7

Comments at a recent question How many tea light candles are needed to equal the heat output of a person? suggest that burning a candle can produce water vaper that masses twice as much as the candle

A typical tea light is around 10g so we'd produce nearly 20g of water from that. That's enough to take the humidity of a cubic metre of completely (and unrealistically) dry air up to 100% (i.e. condensing) at 15C

Realistically there are also ventilation issues (as mentioned in comments, question and answer).

Presumably there are similar results for other portable combustables like, propane, etc.

Does warming an enclosed area with an open flame provide help with drying wet fabric?

  • 1
    Well done for framing this in a way that's on topic here. I was thinking along similar lines but my ideas would have been better at chemistry.se – Chris H Feb 1 '18 at 20:52
  • Depends a great deal on the fuel, ambient temperature, relative humidity, how radiant heating fits in, and what else your fire must heat to heat this air. So much so that it may be difficult to answer. – Harper Feb 1 '18 at 23:11
7

To dry an object, as you know you want the water to evaporate off of it and out of it. Heat is one of the obvious ways to do that.

The dumbed down chemistry lesson

However, if you increase the humidity of the air, you will indeed slow evaporation and therefore slow the drying process. The water molecules leave the object you want dry when some of the surface ones have enough energy to break free from the rest.

To drastically over-simplify it, imagine the molecules as a bunch of sticky balls all vibrating around (think maybe one of those kid ball pits, but all of them extreme bouncy-balls, covered in slow-drying glue, and with a motor at the bottom keeping them all vibrating), vibrating hard enough that they are moving relative to each other (ie: not stuck in place like a solid). The ones that vibrate hard enough can break free from the sticky mess entirely and get away. The water molecules that do that become part of the water vapor in the air. When water boils, that is when all of them are on the verge of breaking free, and many cross that boundary rapidly. For simple evaporation, you have to wait as the energy randomly shifts around in the water until one of the molecules happens to get bumped around just right to have that kind of energy.

Back to the stick-ball analogy: imagine that a bunch of other balls have already broken free of the mass and are wildly zooming around through the air because of all the energy they have. The balls trying to leave the sticky mass will have a hard time doing so if there are too many balls ready to knock it back in. That's kind of like how evaporation works when the air is too humid. The balls (or water molecules) can still get free of the mass into the air, but they now require even more energy to ensure that it happens at a reasonable rate.

The ramifications (and crux of the answer)

So if you raise the air temperature just a little bit but raise the humidity a lot, you're much worse off. In fact, I would suggest that keeping humidity as low as possible is more important than adding heat, assuming the heat is in small quantities of only a few degrees, or even a few tens of degrees. If you can raise the temperature while also keeping the humidity constant or lower, then that will help, yes.

If you can increase the heat a lot though, then it is worth it. If you can increase the temperature of the room to a significant fraction of boiling temperature (close to 100C or 200F), then you're good... except that then it's way too hot, so you're not good. @JiK suggests in comment that some people do tolerate this temperature for extended periods of time, but you won't catch me sitting in a 200 degree F room for an extended time. I certainly could not handle that.

So the answer, as long as you are willing to tend the drying process very carefully and actively, is to dry with a flame and get your fabric close enough to the flame to heat up a lot. But only the wet portions; you will ruin the fabric if you get drier portions near the fire too, or if some of it dries out before other parts and is still too hot.

What I do sometimes is to hold fabric close to a fire, just out of reach of burning myself, until the water starts to come off fast and visibly as steam, maybe even making a sizzling sound, then I pull it back. That dries it out a lot faster. As for using propane, this might work with a propane stove, but I would be wary of that and way extra cautious.

Keep in mind that as you are drying your fabric, the water coming off of it will itself raise the humidity around it. So if you are in a small, enclosed area, each unit of drying will be more difficult than the unit of drying that preceded it.

