Understanding Triple Evacuation
The triple evacuation is meant to be a means of dehydrating refrigerant tubing in a system that has high water content before refrigerant is introduced. This was and is still used on systems that have the potential for water to freeze while vacuuming.
Does water inside the tubing freeze while vacuuming? Do you have to worry about this on most systems? That sounds like a whole other article for another week, but we will discuss some of the topic now! On air conditioning split systems, we usually don’t have to worry about water freezing inside the tubing while vacuuming since there is quite a lot of tubing that exchanges heat with its surroundings. As you remove the air from within the system’s tubing while vacuuming, heat is removed. This is because the heat is stored in the air inside the tubing. If the removal of heat happens too quickly then it is possible for any water left in the system’s tubing to freeze. However, the tubing transfers heat from the air surrounding the outside of the tubing to the inside of the tubing. The tubing can absorb heat surrounding the outside of the outdoor condenser, the indoor evaporator, and the line set which connects the condenser to the evaporator. This will usually be more than enough heat to replace the heat lost while removing the air when using the vacuum pump. Now that we’ve got that covered, let’s get back to the triple evacuation.
So what is the “Triple Evacuation”? For most techs, this question originates from hearing about the triple evacuation during the EPA 608 certification exam review. Presently, the triple vacuum method is listed in multiple installation manuals especially those with smaller tubing such as mini-split systems.
An example of a triple evacuation is when you vacuum an empty system down to a certain level such as 1000 microns, then break the vacuum with nitrogen to a pressure of 5 PSIG for 5 minutes, then vacuum down to 500 microns, break the vacuum with nitrogen to a pressure of 5 PSIG for 5 minutes, then vacuum down to 200-300 microns. After reaching 200-300 microns, the standing vacuum test is performed for ten minutes with the pump isolated and shut off.
The exact micron level at each of the 3 levels along with the nitrogen PSIG level in the written example above may be different than what you may read in each manufacturer's install manual or textbook but the process is the same. The object at the end is to have a system hold under a 500 micron level per EPA 608 guidelines with the vacuum pump isolated from the system. The vacuum gauge will need to be between the valve and the system so the vacuum level on the vacuum gauge is measuring the micron level inside the system and not outside the system. This is referred to as the standing vacuum test and it is typically done for 10 minutes. Some manufacturers may recommend a 300 micron or lower level after the 10 minute standing vacuum test.
The difference between the Triple Evacuation and the Triple Evacuation with Double Sweep is that when you are breaking the vacuum the first two times with nitrogen, you are flowing nitrogen at 3 to 5 CFH (Cubic Feet per Minute) for 5 minutes and not just bringing the system nitrogen pressure up to 5 PSIG for 5 minutes. This helps push any water vapor out of the system while the nitrogen is venting. This seems to be more beneficial than just a triple evacuation with a system pressurization since in the case of a system pressurization, the water vapor has nowhere to go and only a small amount will be vented out when bringing the system to 0 PSIG before performing the vacuum again. Of course, a double sweep cannot be performed on a mini-split unit since it usually only has one service port on the vapor side. In this case, a standard triple evacuation can be performed.
If you are flowing 3-5 CFH of nitrogen while brazing then you are already starting the dehydration process before you even hook the vacuum setup onto the system.
I do believe flowing nitrogen will help the vacuum process on high water content systems but regardless of the vacuum method, if the micron level does not rise during the standing vacuum test, then there are no leaks, air, nitrogen, or water left in the system. If you have a concern about high water content in a system then I recommend running the indoor fan during the vacuum in order to help boil out the water. As a result of the indoor air blowing across the indoor coil, the tubing at the evaporator will introduce heat to the inside of the tubing. This will allow for water to boil and result in a fast accurate vacuum.
The moral of the story is if you are able to pull a vacuum to 300 microns on a system in 5-15 minutes and perform a standing vacuum test for 10-15 minutes without the vacuum rising, there is no need to perform a triple evacuation or a triple evacuation with a nitrogen sweep. The standing vacuum test proves that the system has no leaks, air, nitrogen, or water left in the system. The triple evacuation and the triple evacuation with nitrogen sweep are just two methods to remove water from within the system when it seems difficult to remove the moisture by doing a single evacuation. I believe some manufacturers start with this method in their literature because they are afraid the technician is not performing the standing vacuum test or not performing it correctly.
Here is a video in which I perform a single evacuation and the standing vacuum test. This video is a comparison between a two hose vacuum setup and a single hose setup but it shows breaking the vacuum with nitrogen and later in the video it shows breaking the vacuum with refrigerant leftover in a near empty R-22 bottle.
If you are concerned about frozen water then run the indoor blower motor during the vacuum and during the standing vacuum test to introduce heat to the indoor coil. If the vacuum level rises a bit but then stops during the standing vacuum test, there may be water in the system. If the vacuum level continues to rise during the standing vacuum test then there is a leak somewhere in the system or on the connections for the vacuum setup. Make sure to use only vacuum rated valves to isolate the vacuum pump from the vacuum gauge and hose setup.
I use Appion Valve Core Removal Tools- http://amzn.to/2uYr8WL
Here is a list of the other tools I use- https://www.amazon.com/shop/acservicetech
It would be a rare situation to have frozen water in a split ac or heat pump system (except during winter and in the case of an outdoor packaged unit) due to the thermal conductivity of the tubing.
If you know techs that are still running a vacuum pump for a certain amount of time or without a vacuum gauge, just gently remind them these three things:
1. A fast vacuum can be done with fairly inexpensive tools.
2. You can prove there are no leaks, air, nitrogen, or water left in the system.
3. A vacuum procedure does not have to take a long time to reach the required level and it can be done with less frustrations
People run vacuum pumps way too long and attempt to pull a vacuum without using valve core removal tools. That adds wear and tear on the pump without results. However, newer setups and frequent oil changes can be used to dehydrate a system with high water content to a low micron level fast.
Check out our book “Refrigerant Charging and Service Procedures for Air Conditioning” where we go step by step through the vacuum procedure as well as all the other procedures, checking the charge, and troubleshooting tips used when working with refrigerants. The full outline is available at https://www.acservicetech.com/the-book If you have already purchased our book, be sure to tell local HVACR Instructors about our book and what you think of it. We would love to get the book into the hands of the next generation of HVACR Technicians!
Published: 9/24/2019 Author: Craig Migliaccio
About the Author:Craig is the owner of AC Service Tech LLC and the Author of the book “Refrigerant Charging and Service Procedures for Air Conditioning”. Craig is a licensed Teacher of HVACR, Sheet Metal, and Building Maintenance in the State of New Jersey of the USA. He is also an HVACR Contracting Business owner of 15 years and holds an NJ HVACR Master License. Craig creates educational HVACR articles and videos which are posted at https://www.acservicetech.com & https://www.youtube.com/acservicetechchannel