Anyone that has ever picked up a vacuum pump has asked or been asked this question, and to be truthful it is like asking “How many licks will it take to get to the center of a Tootsie Roll Tootsie Pop?” In the words of the wise old owl, “The world may never know.”
Modern day evacuation techniques are meant to degas and dehydrate a system, cleaning it of contaminants to a level that assures that non-condensibles – and more importantly, moisture – will cause no harm to the refrigerant or the refrigerant oil in the system. Moisture with oil forms sludge, and moisture with refrigerant forms hydrofluoric and hydrochloric acids. All of these can cause permanent damage to the refrigeration system.
How long an evacuation takes depends on many factors in this order, including but not limited to, the size of the system, the level of system contamination, the diameter and length of the vacuum hoses, the presence of the Schrader cores in the service valves, dryness of the vacuum pump oil, and lastly, the size of the vacuum pump.
More important than how long will an evacuation take is understanding when the evacuation is complete. Removal of the air is an easy process, but the removal of moisture is much more difficult and simply takes time. Moisture has strong molecular bonds and does not easily free itself from the surfaces it attaches to. It takes heat energy and time for the bonds to break and a deep vacuum for the pump to ultimately carry that moisture out of the system.
The best advice that can be given, when it comes to evacuation, is to make sure the preparation of the copper tubing is kept the primary priority. Keeping the system clean (contaminate free), dry and leak free during assembly will save far more time on the back end then the uncertainty it will introduce into the time required to clean the system through the evacuation process.
To properly clean (degas and dehydrate) the system, an accurate vacuum gauge is an indispensable component of the evacuation system. The use of an electronic vacuum gauge is the only way to determine when the dehydration process is complete. Using an electronic micron gauge like the BluVac+ Professional and its accompanying application will show you the characteristics of moisture allowing you to easily identify a wet vs a dry system. At 5,000 microns, 99.34% of the degassing has occurred, but the moisture removal is just beginning. If you cannot achieve a vacuum below 5000, it is a good indicator of a system leak, a leak in your vacuum hoses, contaminated vacuum pump oil, etc.
Once you are below 5,000 microns you can be assured that dehydration is occurring and that moisture is being boiled off and removed the through evacuation process. Significant levels of dehydration are not occurring until the vacuum level is below 1,000 microns.
When is comes to the vacuum gauge reading and the actual vacuum level, and an important distinction must be made. Pulling below 500 microns and being below 500 microns are two totally different things. A good vacuum rig coupled to a large pump can overpower the dehydration process, pulling below 500, but not removing the moisture which simply takes time. It is not until the vacuum has been isolated that we can determine the ultimate level of vacuum. Core tools are essential to isolate the vacuum pump and rig from the system when the ultimate vacuum level is being measured. The system needs to hold below the target vacuum to assure that adequate dehydration has occurred.
The following are guidelines for an acceptable standing level of vacuum. For systems containing mineral oil like R22 systems, a finishing vacuum of 500 microns with a decay holding below 1000 microns generally considered acceptable, whether we are talking a new installation or a system opened for service. For the system containing POE oil, like that of a R410a or R404a system, a finishing vacuum of 250 with a decay holding 500 microns or less should be achieved, and never a decay rising over 1,000 microns on an R410a system opened for service. For ultra-low-temperature, refrigeration, a finishing vacuum as low as 20 microns may be required with a decay holding below 200 microns (for these systems, consult the manufacturer if at all possible). Each of these requirements is focused on the acceptable level of moisture remaining in the system, again because at these levels the majority of degassing has already occurred. The time allowed for decay depends upon the size of the system, but generally, 10 minutes minimum with 1 minute added per ton is a good guideline.
The moral of the story is this: A proper evacuation may take 15 minutes, 15 hours, or 15 days, it simply takes what it takes. While removing cores, using large diameter hoses, clean oil, and a properly sized pump will definitely shorten the time required to complete the process, the true time required is a function of the cleanliness and dryness of the system being evacuated.
Evacuation cannot be rushed or shortcut because the consequences are far worse than the lost time in the process. The best and most important thing to remember is cleanliness is next to godliness when it comes to preparation and finally, evacuation. This means keep the system piping clean, your vacuum rig clean, the oil clean, and follow good processes. This is a point that cannot be understated when trying to shorten the time required to complete the process properly.
