Sciencemadness Discussion Board

Creating vacuum in sealed vessel via condensation of vapors

RogueRose - 11-12-2016 at 06:36

A while ago there were some threads discussing novel ways to create a vacuum without a mechanical pump and one way was to heat a vessel (like an old propane tank) filled with air so that the hot air can escape - then close the vent on the tank and allow the tank to cool. This should reduce the pressure in the tank to below ambient pressures.

Someone mentioned that a better way to create a vacuum, or at least a significant drop in pressure, was to place some water in the vessel and heat to boiling - close vent - and allow to cool. The vapor -> condensation transition allows for much greater pressure drop than heating just air to the same point (or even much higher temp).

I'm trying to find out how much water or steam is needed per cubic ft and how much of a pressure difference there will be once the water vapor has condensed back to liquid.

Also, would this work with something like an alcohol or other more volitile liquid? I'm thinking that the energy needed to change from liquid to vapor (and back) may work better with a liquid that doesn't require 540cal/g like water - but would an alcohol (or other liquid) create the same pressure difference?

What would be some other liquids which would be good for this type of application? (note, this is theoretical, I'm not planning on heating a vessel for this - simply research on compounds)

Metacelsus - 11-12-2016 at 09:43

If you boil water in a vessel to fill it with steam, and then seal and cool it, the internal pressure will be the vapor pressure of water at the temperature to which it is cooled. For a table, see: https://en.wikipedia.org/wiki/Vapor_pressure_of_water

A more volatile liquid, such as alcohol, will produce a weaker vacuum (and also, alcohol will cause flammability issues).

[Edited on 12-11-2016 by Metacelsus]

XeonTheMGPony - 11-12-2016 at 11:30

We covered this in another thread, lots of risks, poor resualts.

water in its liquid form and steam is 1:1700 so a small bit will do a large volume, but in the rarefied atmosphere you will only get to a vacuum relivent to its vapor pressure at what the ambient temp is. So to get any usable vacuum for most would require chilling to make it deeper

Gravity pump, fill large bucked with water, put a lid on with 2 hoses, longer hose better vacuum hose lets water out another sucks air in. Better sustained vacuum with out the risks of steam explosion!
vacuumPump1_A0E75E4F-FB5A-310B-69FDA452A160ABE4.jpg - 11kB

What are you trying to do/achieve by using this odd ball method?

As for fluids, silicon oil any thing with a high vapor pressure (Tends to need higher temp to boil)

[Edited on 11-12-2016 by XeonTheMGPony]

RogueRose - 11-12-2016 at 18:12

Quote: Originally posted by XeonTheMGPony  
We covered this in another thread, lots of risks, poor resualts.

water in its liquid form and steam is 1:1700 so a small bit will do a large volume, but in the rarefied atmosphere you will only get to a vacuum relivent to its vapor pressure at what the ambient temp is. So to get any usable vacuum for most would require chilling to make it deeper

Gravity pump, fill large bucked with water, put a lid on with 2 hoses, longer hose better vacuum hose lets water out another sucks air in. Better sustained vacuum with out the risks of steam explosion!


What are you trying to do/achieve by using this odd ball method?

As for fluids, silicon oil any thing with a high vapor pressure (Tends to need higher temp to boil)

[Edited on 11-12-2016 by XeonTheMGPony]


Thank you for the reply! What you stated in the first paragraph is exactly what I was looking for. I'm not looking at creating/using vacuum as applied to chemistry but how it may be used in conjunction with sterling engine type application (again, not exactly the standard sterling engine) - where vacuum can assist movement on either a cold or hot end of a "piston".

XeonTheMGPony - 12-12-2016 at 03:48

look up : Organic Rankin cycle engine.