Tag: nitrogen

 

Flowing nitrogen while brazing and pressuring with nitrogen are both great, but nitrogen in with the refrigerant? Not so much. Nitrogen is a “non-condensable” gas because it cannot be condensed (under normal conditions), but Nitrogen isn’t the only non-condensable.

First, let’s talk about what a non-condensable gas is.

Any gas that does not condense (change from vapor to liquid) under the normal compression refrigeration conditions is called a non-condensable gas or NCG. These would commonly be air, nitrogen, carbon dioxide, Argon and Oxygen.

Non-condensable in the system will result in high head pressure / condensing temperature and occasionally high side pressure fluctuations as well as decreased cooling capacity and efficiency due to higher compression ratios.

The only way to remove non-condensables COMPLETELY in a small air conditioning or refrigeration system is to recover the entire charge and recharge with virgin refrigerant. You can recover the charge, let it sit in the tank for a while and then recover the vapor off of the top into another tank and recharge with liquid only to remove most of the non-condensables but it’s a pretty inexact science.

You can’t remove non-condensables with a line drier and while you do remove air with a vacuum pump you only remove the air that entered the system once you open it. The vacuum does nothing for the refrigerant you already pumped down or recovered as the non-condensables remain mixed with the refrigerant unless you are dealing with large volumes where they can actually be separated and the NCGs removed.

Non-Condensibles Don’t Cause Restrictions 

However…

Non-condensables is often a term used by techs to mean ANYTHING in the refrigerant that shouldn’t be there, such as moisture, solid contaminants and other refrigerants.

Carbon buildup from brazing is a solid contaminant, not a non-condensable. Moisture in the system is moisture in the system, not a non-condensable. A high glide refrigerant blend (such as R-407c) charged in a vapor instead of liquid is a fractionated charge…. not non-condensables

I think you get the point.

When we use a term like “non-condensable” as a replacement for “anything weird going on in the system we can’t explain” then it becomes a useless phrase, like saying a compressor is “bad” rather than explaining the actual fault.

–Bryan

This article is written by longtime contributor and RSES CM Jeremy Smith. Thanks Jeremy!


Nitrogen doesn’t absorb moisture like many techs think that it does and I think that we, as technicians, need to reevaluate the reasons for the “triple evacuation” process.

OK.. Hold on, now. Put down the pitchforks and torches and give me a chance here.

Take a minute or two and read what I have to say and at least I’ll have a head start if you still want to come after me, OK. Fair enough?

First off, let’s start with what nitrogen is. Nitrogen is a chemical element with the atomic number of 7. An odorless and unreactive gas that forms about 78% of earth’s atmosphere.

Nitrogen is not truly inert, it is mostly inert, it is generally non-reactive but it can and does have some chemically reactive properties. They don’t affect much of anything that we work on in the HVAC/R industry so we can safely consider it “inert” for our purposes but it isn’t exactly inert. If it were inert, Ammonia (NH 3 ) would not be possible as nitrogen would not be able to react with the Hydrogen. But I kind of digress from the point I’m trying to make here.

My point is about the use of nitrogen to “absorb” moisture during the evacuation and dehydration of a refrigeration system using a process commonly called “triple evacuation”. Quite a lot of techs seem to think that nitrogen has some special properties because it’s “dry” and it will remove a bunch of water from a system. That’s not really the way things work and I’ll explain why that is if you’ll bear with me a bit. As we work through this together, I’m going to try to minimize the number of confounding
variables. This is both to keep everything consistent and for the sake of simplicity.

When we deal with nitrogen in our case, we will be referencing nitrogen at 70°F and standard atmospheric pressures. We are also going to assume that nitrogen is the primary agent in air that absorbs or holds moisture and that no other gasses in the mixture that is air have any contribution to the ability of air to absorb moisture. This way we can both simplify the thinking and the math involved and we will give nitrogen the benefit of the doubt. Increasing the pressure of nitrogen above atmospheric will reduce dramatically the amount of moisture the nitrogen is capable of holding, so we will keep it at atmospheric again, to give nitrogen the benefit of the doubt.

First off, let’s look at what “dry” means in this context. What is “dry nitrogen”? Industrial grade “dry” nitrogen is 5ppm moisture or lower. For convenience, we will use 5ppm as a reference to maintain constancy and evaluate the worst possible case scenario. This 5ppm gives us a dewpoint of -86°F or 0.0187% RH. Now, we have a definition of “dry” nitrogen.

Like air or any gas for that matter, nitrogen can hold a certain amount of water based on its temperature. For nitrogen, this relationship can be found in the psychrometric charts for air. Let’s look into the psychrometric charts and see just how much moisture nitrogen can actually hold.

