How Long Should You Evacuate a System?

Jim Bergmann · April 19, 2017

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."

Copper plating on a bearing journal due to acids formed by moisture left in the system. A victim of improper evacuation.

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.