Zero is NOT Nothing!

Many times people have remarked that even though they recovered a system to 0 psig or lower, when they opened the system, there still appeared to be some refrigerant. Often the presence of some refrigerant was most noticeable when they started brazing on a system that supposedly has nothing in it, and noxious green flames come out of the joint. This is because zero is not nothing. And no, that is not my “casual” grammar coming out. Don’t assume that because you have recovered a system down to 0 psig, or even into a vacuum, there is no refrigerant remaining. In fact, there can be quite bit of refrigerant in the compressor oil even under a vacuum. Since the refrigerant oil and the refrigerant are miscible, refrigerant dissolved in the refrigerant oil leaves the oil very slowly. The attached video shows refrigerant boiling out of oil removed from a compressor that was removed from a system which was recovered down to 28” of vacuum. The oil continued to boil for hours after being removed.



 This helps explain why system pressure can rise in a system which is left under a vacuum. This also explains why you should make sure the compressor oil sump heater(crankcase heater) is on before recovering refrigerant. If you are planning on recovering refrigerant from a system that has an operable compressor, run the system until the compressor is warm before beginning recovery. The compressor can draw out the refrigerant from the oil faster than your recovery machine. A heat gun applied to the bottom of the compressor can also help a great deal. While you are heating places that trap refrigerant, go ahead and heat the bottom of accumulators, receivers, and filter driers as well. A little time spent warming these areas trap oil and refrigerant will save time during recovery and evacuation. Remember, zero is not nothing!

A Lab Gone Wrong Sometimes Teaches More

Sometimes adversity is the best teacher. Recently one of our first semester students learned more than he bargained for when he had multiple issues with his lab. The lab was to recover the refrigerant from a packaged unit, evacuate the system, and weigh the charge back in. The recovery process went well, but things got off track during the evacuation. The system would not pull down low enough for the vacuum gauge to register. After waiting a while, he decided he must have a leak, so he disassembled his evacuation setup and began to put nitrogen in the system. The leak showed right away – it was on the stem of the Schrader valve. It may have been created while he was connecting and disconnecting hoses. He brazed up the leak, reassembled his evacuation setup, and evacuated the system down to 250 microns. However, when he closed the valves on the core tools the vacuum started to rise. I told him a little rise is normal because the pressure back in the recesses of the system is a little higher than right at the point where you are pulling out the gas. However, it continued to rise past 1000 and kept going. At which point he decided he must have another leak. Once again, he disassembled his evacuation setup and charged the system with nitrogen. When he took off the vacuum gauge, he noticed that the rubber O-ring was missing. Most likely, that was his second leak. He left a note on the system stating the nitrogen pressure and ambient temperature so he can check it on Monday after a weekend of sitting. If the temperature changes, he will have an opportunity to use the gas laws to determine what the holding pressure should be. The lab was really intended for practice in refrigerant recovery, evacuation, and charging. He will get that, but he also receives the bonus of seeing first-hand the effect a leak can have on system evacuation. This also illustrates the value of leak testing and using vacuum gauges. Without the vacuum gauge he would have charged a leaky system and a good bit of the refrigerant would be gone in a few days. Vacuum gauges don’t cost time, they save it. Using a vacuum gauge is the only way of knowing when you have a vacuum and when you don’t. If you can’t pull a deep vacuum, you need to find the problem. The time you save by ignoring the problem will be far outweighed by the time it will cost chasing down the problem later.

Sucking Through a Coffee Stirrer

Have you ever tried using a coffee stirrer as a straw? I have attempted to use a coffee stirrer as a straw when I was really thirsty, needed a straw, and none were available. After all, they do look like a straw: they have a small hole running up the middle. In fact, you can get a little bit through, but the process is slow and frustrating. The restriction created by the small hole in the coffee stirrer just will not allow very much through. When you connect your vacuum pump up through standard 1/4” hoses, leaving the Schrader cores in the Schrader valves, you are essentially forcing your vacuum pump to draw through a coffee stirrer. It’s not impossible, but very slow and frustrating. The restriction imposed by the Schrader valve and small diameter hoses restrict how much gas can come through, slowing the process down. You can dramatically reduce the time it takes them to evacuate a system by removing these restrictions. In fact, removing the restrictions will do far more than getting a bigger vacuum pump. The same pump you already own can probably evacuate systems in half the time you are accustomed to by simply removing the restrictions (assuming you keep the pump clean and change the oil). The biggest restrictions are the Schrader valve cores. You can buy two vacuum rated core removal tools and dramatically reduce your evacuation time, even using the same hoses and gauges  (so long as they don’t leak). The core removal tools allow you to remove the core, evacuate the system, charge the system, and then replace the cores. Next, you might want to use 3/8” or even 1/2” hoses to evacuate rather than 1/4” hoses that came on your charging manifold. That replaces the coffee stirrer with a properly sized straw. Finally, you should consider using a manifold with a larger bore. Any one of these improvements will make a noticeable difference; all together, the effect is close to amazing. Appion sells a Megaflow Speed kit that packages these three components together. Tru-Tech sells a Rapid Evac Kit that contains the hoses, valve core tools, and some Nylog vacuum sealer. They are both a little expensive compared to standard manifolds or 1/4" hoses, but they work much faster. Just ask yourself this – how much would you pay to cut your evacuation time in half.

If you teach air conditioning, you should contact Appion. They are giving away a vacuum pump to schools that purchase one of their Megaflow Speed Kits. While you are at it, ask for one of their recovery machines too. Note: this deal is just for schools – they are not giving away a recovery machine and vacuum pump with every gauge set sold. E-mail sharon@appioninc.com and ask for details on their educational program.

Warning: After you use better equipment you won’t want to return to your old coffee stirrers.