Remember, in this case, we are giving nitrogen 100% of the credit for the ability of air to entrain and hold moisture even though nitrogen only makes up 78% of air. At our selected conditions of 70°F and atmospheric pressure, one pound (by weight) of nitrogen will hold 100 grains of moisture at 90%RH. For reference, one grain is 1/7000th of a pound, so that 100 grains is 0.0143 POUNDS of water. An ounce is 437.5 grains so one pound of nitrogen is capable of holding less than a quarter ounce of moisture at 90% RH at 70°F.

How does that apply to the practice of triple evacuation and using nitrogen to try to dehydrate a refrigeration system?

Let’s start with a couple of more reference points, then we’ll get into the nitty-gritty.

One pound of nitrogen, at atmospheric conditions, occupies 13.8 cubic feet. This means that a 40 CuFt cylinder of nitrogen that is completely full, holds approximately 2.9 pounds of nitrogen.

I like to use hypothetical examples that closely resemble what we see in the field, so let’s set up a hypothetical example as a way to think about this so that we can see how difficult a process removing moisture with nitrogen alone would be.

In our example, we will consider a system which conveniently holds exactly 1 pound of nitrogen at atmospheric pressure and 70°F. Keep in mind that the internal volume of that system is 13.8 cubic feet, so this is a very large system. As a point of reference, 100 feet of 1 ⅛” ACR copper has an internal volume of 0.69 Cubic feet, so this is a very large system we are using for this hypothetical example. Far larger than any residential system one will encounter. The results we get will be scaled down
accordingly on smaller systems.

Into this system, we will introduce one ounce of water. For a little perspective, this is 2 tablespoons full of water. That is not very much water given the size of the system.

What will it take to remove any significant amount of moisture using nitrogen?

The moisture, in our example, is simply a small pool or puddle of moisture, say at the bottom of a trap or simply laying along the bottom of a horizontal section of piping. A victim of poor practices more than likely. This will be a new installation job, so we don’t have any oil in the piping that could cause a problem during the evacuation and removing any other confounding variables from the issue. We just have a puddle of water and we need to get it out of the piping.

For the 1 pound of “dry” nitrogen to absorb the full 100 grains of water that it is capable of holding, it needs TIME. It needs to sit long enough for the humidity to diffuse throughout the entire system and raise the humidity of that entire quantity of nitrogen to 90%. If you only allow it to sit long enough to raise it to 30% or 40% for example, you only pick up between 30 and 40 grains of moisture which is only a fraction of the total maximum that same pound of nitrogen can
hold.

In a system of this size and complexity, the time to diffuse the humidity through the entirety of the system could be several hours or longer. Then, you have to repeat the standing nitrogen step once more, again waiting several hours (or longer) for the humidity to diffuse throughout the system, again. All for a measly 100 grains of moisture. Less than a quarter of an ounce.

Remember the basics here. Everything moves from “high” to “low” so the moisture moves from an area of high humidity to an area of low humidity. This process is a rather slow process. Imagine, for purposes of scale, what effect a small pot of water would have on the humidity of a room if that water were left at room temperature. How long would it take for the higher humidity level close to the water’s surface to have an impact on the humidity 20 feet away? Even if we lowered the humidity in the room to the levels of “dry” nitrogen, the humidity diffusion rate would be very low and very slow. Would it ever have a measurable, appreciable effect on the humidity of that room? This is key to why nitrogen doesn’t really have any impact on evacuation procedures.

While evacuating the system, there is a constant removal of moisture as the water will be constantly boiling, constantly absorbing heat from the surrounding piping and materials, resulting in a faster moisture removal because there are no interruptions in the process.

Well, wait, you say. I’m not saying that nitrogen “absorbs” water, but nitrogen DISPLACES moisture. OK, to a limited degree, this is a fair statement. IF you have a system filled with warm, humid air, a gentle push or ‘flush’ of nitrogen will displace that moisture. I’m not entirely certain how this displacement is expected to significantly accelerate dehydration. It will displace moisture one time. A vacuum pump will do exactly the same thing and do it just as fast.

Remember, the moisture content of air is measured in grains of moisture. The actual quantity of moisture is extremely small.

But wait, I’m not using nitrogen to “absorb” moisture and I’m not using it to “displace” moisture, I’m using it to prevent the moisture from freezing because the vacuum is being pulled so fast that I don’t want that to happen. If this is your position on the use of Nitrogen, I’d first ask you to listen to The HVAC School podcast with Jim Bergmann where he specifically addresses this issue with a resounding NO.

In a normal HVAC/R system under conditions that are not near or below freezing, it is extremely unlikely to freeze moisture due to the amount of heat available in
the environment that will keep any liquid water in liquid form and keep that liquid water boiling under vacuum. This simply is a non-issue.

