HVAC Refrigeration and Triple Evacuation
There is definitely some confusion among HVAC/R technicians about the proper methods of evacuation before introducing the refrigerant to the refrigeration circuit. Let me be clear from the start that my methods of evacuating a refrigeration circuit are based on experience, education and an official guide produced by the US Military and their specifications and proper procedures for evacuating a refrigeration system. It is the proper procedure on any US Navy ship or support ship that a refrigeration system that has been open to the environment be triple evacuated before the refrigerant is reintroduced to the refrigeration circuit. The purpose is to prevent future problems because of contaminants including moisture in the system.
Why Triple Evacuation is Always the Best Method for Refrigeration Evacuation
Before I get into the proper procedure for triple evacuation let us explore why a triple evacuation is always the best way to evacuate a refrigeration system over other methods. First let us find a common point of agreeing simply because I know for a fact there are some who will argue a deep vacuum will suffice when evacuating a refrigeration system. We all agree that moisture in a refrigeration system is bad for the refrigeration system and will eventually lead to premature failure of the system. So we all agree that removing the moisture from the system is an absolute necessity and extremely important simply because moisture will harm the system and cause premature failure. I can hear someone now say that a deep vacuum will remove the moisture especially if you leave the vacuum pump on for several hours and reach a 500 micron vacuum or less. I contend that a person who says that does understand the temperature pressure relationship but only to a limited degree. Here is a conversation I had with someone who thought the vacuum pump would be the best way for a proper refrigeration evacuation:
Pro Vacuum Pump, “What happens to the moisture when you pull a vacuum on a refrigeration system”?
Me, “Some of the moisture evaporates”. (note I said some)
Pro Vacuum Pump, “But according to my pressure temperature chart all the moisture is going to evaporate when I pull that vacuum”.
Me, “You are correct but only partially. Please see the video below and then tell me about your method again”.
So as you can see the vacuum pump did evaporate some of the moisture in the jar but it left residual ice and ice crystals inside the jar. So Mr. Pro Vacuum Pump, do you want to leave ice crystals or ice inside your refrigeration system because you only want to use one method of evacuation? Your deep vacuum method was just blown out of the water and sunk because you failed to follow the pressure temperature relationship water or moisture in a vacuum to the next level. Sure you knew the moisture would boil and some of it would be pumped out of the system in a vapor form through the vacuum pump but did you take it to the next level and think about the freezing effect of the water once you drop the pressure to a certain point? I am so glad I could clear that up for you because there seems to be a lot of misconceptions and uneducated technicians out there claiming to know the vacuum pump method is the only method necessary to properly evacuate a refrigeration system. Sorry to be so snarky about this but there seems to be a lot wrong information available especially in the top search results of most search engines.
Triple Evacuation Procedure
Some very old manufacturer guides for proper refrigeration evacuation will tell you to use refrigerants for a triple evacuation however these manuals predate the Montreal Protocol and the Clean Air Act Section 608 rules and regulations. It is illegal to use refrigerants for the triple evacuation but every HVAC technician or HVAC contractor should have nitrogen tanks for the primary purpose of performing evacuations among other uses.
Tools Needed for a Proper Refrigeration Triple Evacuation
- Manifold Gauges and extra hoses for the vacuum pump and nitrogen tank
- Nitrogen Bottle(s) and Regulator – depending on the size of the system will determine how much nitrogen you will need. For very large refrigeration systems the larger 120 Cfm bullet tanks are necessary however for most residential and light commercial systems a couple of the 60 Cfm tanks will suffice.
- Micron Gauge
- Vacuum Pump – again as with the nitrogen the capacity of the pump will depend on the size of the system being evacuated.
A brief inspection of the system should be made to make sure everything is completed and installed properly. Check the filter driers to ensure they have been replaced and installed in the proper direction (provided it is not a bi-flow) and make sure all the shraeder valve cores are installed and tight. If you were not the installing technician it may be a good idea to speak to the technician (for new systems) to make sure they took great care to protect the piping from contamination and moisture and also used proper techniques when brazing the pipe. After I am satisfied everything is good I pull a vacuum getting the vacuum down into the 100 to 500 micron range. At this point I do not expect a 500 micron reading as this is simply the preliminary vacuum.
Remember, a refrigeration system has oil inside the circuit especially on old systems that have been running and this residual oil has refrigerant trapped inside it. As you pull the vacuum this refrigerant is released and pulled out of the system. This is not illegal and can be referred to as De minmis (EPA Section 608 Rule) for the release of refrigerant into the atmosphere through the vacuum pump.
Once the vacuum has been pulled for a specified amount of time I turn the vacuum pump off and isolate the circuit from the pump carefully watching the micron gauge and the gauge on the manifold for movement. Even on new systems there is usually movement and this is where I introduce the nitrogen charge.
