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'!

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Comment by Bob Blanchette on July 21, 2012 at 10:21am

This is something I think gets skimped on in typical installations, especially new construction. Take it down to 500 microns, leave it 15 minutes to check for leaks, then gas it up is how it typically gets done. Sure there is some moisture, but the filter/dryer will typically catch it. Not ideal by any means, but when you get techs that are working 12hr days...

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