Triple Evacuation vs. Deep Vacuum Method


Deep Vacuum Chart; Carrier Installation Manuals

  Little did I know, call it that HVAC Tech's ignorant moment, there are actually two different types of methods of evacuating a system free from non-condensables.  The Deep Vacuum and the Triple Evacuation methods are not one in the same.  Maybe my confusion started because I kind of used a hybrid to maximize my time on the job site.  Hopefully this could help you!
  The triple evacuation method, developed very early on in our industry, is used with single stage vacuum pumps.  You see, this type of pump can only pull a vacuum to 28" Hg (Mercury).   In order for water to boil off and be removed from the system, the temperature must be above 100F @ 28" Hg.  Since there is no way to make that elusive 500 microns with the older technology, and it isn't always 100F+ when installing A/C's, we used to have this method of removing the moisture with dry nitrogen.  So, you would pull a vacuum to 28" Hg, operate the pump an additional 15 minutes, and then break the vacuum with dry nitrogen until you reached 2 psig (Source: Carrier Installation Manuals).  Since it takes time to absorb the moisture, the nitrogen must remain in the system for an hour.  The technician would then repeat this again, spending close to 3 hours for the effort!  Although very effective at removing moisture, this process is very time consuming and likely not needed with today's two-stage pumps.
  The deep vacuum method requires a two-stage vacuum pump and a quality gauge that can measure microns.  You need to operate the vacuum pump until you reach 500 microns.  Depending on your tools, CFM of the pump, size of the refrigerant lines, and restrictions like Schraeder cores, this can take varying amounts of time.  Recently, I watched a video of a well respected instructor in our industry reach this 500 microns on a small system in only 51 seconds!   Of course, he had the right tools.  Anyhow, once this deep vacuum is achieved, you should valve off the system and verify the system is in fact sealed and free of enough moisture.  No matter what anyone tells you, no vacuum pump, no matter how large, will never remove 100% of the water vapor.  If the system holds for about 7-10 minutes below 1000 microns, than you can charge the system with refrigerant.  If the gauge hits 2000 microns in that time, then there is still too much moisture and you need to operate the pump longer.  Finally, if the microns continue to rise, than you obviously have a leak.
  I personally have unintentionally merged the two methods over the years and it seems to maximize my time spent.  I like to leak check the system first with dry nitrogen.  But, when adding the nitrogen, I like to sweep the system free of air by adding through the liquid line valve and purging on the suction line valve.  For a mini-split, I crack the flare nut on the suction line to achieve this same approach.  I let the system set for at least 15 minutes to assure some moisture was removed.  I'll then purge the nitrogen from the system to less than 10psig and operate the two-stage vacuum with the gas ballast open until I reach 28" Hg.  I then pull a deep vacuum to the required 500 microns and check the system.  Very rarely does the micron gauge rise above 700 or so, and I think it is due to the sweep with the dry nitrogen that I get this done in a quicker manner than most.  If you are concerned about a large number of systems, or completing this during the wet Spring months, you should look into some of the new kits out there that really speed this process up.  Larger hoses, schraeder core removers, and tees make a deep vacuum quick work - and 'time is money'!

2 comments:

  1. If the system indicates moisture, a multiple evacuation with a nitrogen sweep will significantly reduce the amount of moisture in the system. To preform this procedure, reduce the system pressure to between 1000 and 2500 microns. Isolate the vacuum pump with the core tools and disconnect the vacuum hose from the low side of the system. Break the system vacuum with nitrogen introduced at the side port of the core tool. Break the vacuum with nitrogen to that equivalent to atmospheric pressure (760,000 microns) then purge nitrogen through the system at 1-3 psig. from the high to the low side letting it vent out the open port of the core tool. Do not pressurize the system as this will not remove moisture. There is no need to pressurize the system unless you are performing a leak check. Increasing the system pressure will actually cause the water to drop out of the nitrogen similar to that of compressed air in an air compressor. Nitrogen does not absorb water, but entrains it and helps it move along out of the system, allowing the liquid water to warm, evaporate, and increase the water vapor pressure without introducing additional moisture into the system. If the system is drying out you will notice that deeper levels of vacuum are quickly achieved indicating progress in the job of dehydration. If desired or required repeat this process until the moisture is removed. Typically no more than a triple evacuation with sweep are required. If marked progress is not achieved during this process, repeat the nitrogen purge to remove liquid moisture that may exist. If a leak is indicated, it must be repaired before the evacuation can be completed.

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  2. Thanks for this very detailed explanation! It's funny how you can have an understanding how a process can speed up your time on the job, but not completely understand what is happening. Like I had said, I was just a little too ignorant when it can to system evacuation over the years!

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