's going to change to a liquid.

thank you!

hmm okay. I think I was getting confused.
Is heat lost because there is gas expansion, and by expanding gas is doing work hence loosing heat?

(sorry, thermo is not my forte)

 

[circulus vitios]

thank you!

hmm okay. I think I was getting confused.
Is heat lost because there is gas expansion, and by expanding gas is doing work hence loosing heat?

(sorry, thermo is not my forte)

Sorry, I'm not sure.

 

[Wolverines83]

thank you!

hmm okay. I think I was getting confused.
Is heat lost because there is gas expansion, and by expanding gas is doing work hence loosing heat?

(sorry, thermo is not my forte)

Hey think of it like this.

Remember "heat" is really kinetic energy (vibrating molecules). The gas doesn't have to do work to expand...you can simply expand the container (meaning you're doing the work) which would cause the gas to fill that space.

When gas expands (ie, is "less compressed") the molecules are farther apart. This results in less collisions with other molecules, resulting in less energy transfer between molecules which results in a net loss of energy as their "vibrations" get smaller and smaller in amplitude/frequency (like a pendulum finally stopping) until in theory they're no longer vibrating.

Compressing them would do the opposite. It would "smash" the molecules together, creating kinetic energy (heat).

If a physicist looked at this, they'd find problems with it but it's a conceptual aid that works for me.

 

[pm1]

hey,
thanks guys! that helps!

I just get confused when I think about a phase change diagram. If we decrease pressure (keeping same temp) liquid --> gas.

But what you said makes sense, if the pressure decreases, there will be less heat and particles will be further apart, but how come that is a liquid? (since the purpose of the condenser is to make gas --> liquid).

Sorry for keeping coming back to this, I just want to make sure I get it down!

Thank you for all the replies.

 

[circulus vitios]

hey,
thanks guys! that helps!

I just get confused when I think about a phase change diagram. If we decrease pressure (keeping same temp) liquid --> gas.

But what you said makes sense, if the pressure decreases, there will be less heat and particles will be further apart, but how come that is a liquid? (since the purpose of the condenser is to make gas --> liquid).

Sorry for keeping coming back to this, I just want to make sure I get it down!

Thank you for all the replies.

I'm probably wrong, but I don't think that expansion alone lowers the temperature. Heat needs to be exchanged with the surroundings. Condensers typically act as a heat exchanger and so they are "very" efficient at radiating heat (from the working fluid) to the surroundings. This lowers the energy of the working fluid (the gaseous vapor) and allows it to condense into a lower-energy phase, which is a liquid.

 

[pm1]

I'm probably wrong, but I don't think that expansion alone lowers the temperature. Heat needs to be exchanged with the surroundings. Condensers typically act as a heat exchanger and so they are "very" efficient at radiating heat (from the working fluid) to the surroundings. This lowers the energy of the working fluid (the gaseous vapor) and allows it to condense into a lower-energy phase, which is a liquid.

that makes perfect sense!!
thank you SO MUCH!! 🙂

 

[circulus vitios]

that makes perfect sense!!
thank you SO MUCH!! 🙂

http://www.physicsforums.com/showthread.php?t=470311

It's a surprisingly complicated question. I guess my posts don't explain why an expanding gas cools, but they explain why a condenser turns the gaseous vapor into a liquid: because a condenser acts as a heat exchanger, which exchanges heat (energy) with the surroundings.