jiggerjaw - 29-1-2015 at 09:58
How do you calculate the strength of an aspirator vacuum?
ie.
I have X amount of water at Y degrees Celsius
I'm dropping it with gravity through a pipe with diameter Z and I'm creating a vacuum in a container with volume V
I've read that an aspirator vacuum is limited to the vapor pressure of the liquid being dropped but how I'm sure there's a way to calculate this?
Zombie - 29-1-2015 at 10:18
I found this on a thread from this forum...
" posted on 16-8-2011 at 03:38 PM
The theoretical maximum vacuum an aspirator can pull is the vapour pressure of the water when it boils at that temperature. If you imagine the
aspirator is a rearranged kettle, and it's boiling away, the water in it is not going to pull any vacuum because it's happily boiling away at
atmospheric pressure. Practical aspirators lag slightly behind the theoretical maximum.
Another way to imagine it is to think about it practically. The water is exposed to the vacuum it's creating. As the pressure drops around it, it will
try to boil, spraying gas back into the vacuum it's trying to create. The two balance out, so this is the limit of the difference it can keep up
between the two.
At 25C, that maximum will be.... 32 mbar (atmospheric is a bit over a 1000).
That is 24 torr, so 24 mmHg (1 atm = 760), 0.46psi (counting down from atmospheric, 14.5) and finally, 3200 pascals, with 1 atm being 101, 325
pascals."
Here is the complete thread:
http://www.sciencemadness.org/talk/viewthread.php?tid=17250
neptunium - 29-1-2015 at 10:33
every liquid has a different vapor preassure . mercury was used to create a high vacuum in the Sprengel pump.
Water has a higher vapor preassure than mercury therefore would generate a lesser vacuum than mercury in the same system.
the math is a bit complex but the constant are usually found in science book .
at 0c the vapor pressure is 4.5 Torr
meaning that water about to freeze under a pressure lower than 4.5 torr will boil away.
P=exp(20.34-(5132/T))
P is the vapor pressure at
T the temperatur in Kelvin
there is all kind of table with various temperature and pressure for water and other liquids availlable
on the net and in every books of physics .
Chemosynthesis - 29-1-2015 at 13:04
As stated, you generally just use a nomograph or estimate between 10-20 torr above the vapor pressure of the liquid passing through the aspirator, at
its current liquid temperature. It's worth noting that, as said, the vapor pressure of your liquid at a temperature is the theoretical maximum
efficiency of vacuum pulled. No need for any more complicated fluid dynamics or Venturi effect calculations under the vast majority of situations, and
I am assuming single-state, liquid aspirators such as in most hobby chemistry uses.
Praxichys - 29-1-2015 at 13:32
It would be neat to make a vacuum pump that used ionic liquids or deep eutectic solvent systems as the working fluid for an aspirator vacuum pump.