Amazing article thank you
Hell does sea level have to anything when pulling a vacuum. I live at 1540 meters above sea level and I can’t reach a full vacuum of -30. The vacuum seems to hold so I’m thinking it’s not a leak
You actually are reaching a full vacuum of -30, the problem is that you’re zeroing your gauges anticipating full atmospheric pressure at sea level not the low atmospheric pressure (or partial vacuum) found at 1540 meters.
Atmospheric pressure (or lack thereof) at high elevation does not have anything to do with an Absolute pressure measurement such as microns or psiA, but it has everything to do with how you “zero” your gauges weather they’re analog or digital. A typical mechanical manifold gauge or your favorite digital gauges such as Fieldpiece, Testo, JB, etc. are typically measuring in psiG (psi Gauge) not psiA (psi Absolute) and zeroing them at high altitude creates a false impression of the actual pressure. Where an absolute pressure measurement (like a barometer) would allow you to see changes in atmospheric pressure not only as weather fronts pass, but particularly as your elevation changes.
For example, imagine you were to zero a gauge at sea level where you have full atmospheric pressure. Then you immediately drive up Pikes Peak with a mechanical gauge sitting next to you on the passenger seat NOT CONNECTED TO ANYTHING. You would actually see the gauge needle go into the vacuum range (below 0) further and further as you drive up the mountain. That would be normal AND ACCURATE as the atmospheric pressure is in fact DEcreasing as your elevation INcreases. At the top of Pikes Peak (elevation over 14,000′) the drop in atmospheric pressure would cause your gauge to drop to around -12″ because there is less pressure in the atmosphere around you and your gauge. Compared to atmospheric pressure at sea level, you’re actually already part way into a “vacuum”. Now remember, full vacuum is -29.92″, at that point there is nothing there, zip, zero, zilch, nada, NOTHING. And when there is nothing there, you cannot take anything away from it, because there is nothing there to take. The problem arises if you were to get to the top of the mountain (or your home at 1540 meters) and re-zero the needle. At this point you just moved the needle from that partial vacuum reading (which was correct) up to 0 and in doing so you moved the goal posts for your vacuum to a point that you physically cannot reach. Remember you cannot take something from nothing. It would be like pulling a vacuum to -30″, setting the needle back to 0, then trying to pull a “double vacuum”… There is no such thing, you’d be trying to take something from nothing.
If you’d like to compensate your mechanical low side gauge for high elevation, connect it to a known good vacuum pump and turn it on. It should only take a second for the pump to pull full vacuum on the gauge. Then set the needle to -30”. When you disconnect the hose from the pump, you will see your actual atmospheric pressure at that altitude which will be less than 0 psiG.
I’ve noticed with a filter-drier that the vacuum won’t hold. I’ve checked the connections and if I remove the flared filter with a brass flare coupler everything vacuums down perfectly and holds. Once the filter is introduced it’ll vacuum down below 100 microns but won’t hold and rises slowly but steady above 19000 microns. I perhaps am holding at a level higher than 19000 but I can’t see that with my gauge. I vacuumed for 3 hours and flowed nitrogen to melt ice if present but still the same. I also checked the connections under pressure and it holds. How long does a filter-drier take to vacuum? Is it off-gassing?
Hi Jonathan,
I remember talking to you about this on the phone last year, this comment fell through the cracks. The only variable in the situation you just described is the filter drier. The factory bakes them in an oven to dry the desiccant and either yours was not thoroughly dried or it picked up a lot of moisture between being opened and installed and now it’s outgassing under vacuum. As I recall, you pulled vacuum on it while heating it with a hair drier (a heat gun would have been better) and that dried it out and then it held vacuum.
Bobby, yes I followed everything you guys instructed and after putting some heat to the filter-drier everything vacuumed down beautifully with the hair dryer. Didn’t have a heat gun available but over time the hair dryer worked great. I really appreciate all your help. Thanks for educating everyone. Knowledge is power and more techs could use a primer on vacuum and outlier events.