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Flow and Pressure

  1. Dec 28, 2012 #1
    I am an amateur in engineering and new to this forum, so please excuse me if I am posting this in the wrong location. I am doing a little project outside of school that deals with water generation.

    The project consists of a coolant that travels through a network of pipes and cools a coil to a desired temperature. For my coolant I am using dry ice(solid Carbon Dioxide), because it directly sublimates from a solid into a gas.I ran into a little problem during the execution of my project. Basically, what happened was, when I placed the dry ice within my containment chamber( which by the way is a homemade flask that consists of a water bottle placed within a 2 liter bottle with the gaps filled in by insulation foam), the rate of sublimation was too slow to spread through the entire network of tubes and cool the coil.

    As a result, I need to somehow make make the cool air travel through the tubes. I have also considered the application of a fan, but it seems impractical on a large scale to use a battery and its not really applicable for an experiment that is meant to be low cost. So I explored the idea of adding air via a bicycle pump into the chamber and "pressurizing" it to force the cool air into the tubes and cool the coils. That didn't work(BTW I am also using tire valve stems), in fact, I ended up overheating the bicycle pump, and exploding the tube off.

    With that in mind, I tried the same experiment in a more smaller environment, where I tried it with a simple water bottle and I observed that the bottle expands when the air is pumped but then contracts when the cylinder within the pump moves up. Essentially, I was expecting the bottle to work like a pressurized inner tube releasing air at a high force.

    As a result, I retried the experiment but with one little change, I filled the bottle half with water and pumped air into it, and pressure began to build to the extent where the bottle would eventually burst. I want to know why this happened(water vs empty bottle) and also can anyone suggest to me how I may circulate air within the tubes without the use of a fan?

    *Sorry I wrote a lot, Thank you in advance
  2. jcsd
  3. Dec 28, 2012 #2

    Simon Bridge

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    It sounds like the bottle lets the air out slower than the pump supplies it, but the pump only supplies air on the downward stroke.

    I don't understand - how was this different? What were you hoping the water would do?
    You can cool pressurized air .... keep the air in a large sturdy pressure vessel. I have used a 40L drum for this myself. You can pressurize it with a bicycle pump and then release air slowly through a valve when you need it. Or you could just pump your existing setup more slowly.
  4. Dec 29, 2012 #3
    Well, I added water because I read online this is how they make pressure rockets:

    Since they make it like so, I was thinking that if water is what causes the build-up of pressure, perhaps if I add water to a water bottle, then maybe it would result in some pressure build-up.
  5. Dec 29, 2012 #4

    Simon Bridge

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    Water is used in bottle-rockets for reaction mass.

    All it is doing in your case is occupying volume - so the higher pressure builds up in fewer strokes of the pump. Do you understand how pressure forms when you pump air?

    I still don't understand what the difference between the two cases was.
  6. Dec 30, 2012 #5
    No, I think that was the difference, "higher pressure builds up in fewer strokes of the pump" with the water in place. Ok, I understand.

    Thank You
  7. Jan 1, 2013 #6

    Simon Bridge

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    ... because the water takes up some of the volume in the chamber. Put the same amount of air into a smaller volume and you get a higher pressure.

    (Just making sure.)
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