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Will my wet clothes dry if I hang them under the sun, and if there is no air around the clothes? In other words, do I need both air and heat to dry wet clothes, or is heat alone (in the imagined absence of any air) enough to dry wet clothes? Related question : will wet clothes dry with only the suns heat, but when placed in a vacuum? Please note - I am trying to dry my clothes differently on earth, and not in outer space.

The Seeker
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    What is the difference between "Will my wet clothes dry if I hang them under the sun, and if there is no air around the clothes?" and "Related question : will wet clothes dry with only the suns heat, but when placed in a vacuum?"? – JiK Apr 07 '15 at 09:32
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    I don't understand the difference between the first and the second question, but no, you don't need air for the clothes to dry. In fact, it will dry faster if in vacuum, because the water will start to boil in zero pressure, even if the temperature is not 100º C. This is sort of what happens to comets; when they get close to the Sun, they start to evaporate, and the released gasses and ions is what we see as their tails. – pela Apr 07 '15 at 09:34
  • @pela I think you should post this as an answer, since it is better than the 2 existing ones – Noldig Apr 07 '15 at 10:41
  • @Noldig: Okay :) – pela Apr 07 '15 at 11:10
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    Vaccum alone can be used for drying on earth. Usually with food, but here's a discussion of using it for clothes: http://www.thenakedscientists.com/forum/index.php?topic=25084.0 – pjc50 Apr 07 '15 at 16:08
  • Question to "The Seeker": Are you by any chance Greek, and does your first question, "no air around" actually mean "no wind around", i.e. moving air, or air current? My Greek officemate said that this could be the case, since it would be called the same in Greek. If so, I'll update my answer. – pela Apr 08 '15 at 12:29
  • If there really is no air they will dry quite rapidly, as the water boils away. If there is still air around the clothes (no wind) they will dry so long as the dewpoint of the air is below the temperature of the clothes. The sun, of course, raises the temperature of the clothes. – Hot Licks Apr 08 '15 at 13:10

7 Answers7

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I don't understand the difference between the first and the second question, but the answer is "No, you don't need air for the clothes to dry".

In fact, it will dry faster if in vacuum, because the water will start to boil in zero pressure, even if the temperature is not 100º C. In fact, at zero pressure, water cannot exist in liquid, but will evaporate if the temperature is above roughly 200 K (i.e. –73º C), and freeze if it's below.

This is what happens to comets. Comets are clumps of ice and dust. When they get close to the Sun, they start to evaporate (or sublimate, to be more precise), and the released gasses and ions is what we see as their tails.

pela
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    I want one of these - a clothes drier that's just a chamber that creates a vacuum, tumbles the clothes a bit, and the moisture is gone. Wouldn't this even save on energy? – JTP - Apologise to Monica Apr 07 '15 at 17:39
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    Good luck creating that vacuum without mangling your clothes. Essentially what you're doing here is pulling the air and water out of your clothes, which is what the rotational spinning does anyway. – Snakes and Coffee Apr 07 '15 at 18:25
  • @JoeTaxpayer I thought the answer was a resounding no, but the energy to create a 7 cu feet vacuum against the atmosphere is only 5 watt hour (!). But don't forget you need to keep "replenishing" the vacuum as the water sublimates and pumps are inefficient. Still, it's not obvious that it wouldn't work. It probably is, however, far to expensive to build a vacuum dryer. –  Apr 07 '15 at 18:30
  • @JoeTaxpayer: But it's so much cheaper (and easier) just to hang them on the clothesline. – jamesqf Apr 07 '15 at 18:39
  • When you say that water will freeze (and stay frozen, I presume) below 200 K, do you mean that its vapor pressure actually drops to 0 exactly. or merely that sublimation becomes unmeasurably slow even in a vacuum below the theoretical vapor pressure? – hmakholm left over Monica Apr 07 '15 at 19:48
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    Sublimation shouldn't become unmeasurably slow, even in a perfect vacuum. Decreasing the pressure lowers the boiling/sublimation point less and less, converging asymptotically to 200 K. I must say that I don't remember the exact value, and I can't seem to find a reference for this, but I think it's of this order. – pela Apr 07 '15 at 20:49
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    Interesting idea with the vacuum dryer but it's not going to work. The problem is that as the water evaporates it draws off a lot of heat. The clothes are going to get very, very cold, limiting the evaporation rate. Normally this is countered by either radiant heat or a heated surface but clothes won't transfer this heat around well enough. – Loren Pechtel Apr 08 '15 at 02:18
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    @LorenPechtel: That just means you need a work cycle. Pull the dryer vacuum - water evaporates below 300K. In the condensor, you let the air recompress to 1 atm. The water now condenses out of the air, and exits the dryer still cold. Reheat the dried-out air, and let it back in. This cycle certainly works, it's a common design for desalination plants. – MSalters Apr 08 '15 at 12:08
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    @joetaxpayer forget the silly clothes, I want a vacuum evaporator in the kitchen, it can do all kinds of fun stuff – rumtscho Apr 08 '15 at 12:45
  • @rumtscho - Isn't that how the latest food storage containers are working? Vacuum really slows down the rotting process. – JTP - Apologise to Monica Apr 08 '15 at 12:51
  • @JoeTaxpayer I am not aware of vacuum storage being used anywhere in food production, although the food industry probably has the means to do so. I suspect it would do terrible things to texture. But the lucky few who have access to vacuum evaporators use them for flavor extraction, as in http://modernistcuisine.com/2011/11/vacuum-concentrating-part-2/ – rumtscho Apr 08 '15 at 13:05
  • @rumtscho - there are food storage containers, available to the consumer now, that have a valve for a vacuum unit, part of the kit, to remove the air. It's been out at least 6 months by now. – JTP - Apologise to Monica Apr 08 '15 at 14:10
  • @JoeTaxpayer interesting, do you mean that the containers are rigid? There are "vacuum sealers" which pack the food in plastic bags, they are good for preventing freezer burn but don't provide increased shelf life by themselves. – rumtscho Apr 08 '15 at 14:15
  • @rumtscho - Ok almost two years see VacuVita - Vacuum Food Saver System – JTP - Apologise to Monica Apr 08 '15 at 14:30
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I think when you say "no air" you mean "no wind" In modern greek too "air" can mean "wind" and and also the content of the atmosphere.

