Sunday, August 5, 2012

Zero is Not Nothing!

The other day a technician asked me why there appeared to be refrigerant coming out of a compressor that he was de-brazing. He had recovered the system refrigerant, so where was this refrigerant coming from? First I asked now deep a vacuum he pulled with his recovery machine. He had pulled down to 0 psig. I pointed out that 0 is not nothing. Even though I am from Georgia, “0 is not nothing” is in fact not a double negative. Remember that there is still 14.7 psia worth of pressure in the system at 0 psig, so 0 is really not nothing. Something is still in there. You would have to pull a deep vacuum to get close to nothing. Additionally, the compressor oil holds lots of refrigerant. So if you want to reduce the refrigerant coming out of the system, you need to go lower. For a system that has an operating compressor, operate the system until the compressor gets warm before starting recovery. No recovery machine can pull refrigerant out of the oil as fast as you can by operating the compressor. However, if you are changing the compressor, that probably won’t work. Instead, get out your hair drier and heat up the bottom of the compressor with hot air. While you are at it, heat the accumulator and filter drier as well. If you don’t plan on dining out while the recovery machine operates, use core removal tools to take the Shrader valve cores out to reduce restriction. Trying to recover through a Shrader valve with the core in is like drinking a milkshake through a coffee stirrer –not impossible, but really slow. But back to the 0 is not nothing story. 

I have done an experiment several times now that shows how much refrigerant can be left in the compressor oil. After recovering a 2 ton R22 packaged unit down to approximately 5” hg vacuum, we let it sit to see that it would not rise above 0 psig. Convinced that we had achieved 0, we operated the unit. After a few minutes of operation, the system was operating at 150 psig on the high side and 25 psig on the low side. There was a lot of refrigerant in the compressor oil. Now I must warn you that compressor manufacturers would frown on my experiment – so don’t do this at home – use someone else’s compressor! Operating a compressor on “nothing” is a sure way to kill it. One last safety tip: instead of de-brazing the compressor, try cutting it out. No fumes, no flames, and far safer if you mistakenly left a lot of “not nothing” in the system.

3 comments:

  1. Good point on cutting the compressor out. This is especially important as scroll compressor manufacturers state that refrigerant can become trapped in the head. That compressors should always be cut out.

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  2. Some really interesting posts on this site! Wasn't sure how to reach you, but wanted to send you a message. I think it's a great idea to have a reference for those looking to learn about HVAC/R.

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    Owens Corning

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  3. The information is really great. I observe many websites regarding HVAC issues, but I found this blog very interesting.
    In last summer I faced the refrigerant leakage problem with my AC unit. But then I hared a good technician and he resolved my problem.
    Many thanks for this informative post.


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