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B Which will stay colder longer?

  1. Apr 8, 2016 #1
    I am currently building a water cooled vest for the hot days here in Phoenix Arizona. Some days in the summer can reach upwards of 115 degrees F, and I would like something to keep me alive if I go out to ride my Motorcycle or just work out in the backyard.

    My current design is a vest with 1/4 clear vinyl tubing laced throughout that all leads back to a 2 liter water bladder filled with a 90% water 10% ethanol solution (so I don't have to worry about algae). A 12v DC pump keeps the water moving.

    Inside the water bladder are removable cold packs.

    My question is what would stay colder longer.

    Cold packs filled with the same 90% water 10% ethanol
    Cold packs filled with 100% ethanol

    My thought is, pure ethanol's freezing temp is drastically lower than pure water, -173.2°F vs 32°F.

    The average freezer temp is 0 °F.

    But pure ethanol does have a lower heat exchange rate than water. Does that mean that ethanol would keep the water slightly cooler than ice for a longer period?
  2. jcsd
  3. Apr 8, 2016 #2


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    I would tend to lean towards cold packs filled with 100% water and frozen, since you want it to take the largest possible amount of heat to warm your cold packs up. If you're using a water/ethanol mixture, the freezing point will be depressed, so you'll basically just be warming a liquid up from 0Fish to 100+F, while if you're running pure water, you have to warm the ice up from 0F to 32F, then provide enough energy to cause the phase change from ice to water, then warm the water up to 100+F. The energy needed to melt ice is really pretty substantial compared to the energy to just warm up liquid water, so this should give you the best results.

    Also, water has a significantly higher heat capacity than ethanol by about a factor of 2, which further makes the case against using ethanol in the cold packs.
  4. Apr 8, 2016 #3


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    Liquid water has a specific heat capacity of about 4.2 kJ/(kg K)), and ice about 2.0 kJ/(kg K). That is the amount of energy it takes to raise the temperature of water. Water also has an enthalpy of fusion of about 333 kJ/kg, which is the energy it takes to melt water.
    Liquid ethanol has about 2.5 kJ/(kg K), which means it can't absorb as much heat before heating up.
    You are better off with water
    Of course, these values depend on temperature, but not too much.
  5. Apr 8, 2016 #4
    Ahh, I didn't realize that there was an extra cost needed just to cause the phase change from solid to liquid. That's cool
  6. Apr 8, 2016 #5


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    Use ice cubes, crushed ice, or 'freezer bricks' (sold in grocery stores, large general merchandise stores, online at the major online retailer whose name starts with "A").

    Or use a canvas bag for the container; it works as an evaporative cooler as the water seeps out of it. (Works best with a significant air flow.)

    You can also use plain water rather than a water/alcohol mix. The algae can be cleaned by soaking in a vinegar or bleach solution when needed. Depending on materials, one or both of them may damage the pump though.

    A bonus to having a refillable container is you can refill it at a drinking fountain when needed, especially if the fountain is chilled!
  7. Apr 9, 2016 #6
    You will not get very good thermal conductivity between the vest and the cold water using Tygon (PVC) tubing. Most plastics have very low thermal conductivity. Blown film producers use about 10-15% CaCO3 to improve frost lines and bubble stability. The CaCO3 increases the thermal conductivity of the film, thus causing the bubble to crystallize sooner.
  8. Apr 11, 2016 #7
    Rather than worry about how long the system will stay cold, have you thought about building
    a small evaporator chiller into the system?
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