So if you hang clothes in the same sun but with no wind to supply convection, the clothes will try slower than when a wind is blowing, due to convection. Convection replaces the saturated air close to the clothes with drier air and accelerates drying.

For "no air" meaning "vacuum" look at the other answers. In vacuum plus sun they will dry faster, because the higher temperature evaporates H2O faster. They will dry up quite well in the shadow in vacuum too, because vapor pressure is non existent in vacuum and H2O will evaporate as fast as the geometry of the clothes allows anyway.

anna v
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    As the question is unclear, maybe it should be added that if air replacement is prevented around the clothes (e.g., putting them in a small glasshouse), then the drying will take much longer even if the temperature grows quite high, because the air in the glasshouse will be saturated in water vapour. Drying may even stop completely depending on the glasshouse volume and quantity of water. – Joce Apr 07 '15 at 12:07
  • No, in vacuum the limiting factor will be how fast you can warm them up as the vacuum will chill them to the point the vapor pressure is extremely low. Given the terrible thermal conductivity of a pile of cloth this is going to be severely rate-limited. – Loren Pechtel Apr 08 '15 at 19:28
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Disclaimer before I get started: A perfect vacuum is impossible. As I answer your question, I will take your use of the word "vacuum" to mean "a chamber with an air pressure arbitrarily close to 0 Pa." When I use the word "vacuum" in my response, I mean the same.

Your clothes don't need the air in order to dry, and in fact, will dry more quickly.

You've often heard that the freezing point and boiling point of water are 273 K (0 degrees C) and 373 K (100 degrees C) respectively, although that's somewhat sloppy language. The reality is that the freezing point and boiling point of water are 273 K and 373 K at standard atmospheric pressure. The phase of a substance is not only a function of its temperature, but also of the pressure being exerted on it. In the case of water, by increasing the pressure on the water, you can keep the water in the liquid phase for temperatures well above water's normal boiling point. Some nuclear power plants exploit this fact to keep its coolant in the liquid phase, even after it has been heated by the reactor.

But, the reverse is also true, which is what your question is really about. By lowering the pressure on water below standard atmospheric pressure, you can "boil" your water at temperatures in which it would ordinarily remain a liquid.This videoshows room temperature water that is boiling, because it is inside a vacuum chamber.

For your hypothetical wet clothing in a vacuum, the water would rapidly evaporate from the clothing, due to the reasons you've stated above. Of course, any heat that you were to also add to your clothing would also speed up the process, although it would likely be overkill. Assuming your article of clothing is laid flat to maximize surface area, and you have a pretty good vacuum, the process would go pretty quickly, even at low temperatures.

Sean
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    You don't need to go to a nuclear power plant. Car engines operate with the coolant at temperatures higher than its 'standard' boiling point, at an elevated pressure maintained by a spring regulator, usually in the rad cap. – greggo Apr 07 '15 at 21:04
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I think: Without air arround, you have some kind of a black body. Depending on the distance to the sun your body will reach a constant temperature. If this temperature is above the boiling point, you wet clothes will become dry. Because the gravitational attraction of your clothes is much smaller than the pressure of the water vapor. Thus the water molecules will be emitted.

Matthias
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No air means no vapor too. So without air your clothes will dry more easily, because the wetness will vaporize more easily.

Jimmy360
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Thaina
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  • Why will water vaporize easier without air? – Steeven Apr 07 '15 at 10:40
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    @Steeven because low pressure. Like you could easier to boil water on high mountain. No air mean zero pressure and if you just place water it would boil there – Thaina Apr 07 '15 at 10:43
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    Ah, I see. But you also loose any diffusion into the air from the solubility. Not sure which advantage overcomes the other. – Steeven Apr 07 '15 at 14:15
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Your clothes would dry very quickly in a vacuum, assuming that the temperature is still one that one would find on earth. This is because the water would boil out of your clothes. On earth normally, boiling takes a lot of heat energy. This is because of the air pressure. In your scenario there is no air pressure. so the water will boil easily.

Jimmy360
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Is it possible that when you mean an absence of air, you mean to say an absence of moving air? If so, then there is a difference to how fast clothes will dry.

If there is no moving air, then the only considerable means of heating up the clothes and the moisture on the clothes is the thermal radiation from the sun. The radiation heats up the moisture and it becomes water vapour.

This water vapour will either linger around the clothes, or it will slowly be transported away by natural convection currents, depending on the temperature of the surrounding air. Either way, water vapour is not moving away quickly, and, due to a higher concentration gradient of water vapour, you can expect some of the vapour to recondense onto the clothes.

On the other hand, if there is wind, the speed at which your clothes dry increases. This is because wind carries any newly formed water vapour away quickly, minimising the concentration gradient about the clothes.

In addition, the collision of air molecules with water on the clothes may have enough energy to drag the water molecules away into the air current.

The only downside to there being an air current is that is enforces forced convection between the clothes+moisture surface and the air current. The clothes and the moisture will be slightly warmer that its surroundings due to radiation from the sun. This heat is sapped slightly by the cooler convection currents, will allows less of the water to vaporise. This, however, is overshadowed by the other two mentioned effects.

So, wind dries things quicker than by still air.

Apologies if I misinterpreted the question.

Involute
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