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Why does condensing steam in a fixed volume produce a partial vacum (Early steam engi 
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#1
Jan2513, 08:54 AM

P: 64

Hello everyone,
Could someone help me with the following question: I'm trying to find out the equations that govern this process; why if you condense steam in a cylinder is a partial vacum created (such as in early steam engines)? The pressure difference then causes the atmosphere to push down a cylinder? Any suggestions much appreciated James 


#2
Jan2513, 09:40 AM

P: 905

Google "vapor pressure".
Left alone in sealed box, a cup of water will evaporate until the partial pressure of water vapor in the air is high enough that the rate of evaporation equals the rate of condensation, resulting in an equilibrium. This partial pressure will depend on the temperature of the box and is called the "vapor pressure" of water at that temperature. Boiling occurs when the temperature of the water is high enough so that its vapor pressure is equal to atmospheric pressure. At this point an equilibrium cannot be attained within a leaky container. Water will turn into steam faster than it can recondense and steam will come out through the leaks. Significantly, in a chamber that is almost sealed this eventually drives all of the air out of the chamber leaving nothing but water and water vapor. If you seal the chamber and reduce the temperature then the vapor will condense until the pressure in the chamber is equal to the vapor pressure of water at the reduced temperature. You can google for a table of water vapor pressure as a function of temperature. I believe that there are also formulas that can predict this pressure, but my chemistry education does not reach that far. 


#3
Jan2513, 10:02 AM

PF Gold
P: 330

For Water the eq. is: 


#4
Jan2513, 12:51 PM

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Why does condensing steam in a fixed volume produce a partial vacum (Early steam engi



#5
Jan2513, 02:36 PM

P: 64

Thanks for your replies, you give me a good start at reading further on this. Thanks



#6
Jan2513, 03:00 PM

P: 64

I was trying to apply the gas laws, but vapour pressure is something I'm not familiar with.



#7
Jan2513, 03:03 PM

P: 64

Also what is the situation if the cylinder is completely sealed?



#8
Jan2513, 03:14 PM

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#9
Jan2513, 09:52 PM

PF Gold
P: 330




#10
Jan2513, 11:34 PM

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#11
Feb2213, 05:47 AM

P: 48




#12
Feb2213, 06:32 AM

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#13
Feb2213, 06:35 AM

P: 48

My bad, misinterpreted the question



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