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 microns with a decay holding below microns is generally considered acceptable, whether we are talking about a new installation or a system opened for service.
For the system containing POE oil, like an Ra or Ra system, a finishing vacuum of with a decay holding microns or less should be achieved—never a decay rising over microns on an R10a 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 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. Although 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 that 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.
Notify me of follow-up comments by email. Notify me of new posts by email. This condition is likely to occur only briefly during the initial stage of evacuation of most residential or light commercial systems. In most cases, techs are already using pumps that are much higher capacity than their rig will support. This is why using the largest possible evacuation hoses of the shortest practical length is one of the best things you can do to reduce evacuation times. Using dedicated vacuum hoses like TruBlu from Accutools will help consistently achieve deep vacuum quickly.
Vacuum hoses are designed specifically to hold under deep vacuum and to keep moisture and contaminants from bonding to the inside. A typical refrigerant hose used for recovery and charging has been exposed to refrigerant, oil, moisture, and other contaminants. The micron gauge should be located as close to the system as possible for an accurate reading.
When you place a micron gauge at the pump, it is measuring the pressure of the pump, which can be drastically different than the vacuum level at the system itself — especially at the furthest point in the system from where the pump is connected. We find that the best place to locate the vacuum micron gauge is either on the side port of the suction CRT when vacuum hoses are connected to both service ports, or directly on the liquid line port when using a single hose on the suction port.
After degassing and dehydrating the system as deeply and as quickly as possible, isolate the system and micron gauge from the pump and hoses and confirm that it will hold the vacuum. This is easily done with CRTs by closing the valves after achieving the target vacuum level. Sometimes, air can get trapped in the ball valves of the CRTs and cause a significant rise when closing them. Slowly close and open the CRTs a couple of times before starting the decay test.
In general, on a newly installed residential split system, you should be able to pull to a target vacuum of microns or less. Then, close the CRT valves. When servicing an existing system, especially when the entire system, including the compressor, is being evacuated, a target vacuum level of microns and decay to under microns after a minimum of 10 minutes of isolation is more realistic.
If there are any leaks, trapped refrigerant, or moisture, then these targets will be impossible to achieve with typical residential or light commercial equipment up to 5 tons in capacity.
This allows nitrogen to carry unwanted vapors out of the system by displacement. It does help to displace the air and the moisture in the air, which is worthwhile in some instances. You may hear that pulling a vacuum too quickly results in water freezing in the system.
While it is true that water can freeze under vacuum, this only happens in real life when significant liquid water is in the system and when ambient temperatures are already near or below freezing.
In low ambient conditions, it is advised to use a heat gun to warm the accumulator and evaporator and turn on the crankcase heater to help drive out the moisture.
Pulling a deep and fast vacuum is always a good idea, and in a very wet system, using a heat gun and periodically sweeping with nitrogen will also help. Some techs say that pulling a vacuum below microns can damage the compressor oil.
Using this method, you will find that newly installed residential equipment can be pulled down to under microns in under five minutes with another 10 minutes for the decay test. This is a combination of time savings and best practices for a win-win result for you and the customer. Want more HVAC industry news and information? A good vacuum rig coupled to a large pump can overpower the dehydration process, pulling below , 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 microns with a decay holding below 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 Ra or Ra system, a finishing vacuum of with a decay holding microns or less should be achieved, and never a decay rising over 1, microns on an Ra 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 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.
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