But wait, Jeremy, you say. I KNOW nitrogen helps speed up my evacuation because I use it all the time and it makes my evacuation go faster. I’m not exactly sure what to say in cases like this. I think I’ve made a pretty convincing case that nitrogen doesn’t really assist the evacuation process. The math and the science are really on my side here. In many cases, particularly in the service world where systems are ‘contaminated’ with refrigerants and other gasses, nitrogen can just kind of recalibrate the micron gauge and cause it to show you a true reading rather than one that is distorted by a non-nitrogen based environment.

The last commonly used reason for triple evacuation is “the book says so” Yes, it generally does. If your installation manual, your customer, your employer, your foreman or manager or anyone else that has authority over the job you are doing or your job, in general, says to do a triple evacuation or any other process or procedure that you deem useless, unless that process is unsafe for your person, DO IT. In no way, shape or form am I encouraging technicians to refuse to do work as they are expected to do it because some random dude on the internet said so.

To conclude, yes, nitrogen does absorb some moisture. It really isn’t as much as most people like to think it is when you break it down and look at the math. It also takes a lot more time to have an appreciable impact than most techs are willing to give the process for what little benefit it can offer.

So, what is the takeaway here? Why did I spend a bunch of time working on this, researching the math and laying out these ideas in a way that is (hopefully) easy to read and to digest? Do I honestly expect manufacturers to change their processes and procedures based on my couple of pages? Nope. I don’t. If anything, my goal here can be stated in a single word.

THINK!

Think about what you are doing. Think about why you are doing it. Evaluate every process, technique, procedure, and idea that is given to you. Don’t take something at face value just because you trust the person telling it to you or because you like him or he’s the boss. I’m not trying to organize some industry-wide revolution, so don’t tell your boss that I said he was wrong for making you do a triple evac. Don’t tell the Daikin or Mitsubishi rep on a VRF startup that I said that triple evac was stupid and that you shouldn’t do it. I haven’t said these things. What I hope you can take away is a little thoughtfulness about the deeper reasons you’re doing the things that you’re doing.

— Jeremy

P.S. – Bryan here. I was talking to Eric Mele about the subject of purging with nitrogen and triple evac. While it’s pretty clear that purging with nitrogen or breaking with nitrogen is going to do very little to help with liquid water contamination it does help you get a more accurate reading on your vacuum gauge (like Jeremy stated). It has one other impact which is that it ensures that a larger quantity of molecules left over in the system will be “dry” nitrogen rather than Oxygen or water vapor (which are unwanted) simply through “homogenous dispersion”. This simply means that the more times you run nitrogen through the system the greater the concentration of nitrogen molecules vs. other stuff even under deep vacuum.

The undisputed conclusion is that proper assembly practices and deep vacuum are keys to keeping the nasty stuff out of a running system.

Two days prior to this article being published I sent one out about the popular fallacy that nitrogen “absorbs” moisture. That tech tip went out at 7 PM eastern time like usual, and I was sitting on the couch watching something on the Food network (like usual).

At 7:10 PM I get a call on my cell and I look down to see the name Jim Bergmann displayed boldly on my screen. Whenever this happens it means only one thing… Jim read my tech tip and he has something to say about it.

“What did I say wrong THIS TIME” I mumble sarcastically into my iPhone

It turns out it wasn’t what I said, but rather, what I had forgotten to say that cause Jim to speed read, then speed dial.

So this tech tip is really Jim’s, even though my hands are the ones typing the words. He had a really good point to make about sweeping nitrogen BEFORE pressurizing with nitrogen.

Air is mostly made up of nitrogen, oxygen, argon and water vapor. The nitrogen and argon are inert and while we don’t want much of them inside a refrigeration system they don’t react with the oil, refrigerant, and metals in the system like oxygen and water vapor can (and often do).

When we call nitrogen “dry nitrogen” we just mean that it is nitrogen vapor alone with no water vapor or oxygen mixed with it. When we flow nitrogen at 2-5 SCFH during brazing we are displacing the air or “atmosphere” with nitrogen that contains no oxygen or water vapor that cause the nasty flakes of carbon to build up.

Before we start flowing at low levels we should first Purge or “Sweep” the system with nitrogen so that all the air is displaced out, to begin with. This should be done at a reasonable low pressure of 3-5 PSIG to help ensure that we don’t condense the moisture in the system into liquid water.

Let’s take a quick pause there… You may ask

Why on earth would pressurizing the system with nitrogen lead to liquid water condensation?

This occurs for the same reason that water condenses inside an air compressor. When you squeeze together those water vapor molecules with pressure the dew point temperature increases… until finally, it condenses into liquid water inside the system.

