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Flowing nitrogen while brazing and pressurizing with nitrogen are great practices. Putting nitrogen in with the refrigerant? Not so much. Nitrogen is a “non-condensable” gas because it cannot be condensed (under normal conditions). However, nitrogen is just one of the non-condensables.
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. As you can see below, our atmosphere is full of them.
Non-condensables in the system will result in high head pressure/condensing temperature. They may also cause high side pressure fluctuations. The end result is 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 from the top into another tank. Then, you can 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. 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; the non-condensables remain mixed with the refrigerant unless you are dealing with large volumes. Then, the refrigerant and NCGs can actually be separated (and the latter removed).
Non-Condensables Don't Cause Restrictions
However…
“Non-condensables” is often a term that techs use when referring to ANYTHING in the refrigerant that shouldn't be there, such as moisture (shown below), 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 as 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,” it becomes a useless phrase. It's like saying a compressor is “bad” rather than explaining the actual fault.
—Bryan
P.S. – HERE is a true story about what can happen when non-condensables are indeed a problem. In many cases, issues caused by those gases are preventable.
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Most of my hard core learning came in the early days of my career. I was thrown into many situations I probably had no business being involved in and like most young apprentices, there were more than a few royal f@#k ups. I remember vividly, my first encounter with non condensables in a system and you guessed it, I put them there or more accurately, failed to remove them.
The Job
We were installing a 5 ton Liebert Challenger for a small server room. The condenser was outfitted with an OROA flood back control, to build condenser pressure in low ambient conditions. The job went well for the most part but it did include a fall from a ladder, a compound fracture, a hospital visit and fusible plug that was accidentally torched that melted the solder within it, but those stories are for another day. On the lighter side, myself, Carlos and Mike were the install crew and when I told the boss about the fusible plug incident he asked who torched it. Growing up with the phrase, "snitches get stitches", I told him I'd rather not say. But, like most bosses he demanded to know. My response, "well, it wasn't me or Carlos". Technically speaking, I fed Mike to the wolves but I never mentioned his name once!
The Start Up
We were on the back nine approaching the 18th hole (evacuation of the system). After pulling a vacuum overnight we added a holding charge, pounding liquid into the liquid line until the system stopped accepting it. The power supply was checked and verified as correct. Upon initial start up, instantaneous high pressure fault and no it wasn't a faulty pressure switch, the pressure was actually 400 psi plus on the R22 system. After a few hours of this and that, I finally made the call to the office, I needed some help.
The Grumpy Senior Tech
The grumpiest, but smartest of the bunch "super tech" showed up and I was thankful because I knew he would get the situation sorted out. After an hour or so of surveying the site and installation he went straight to the condenser mounted on the roof, stuck a hose on the service fitting at the highest location and blew off about 30 seconds worth of gas (which I am not recommending anyone do). We went back down to the indoor unit and started it up, no longer did it instantly pop the pressure control, it ran...but not so well. He explained to me that there was air still in the system (non condensables). I was dumbfounded because of our 14 hour evacuation. It turns out that on a system with a flood back control and liquid line solenoid valve, we must energize the solenoid during evacuation... If not, we will be unable to remove the air trapped between the two devices. The entire charge was removed, the evacuation was executed once more, with the solenoid energized. A fresh batch of R22 was charged into the system and we called it a day....A long day!
What Are Non Condensables
Non condensables are gases such as air or nitrogen that can not be condensed during the refrigeration cycle. They move into the condenser and cause issues within a system, but lucky for us, those issues have symptoms.
Symptoms Of Non Condensables
We talked about the elevated discharge pressure, this is due to the fact that the non condensables are talking up needed space within the condenser coil. The elevated pressure in the condenser will cause higher than normal compression ratios as well. Due to the increased condenser pressure and saturated condensing temperatures, the condenser temperature difference or condenser split will also be high (difference between condensing temperature and ambient temperature). Others things to look for...possible increase of suction pressure, high compressor amp draw and higher than normal subcooling. Use best pratice evacuation procedures to ensure all non condensables are removed for a healthy operating system.
Some lessons can't be learned from books, you must breathe in the mistakes and embrace them as learning experiences. Happy HVACing!
Check out the link to my YouTube channel for more tips, tricks, and troubleshooting videos and check out the The HVAC Know It All podcast here or on your favourite podcast app.
Gary McCreadie
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