Being careful not to exceed the manufacturers recommendation for maximum pressure I pump the pressure up to 300 to 400 PSIg and find a sweet spot on the gauge for pressure and watch the pressure carefully making note of the pressure reading.
I leave this amount of nitrogen inside the circuit typically for at least an hour or more. When I return I check the nitrogen charge to make sure the pressure remained the same as when I left it. This ensures I do not have any leaks. Then I release the nitrogen charge watching the gauge carefully till it reads zero and then I return the vacuum pump to the refrigeration circuit and turn it on.
It is important to isolate the vacuum pump from the refrigeration circuit when there is a lot of nitrogen pressure in the refrigeration circuit. The pressure from the nitrogen will blow the oil out of your vacuum pump if you do not isolate it from the circuit. I use ball valves in the hoses to isolate the circuit.
On this vacuum pull you should be able to realize a 500 micron vacuum pull unless the system is old and has a lot of residual oil inside the circuit.
If the old system had a burn out (compressor) then the old refrigerant should be flushed from the system using appropriate flushing techniques. Do not try to start the new compressor without first flushing or replacing the old piping lest you will be replacing the newly installed compressor again very soon.
Pull the new vacuum to 500 microns leaving the vacuum pump running for more than an hour. Watch the micron gauge and make sure it is staying at 500 microns. If you have problems with this check all your connections where your hoses connect to the refrigeration system. It is also good to make sure your hoses have excellent seals in the tips to make sure you have a tight connection. If your vacuum is holding good then reintroduce a nitrogen charge. I typically allow this new nitrogen charge to remain in the system for 10 to 15 minutes checking the pressure and then allow it to purge out of the system again carefully watching the pressure so when it hits zero I reintroduce the vacuum pump to the system and pull a final deep vacuum. Depending on the time of day I will either go run a service call or take care of business elsewhere or go home and leave the vacuum pump running all night (provided I know that no one is going to hijack my vacuum pump or any other tools I leave there).
Wrap Up: Triple Evacuation Procedure
Once that final vacuum has been pulled and it holds at 500 microns for more than 15 minutes then I release the refrigerant charge into the system and perform a proper start up of the HVAC system whether it is a heat pump or and air conditioner. That is it and that is all there is to doing a proper evacuation of a refrigeration system. I have never lost a compressor yet because of improper evacuation methods. In fact, I have been behind other technicians who replaced a burned out compressor only to have their replacement compressor burn out in a short period of time after the replacement and I did a thorough job of flushing the system and doing the triple evacuation and I have never had a call back for a burn out.
That is what you call doing the job right the first time and making sure the system will be reliable for use in the future. When you are 500 miles out to sea and underwater the last thing you want to worry about is the refrigeration system failing and all the food for the crew going bad. This leads one to do things right and proper in the first place.
What you Need to Know about the Basics of Using the HVAC Refrigeration Triple Evacuation Method:
- a 500 to 1000 micron vacuum held for more than 10 minutes equals a leak free and dry system. I say go a step further and wait 15 to 20 minutes before declaring the system leak free and dry. And always remember the refrigerant trapped in the oil will boil off causing the micron gauge to rise on the first and possibly second vacuum pull.
- If your micron gauge keeps rising then you have a leak in the system. Find and repair the leak before moving forward.
- Many people refer to the nitrogen as dry nitrogen. That’s fine but in my technical knowledge base the nitrogen is mainly used to break the vacuum and raise the pressure thereby melting any ice crystals or ice left in the system from pulling a deep vacuum (for an explanation of this see the video above). For whatever reason there are some seemingly intelligent people out there claiming the dry nitrogen absorbs moisture which it does not, it simply raises the pressure thereby the temperature and melts any ice thereby facilitating the moisture being removed by further process of the triple evacuation method. That is either evaporating the moisture with the vacuum pump or force purging the moisture out of the system with the excess nitrogen pressure. Never ever ever leave any moisture in the system for any circumstances! Qualifying that statement – a system that holds a 500 micron vacuum after performing the triple evacuation will have less than 10 ppm of moisture in it. There are too many technicians out there that say, “Oh the filter drier will get any residual moisture left after pulling a single deep vacuum (with no nitrogen purge)”. That is unacceptable in any circumstance for any type of refrigeration system especially systems using modern refrigerants that utilize POE’s (oil) in the refrigeration system for lubrication.
- When you pressurize the system with nitrogen (100 to 300 PSIg*) and the pressure does not remain constant then you have a leak. Find and repair the leak before moving forward otherwise you will end up with a refrigerant leak.
Me Thinks of the Old Axiom: Do the Job Right the First Time so there will be No Second Time
*Always follow manufacturers recommendations and never exceed the rated pressure for the system.