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  1. #1
    Join Date
    Oct 2004
    Posts
    3,400
    A little physics demo...

    Use a variety of vapor refrigerant to inflate a few balloons. Use the appropriate color balloon for each refrigerant, if you can. Tie each balloon.
    Place each of them in a small ice chest with a block of dry ice, ahead of time. A pound or two is enough. The balloon must be in contact with the dry ice. The balloons will shrink to nothing as the vapor refrigerant condenses into liquid. You can see it through the balloon. Quite a bit of liquid collects in the bottom of each balloon.
    Remove them, one at a time, and toss one to each of your students.
    They will re-inflate as the refrigerant warms & boils.

    You can put them back into the ice chest, and let them shrink again.
    Work this into a discussion about gas laws, conservation of energy, state of matter, superheat, subcooling, saturation, whatever...
    Try it with a balloon inflated with air, nitrogen, CO2, whatever else you have. Some will shrink, some won't.

    You can tell a noticable difference in the weight of the balloon, if compared with one inflated with air.
    Try throwing them. R-134a is really heavy.

    Official disclaimer: The refrigerant will absorb heat at a great rate. Don't let them frostbite themselves.
    When you get finished with the demo, recover the refrigerant back into your DOT approved cylinder.
    You figure out how. I did.

    An interesting variation is R-290.
    You can smell it through the balloon, and it will leak down in a short time. They will want to know why.

    Here's an interesting link about R-290:
    http://www.danfoss.dk/pdf/installato...d/cn60f102.pdf

    In addition, you can use the dry ice for other demos. I am amazed at how many of my students have never seen dry ice.

    I saw this done with liquid nitrogen at NASA, but I don't know of anybody else that does this with dry ice.

    Try it, and let me know how it works for you.

  2. #2
    Join Date
    Sep 2004
    Posts
    3,157
    you can use the dry ice for other demos. I am amazed at how many of my students have never seen dry ice.


    well if that amazes you, take a little bit of that dry ice and shove it in a two liter soda bottle - a little water in the bottle, 1/2 a cup or so-- put the lid on the bottle - set it out in the parking lot- and you and your students will be amazed

  3. #3
    Join Date
    Sep 2004
    Posts
    3,157
    So , if you would please , explain why or under what conditions would I want to use R-290. Iunderstand that it would only be flammable if or when air is present, but it seems that there are much safer products to use that operate at or around the same temps

    bwal2: btw I to am a student dying to learn all that I can about this busniess, so this type of info is great

  4. #4
    Join Date
    Oct 2004
    Posts
    3,400
    Originally posted by ct2
    So , if you would please , explain why or under what conditions would I want to use R-290. Iunderstand that it would only be flammable if or when air is present, but it seems that there are much safer products to use that operate at or around the same temps

    bwal2: btw I to am a student dying to learn all that I can about this busniess, so this type of info is great
    Did you read the Danfoss link?

    Little refrigerators, in Europe are using propane as a substitute for cfc's. From the factory. We went a different direction here.

    You probably would not use it as a refrigerant here, but some of it's properties resemble r-22, except for the obvious fire/explosion risk.

    In actuality, propane is probably not as dangerous as gasoline.
    (Gasoline @ 17,500 btu/pound vs Propane @ 15,000/pound)


    Have you ever had a propane bottle refilled?
    They bleed a little gas out of the cylinder to cool it & lower the pressure. This speeds the transfer process. Before EPA, the same thing could be done with r-12, or any of the other common refrigerant. It was commonplace.

    Using propane as a demo, in properly vented areas, makes an impression on those that have trouble visualizing low temperature boiling. Liquid propane can be poured into a styrofoam cup. It will soon cool the cup & air around it to the saturation point. The boiling process slows & almost stops. A thermometer can be placed in the clear, calm, liquid. It will boil violently, then stabilize.

    (It doesn't have to be a full cup. Just a little in the bottom of the cup will do.)

    We used to do this with cfc refrigerants, before they were regulated.

    Drop a penny into the liquid.
    Pour the liquid.
    Don't touch it.
    It will frostbite.
    Safety glasses, gloves, ventilation, BRAIN IN GEAR, are all necessary.

    BTW, this "brainstorm" came to me by accident. A student walked in carrying a disposable propane cylinder he had dropped. It was cracked & leaking. Propane was coming out anyway, so I took it outside, in a "safe" area, & monitored it as it bled down. The tank started to freeze up & you could see the liquid level. I knew what was happening, but my first-year students got to witness it & ask questions. I turned it upside down, so that liquid dripped out, & caught some in a cup.

    Yes, there is danger involved.

    You make the call.

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