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System Evacuation Question

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    System Evacuation Question

    Theoretical Question:

    When evacuating a system, any water turns to vapor. Where does it go? I don't see how the vacuum pump "sucks" it out as there is no flow going on; it's a static condition.

    Given that, once a pump gets the system to its highest vacuum wouldn't turning off the pump have no affect on the evac procedure? If there is no leak in the system the vacuum stays the same whether or not the pump is running.

    Moisture in the system changes to vapor just like liquid refrigerant boils to gas, when that happen the volume is much greater than the volume of water.
    You should be able to detect exhaust from the outlet of the vacuum pump until it pulls to the maximum. It is not a "static condition" until it can pull no more.
    Once max vacuum is reached, the reason you let it sit is two-fold. First is to allow any moisture to boil, which will show itself in a raising pressure. 2nd is to confirm there isn't a leak, which will also show a rising pressure. Experience will tell if it is a leak or moisture. Leak will continue to rise all the way to 0 if left long enough. Moisture will cause a rise that levels off. drawing down again, will start the boiling again and more moisture will boil. It takes a very low vacuum to boil at room temp
    see chart here:
    http://farm3.static.flickr.com/2526/3850655888_b365f16cb9.jpg This is in archives of once just called ACSource.com forums, useful here I hope,

    A complete vacuum means nothing, no gas is in the system. At 29.3" of vacuum most all moisture will turn to gas, and if enough time is allowed for it all to boil (remember the phase change absorbs heat, so you need to allow time for more heat to soak into the system) what little may remain can be absorbed by the new drier bed.
    Unless you are leaving the hand wheels closed on your manifold, gas passes from the system thru the gauges to the vacuum pump.


      Thanks! Yeah, that chart has always made me think about if I am ever getting all the moisture out. I'm at 4,000 ft and can only pull 25". But, on this particular project the vacuum is holding steady. So maybe I do have it completely evaced.


        at only 25" you'd have to get the system to 130 degs to START boiling off the moisture. Either you need a new vacuum pump or better gauges, either way you should get lower than 25"


          Not at 4000 ft elevation. 25.8" is the max theoretically possible. I ran the engine to warm up everything to help but can't say for sure if all the AC components were up to that temp.


            Yeah, sorry forgot about your altitude. The boil chart is for sea level. Somewhere there should be a chart in absolute pressure, which would be accurate at altitude. (PSIa) instead of PSIg
            Last edited by Cornbinder89; 06-30-2019, 07:17 PM.


              Good stuff CB89. 4,000 elevation - OK. Admitted I've been but never worked at that or more. Basics would say at SEA LEVEL 29.92Hg is max. Reducing that by ONE for each 1,000 ft. of elevation seems true untested I'd have to be in that?

              Zero is zero by common sense are not getting more out of ZERO of anything?

              Easy to forget some trivial stats: 90+ % of people live below 1,000 ft. elevation. Sea Level PRESSURE is what we live in is 14.7 PSI seems like a zero # to us? In theory a flat tire but has air is zero. Put it in a vacuum should have 14.7 PSI? How would we test that? It has to be true we just forget we live and work in pressure. Just do keep it in mind it does change boiling points.

              IMO and untested live on video to show it is water or H2O for the example is able and does change from solid to vapor without passing thru being a liquid! Takes much longer but would raise reading of held vacuum - would have to.

              To speed things up vacuum A/C when warm either make it warm or IMO more practical also do it with a warm engine. IT TOTALLY MATTERS, moisture and refrigerants are havoc to A/C,
              MetroWest, Boston