evacuation Archives - HVAC School

First I want to give credit where credit is due. This post is made possible by the fantastic demonstration video by Neil Comparetto that I embedded below.

Before you get bored and stop reading I want to give some conclusions. Ice can form in a vacuum, but I still advise pulling a fast, deep vacuum. Now… keep reading to see why.

The statement that is often made by techs is that pulling a vacuum “too quickly” can result in freezing of the moisture inside the system and reduction of evacuation speed. This conversation usually occurs when another tech is demonstrating a SUPER FAST evacuation, or by a tech who is advocating for the consistent use of triple evacuation.

Neil’s video proved that pulling a deep vacuum quickly can result in freezing with even a small amount of liquid water present. He also demonstrated that this is possible with a typical HVAC hookup and that it can pull down to 500 microns with substantial ice present. This was done in 50 degree shop, with a glass jar (insulator), with relatively low internal volume, pulled down using a vacuum pump direct connected with large hoses.

So he proved that under certain circumstances, ice can form and cause a real problem with evacuation speed and even trick a junior tech into thinking they pulled a proper vacuum when they did not.

Now before you get too excited

In a typical system with larger internal volume than a jar this experiment doesn’t replicate the same results unless of course the ambient temperature is already near or below freezing.

So what causes this (Water freezing under vacuum) and how do we prevent it?

Water, like all substances, changes state due to the molecular density and configuration based on the pressure surrounding the molecules and the temperature of the molecules (average molecular velocity). When we pull a vacuum on water there are two opposing forces, on one hand we are DECREASING the pressure which leads to evaporation and then boiling, but as the water boils it begins to lose heat because that molecular energy of the highest velocity molecules are being evaporated and removed by the vacuum pump, thus reducing the average molecule velocity (temperature). The key reason why it can freeze in these experiments is because the heat rejected through evaporation / boiling is significantly higher than the heat ADDED through the sides of the jar which is why it starts by flashing, then boiling, then back to liquid then freezing. You are WATCHING the change in energy state in real time as the available heat in the jar and added through the walls is overcome by the heat REJECTED from the boiling liquid and out the pump.

Now, if the pump was left on the ice long enough, the ice would eventually all SUBLIMATE (change directly from solid to vapor) but the rate at which that would occur is based solely on the amount of heat being added to that ice through the walls of the jar. This addition of heat is equal to the differential in temperature between the ice in the jar and the temperature around the jar.

The deeper the vacuum pulls the colder that ice will get, which will increase the differential between the ambient around the jar and the ice temperature inside.

If we hit the jar with a heat gun, the ice would melt quickly because we are ADDING a huge amount of heat very quickly. In the same way, if the ambient temperature in Neil’s shop was higher or if the vessel was made of copper (conductor) instead of glass (insulator) it would be less likely that ice would form and if it did form it would sublimate more quickly.

Here are my current conclusions based on this video (and many others), good science and practical field experience

It becomes increasingly more important use a heat gun on components and sweep nitrogen as ambient temperatures drop

Liquid water inside a system should be exceptionally rare, follow good copper handling practices and don’t work with open copper in the rain.
When the internal volume of the system you are evacuating is very small it would be easier to create ice (ice machines, ductless linsets etc…) use more caution in these circumstances by employing triple evacuation / breaking the vacuum with nitrogen / sweeping with nitrogen.
Most important is to valve of the micron gauge from the pump and watch for rapid increases. If you have ice you WILL see and quick increase when the micron gauge is isolated from the pump in the system.
Use a quality micron gauge that can show you a decay / leak rate so you can easily be aware when there is an issue.
When you are pulling a vacuum on most systems, during warm ambient conditions you are RARELY if ever going to make ice in a system under vacuum.
The conclusion is NOT to pull a SLOWER vacuum, it is simply to use heat and breaking the vacuum with nitrogen to get the moisture removed.

Here is the video-

— Bryan

All Videos, Tech Tips, Videos

Testing the Leak Rate on Your Micron Gauge Itself

July 9, 2018 By Bryan Orr 1 Comment

As a technician gains skill they will learn that regularly testing your tools is a huge part of success. It isn’t long in the field before techs find out that just because a meter or gauge gives a particular reading it doesn’t ALWAYS mean it is correct. Vacuum is one of these areas.

Everything in an air conditioning and refrigeration system leaks to some extent, our job isn’t to eliminate all leaks, our job is is to reduce the leakage rate to as low as possible. When using a sensitive micron gauge we find that isolating an assembly and checking the “decay” or standing leak rate is a great way to test and ensure that a system has minimal leaks and moisture. The challenge is that all of the connections in your rig leak and even the vacuum gauge itself leaks.

Some techs attempt to test the leak rate on micron gauge by connecting it to a core tool and then straight to the pump, evacuating the gauge down to very low level and then valving off. If you do this, you will find that every commercially available vacuum (micron) gauge shoots up pretty quickly. This is because the VOLUME of the gauge and coupler are so low that ANY leakage whatsoever has an enormous effect.

In this video Ulises Palacios shows us how to use an an empty recovery tank to better test the leak rate of a vacuum gauge rig.

It is certainly important to test all of your vacuum rig components, just remember that volume makes a huge difference when decay (standing vacuum) leak testing.

— Bryan

Tech Tips

How Long Should I Evacuate a System?

June 4, 2018 By Bryan Orr No Comments

This article was written by Jim Bergman with MeasureQuick

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, 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 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 5000 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 5000 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 1000 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 1000 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 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 in 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.

Jim Bergman
MeasureQuick

Tech Tips

Replacing a Piston w/ TXV and Evacuation

December 5, 2017 By Bryan Orr No Comments

We get a lot of questions about both evacuation procedure and TXVs so last week we produced videos on both topics including –

Before and after testing of piston vs. TXV
Using the Bluvac Measurequick app
Use of core remover tools for evacuation
flowing nitrogen process
creating an external equalizer port and much more

If you haven’t hit subscribe on our YouTube channel yet would you mind taking the time to do it today? it would be greatly appreciated. You can do that HERE

P.S. – I will be at the Rectorseal booth 2545 in Chicago at the AHR conference on January 23rd at 2PM demonstrating the new Pro-Fit flaring tool. If you are at AHR come by and see me and sign up for a chance to win a free Pro-fit!