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Outside Reading

Home and work have conspired to leave no time for a post this morning – man, I wish those two wouldn’t team up so often; they’re supposed to not even be friends or anything.
But you can head over to the science/medicine blogfest called Tangled Bank and find a lot of good stuff, written by a lot of good people that I need to blogroll. Enjoy, and I’ll see everyone on Monday.
(Oh, one other thing on the outside-reading front. Glenn “Instapundit” Reynolds mentions that he has a law-school colleague who tried to build a Dune-style “stillsuit” some years back, and adduces this as proof of her geek credentials. Point taken, but let me pull my papers out for inspection. A more technically inclined geek would have realized the physics problem that Frank Herbert sort of, er, skipped over. The major point of sweating in hot weather is evaporative cooling. What happens when you trap that moisture and try to recycle it? Condensative heating, that’s what, which you don’t hear about as much but is as real as the laws of thermodynamics can make it. A stillsuit would cook you like a crock-pot.)

7 comments on “Outside Reading”

  1. Doug Sundseth says:

    “A stillsuit would cook you like a crock-pot.”
    Well, either that or it would shine like the beacons at an old Hollywood premiere in the infrared. With enough wasted energy you could cool the inside down with a heat pump. It’d sort of put a damper (so to speak) on cuddling, though.

  2. Jim Hu says:

    Condensation heating can’t exceed evaporative cooling, so you’d only cook to the extent that you’d cook if sweating was blocked by drugs. This can be a fair amount, probably enough to induce heat stroke, but I doubt that it would reach crock-pot levels.
    If the condensation can happen on the outside of an insulating barrier, then the stillsuit is a heat pump. This is how I’d try to design it, I think.

  3. Mark says:


    Nice idea, but a heat pump needs somewhere to put the heat. Without evaporative cooling, you’re limited to convection (doesn’t work if the air is hotter than the person in the suit) and radiation (doesn’t work if the sun is out).

    If the outside of the suit can’t get rid of heat, moisture won’t condense on the inside of the suit, and pretty soon it won’t evaporate from your skin either. The insulating barrier reduces absorption of heat from outside, but that’s not the problem.

    What you really need is a heat sink, such as an inner layer filled with water. Put the insulating barrier over that to make it last longer. Sweat evaporates from your skin, condenses on the heat sink, and is collected. At night, you turn the suit inside out. Now the insulator keeps you warm, and the heat sink cools off. (If you’re the Fremen in Dune, and you have a network of camps, you can store cold water underground at each camp and just refill the suit every night. The hot water would go back into the cistern and eventually cool down by contact with the walls.)

    If you’re burning 2000 kcal/day, and you start with ice cold water, you have to carry 54 liters of water in the suit. That’s a layer about 3 cm thick, not including the insulation. It’s also a lot of weight, but it’s water, so in an emergency you can drink it.

  4. Doug Sundseth says:

    “Nice idea, but a heat pump needs somewhere to put the heat. Without evaporative cooling, you’re limited to convection (doesn’t work if the air is hotter than the person in the suit)…”

    Incorrect. You need to raise the temperature of the convective surface (not the body) above the air temperature.

    “… and radiation (doesn’t work if the sun is out).”

    Also incorrect. You can put a reflector on the top and the principle radiative surfaces on the bottom (paint them black in IR).

    For empirical evidence that it is possible to use work to cool things below air temperature in bright sun, see pretty much every home in Phoenix. Evaporative cooling (swamp coolers) work, but so do air conditioners. The fins on the AC units get really hot, though.

  5. Mark says:

    “Incorrect. You need to raise the temperature of the convective surface (not the body) above the air temperature.”
    True, and with a heat pump in the suit that would be possible. What I should have said is “That’s not a heat pump.” Without a heat pump, the convective (or radiative) surface will never be hotter than the person in the suit.

  6. Jim Hu says:

    What is needed and not provided by Herbert is the power for the compressor/condensor. I don’t think the amount captured from movement is enough. In principle water can be the refrigerant, but the problem is really that its boiling point/condensation point is too high.
    To get the cooling from sweating, you’re allowing the water to escape into a very large volume (the environment)…it only works because of the concentration on the vapor side of the equilibrium is low – trust me, on a humid day in Texas, sweating makes you wet, not cool. Arrakis was a very low humidity environment…but if you close the system, then you’re potentially creating a high humidity in the stillsuit.
    So to recapture the water while retaining the cooling you’d need to pull a really large volume of relatively low humidity vapor through the condensor, which means putting in a lot of energy.
    I admit that when I read Dune, I didn’t envision the stillsuit as a Michelin man with a large powered backpack! It would probably be more like a spacesuit with umbilical ductwork to a small trailer containing the compressor and condensor…and while it might be noisy enough to attract sandworms, you’d never be able to ride them.
    And of course, it would be more efficient just to connect to a real heat pump A/C unit with a more suitable refrigerant.

  7. markm says:

    “I don’t think the amount captured from movement is enough.” I suspect this could be proved mathemetically. Anything captured from movement is energy extracted from the wearers’ muscles. Muscle energy ultimately comes from the oxidation of sugar, in a process that (like combustion engines and even fuel cells) is far less than 50% efficient. That is, for each joule of useful energy, you get several joules wasted as heat. In a refrigerator or other heat pump, one joule of mechanical input energy can move more than one joule of heat, but the ratio is limited, especially when the output temperature has to be high compared to the input temperature. I don’t have the exact numbers, but I suspect the net result of trying to drive a heat-pump style cooler with your muscles is that you get tired and even hotter.

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