By sweeping the system with low-pressure nitrogen for 30 seconds or so you help displace and carry out that air and it’s water vapor with it before it has a chance to condense. Then you flow nitrogen while brazing, finally you are ready to pressure test.

What if you did no brazing?

In cases where you opened a system and only made repairs to threaded fittings, or used a low-temperature solder that doesn’t require flowing or installed a ductless or VRF system that has no brazed connections or used Zoomlock…

In that case, you would still want to do the nitrogen sweep BEFORE you pressure test. This will help decrease your evacuation time and keep your pump oil cleaner, longer.

So there you have it… from Jim’s mind to my ears, to this article, to your brain. Pretty good stuff.

— Bryan

 

There are many examples of teaching using metaphor to help someone get a grasp of how something works without being EXACTLY correct.

Some examples are how we often use water flow to explain electrical flow or refrigerant circuit dynamics. It’s enough like the way it works to get our heads wrapped around it but there are many differences and the metaphors eventually break down.

This is definitely the case with air and nitrogen “absorbing” water

I’ve done podcasts and videos about how air can “hold” less moisture when it is cooler and more when it is hotter. You have likely heard old school techs talk about triple evacuation and sweeping with nitrogen to “absorb” the moisture from the system.

News Flash, Air and Nitrogen DO NOT absorb or hold moisture… They ignore one another at parties and they certainly don’t shake hands.

Water vapor in the air behaves much like all the other gasses contained in the air with the notable exception that water exists in both vapor and liquid states at atmospheric pressure and temperature.

When the temperature of water vapor is higher, a higher percentage of the air by volume can CONTAIN water vapor, but the air itself isn’t what is holding it. It does interact with it as the molecules move and bounce around and the percentage of water vapor in the air does impact the mass/weight of the air by volume (water vapor weighs less than dry air) so there are certainly impacts to the makeup of the air based on moisture content.

The percentage of the air around us that is moisture can vary from almost zero In cold arctic & Antarctic climates to nearly 4% in hot, tropical climates.

When teaching it we speak as though the air is a sponge and the hotter the air the bigger the sponge. This certainly helps us remember but it isn’t really how it works. In reality water in the air is all about the saturation temperature and pressure of the water and the air has little to do with it.

By Greg Benson

This is the same sort of thinking when a tech is having a hard time pulling a vacuum and they add dry nitrogen to the system to “absorb” the moisture. First off, you will want to sweep the nitrogen through the system, not just pressurize. Secondly, the nitrogen has no special properties that allow it to “grab” moisture. It can entrain the water vapor using Bernoulli’s principle, it will warm up the system a bit, it will certainly add in a bit of turbulence which can help move the oil around and potentially release some trapped moisture… but nothing more than that.

Don’t get me wrong, there is nothing wrong with sweeping with dry nitrogen, even better to use a heat gun and warm the compressor crankcase, receivers and accumulator and coils during a deep vacuum on a large system to help speed up the vaporization of moisture.

It doesn’t change the fact that air and nitrogen don’t “hold” moisture.

— Bryan

 

My technician (and brother in law) Bert made a good point the day (It’s hard for me to admit it, but it’s true). When he needs to open the refrigerant circuit to make a repair regardless of whether he is recovering or pumping down, he pulls out his nitrogen tank and his regulator (We like the VN500 shown above).

Once the refrigerant has been fully pumped down or recovered, instead of opening the system to the atmosphere and exposing it to air and moisture, he simply puts it on “BRZ” mode and introduces a very low flow of nitrogen. Now when he cuts into the system to replace a line drier, or a coil, or a compressor, or an accumulator (you get the idea) the system will stay dry and it will be less likely that anything undesirable enters the system. You simply connect the regulator to your center hose and direct the flow to the high side, low side or both depending on what part of the system you have open.

Once the system is all dry fit into place you are then ready to flow nitrogen while brazing, pressure test and even triple evacuate if nitrogen is needed for that.

The biggest hurdle to getting techs to flow nitrogen while brazing is getting the nitrogen tank off the truck. If you get in the habit of connecting nitrogen before you ever cut or open the lines it even further reduces the chance that you “forget” and increases the chances that your system is clean and dry.

Just a thought (from Bert)

— Bryan

Flow_Nitrogen

In this episode of HVAC School Bryan talks with Tim Bagnall about flowing nitrogen. We discuss:

  • The Proper tools and flow settings for brazing
  • How the pressures should be set to SCFM and not PSI
  • The possibility that geography may contribute to scale
  • How to flow nitrogen in a practical way
  • And More…

As always if you have an iPhone subscribe HEREand if you have an Android phone subscribe HERE

 

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