  • 7
    This answer seems to be very misleading as it doesn't talk about the difference between relative and absolute humidity at all. Air at 30 °C can hold almost twice as much water vapor as air at 20 °C, that's by no means a small difference when it comes to drying. Very often a process that increases the amount of water in the air and increases temperature is still useful for drying. – JiK Feb 1 '18 at 22:42
  • 3
    You are not automatically dead if the temperature of the room is near or even above the boiling point of water. A lot of people do that every week. – pipe Feb 2 '18 at 9:37
  • 1
    @JiK The context of the question is talking about warming the air and increasing the humidity to 100%, or close to it; that drastically reduces your drying potential. If you read my answer and think of small humidity changes, then yes, you are correct, or if the humidity is already very high. So your point is valid. I will try to come up with a better way to present that and provide an update. I might not be able to get to that until Monday or Tuesday next week though. – Aaron Feb 2 '18 at 16:12
  • @pipe If by "automatically" you mean immediately, then yes. For drying fabric, you're likely exposed longer. Generally, the sauna is below boiling point. They can be quite hot, yes, but I have not heard of people keeping them above boiling temperature for more than brief periods over the extended usage. Prolonged exposure to such temperatures is unhealthy. In fact, prolonged exposure to that temp in otherwise normal conditions can be detrimental to your health, but in the sauna your dehydration is generally reduced by the excessive humidity. Also, you're probably not drying things in a sauna. – Aaron Feb 2 '18 at 16:25
  • @Aaron re saunas: (a) I don't know what you mean by "brief period" but over 100 °C isn't that uncommon for a normal sauna in Finland (although majority is around 80 °C). (b) High humidity makes dehydration worse. And excessive humidity in a 100 °C sauna hurts, hot saunas tend to be quite dry on average (some people like when it hurts but generally not for long times). (c) Many people dry things in a sauna, and it would be perfect if not for the fire hazard caused by clothes falling on the stove. – JiK Feb 2 '18 at 18:01
4

I'd have to say No, not based on the water vapor in the exhaust from the candle, but based on the fact that if the area is enclosed and not very large, you can only dry a very small amount of wet fabric no matter whether you're using a heat source or not. Unless you have a way to dehumidify the area and store the condensation.

Heat will obviously help with drying in an open system. Warmer air holds more water vapor, and warmer conditions hasten evaporation. But it doesn't take much water to saturate the air in a small, enclosed area and prevent further evaporation and drying.

Some means of rapid dehumidification, and sequestering the liquid water which condenses out, would help with the drying of the fabric and every other surface in the area, but you still wouldn't want to be inside of there.

You're going to want ventilation. If you have that, then a candle is likely to help quite a bit. Not to mention, protecting any living, breathing occupant of the same area from suffering carbon monoxide poisoning.

3

Almost all common fuels produce water when burnt, as they're made up of hydrocarbons. That is to say the hydrogen in them combines with oxygen to form water. The extreme example is methane, which has 4 hydrogen atoms to one carbon

In a completely sealed but poorly insulated space (an approximation to a tent with all vents closed) an equilibrium will be reached as water condenses on the walls (the outside will be cooler than the inside due to the heat source. The production of water amounts for a lot of the energy released per unit mass of fuel. Eventually the process will be limited by oxygen depletion.

In a completely open environment the air isn't heated appreciably - or rather the hot air convects away, taking the water vapour with it. But radiant heat is effective. That's what happens when you dry things in front of a fire. If it's raining you can often arrange your camp so that the things you need to dry are sheltered from the rain but receive radiant heat from the fire.

The intermediate case is interesting: enough ventilation to carry away the combustion products and replace the oxygen (in fact to carry away the water evaporated from your wet clothes). This is clearly possible as anyone who's finished a wet hike at a pub with an open fire can confirm. The fireplace and chimney are important, as they transfer heat to the room while venting the combustion products and sucking in fresh air. A wood burning stove is similar but more efficient. Whether we could arrange such a setup in an outdoors situation is another matter.

The common exception to these fuels is charcoal. It's not actually pure carbon but near enough. So next to no water is given off in burning. And that's very closely related to why it's so hazardous to burn indoors. It's not very energy dense (so you burn more), and consumes a lot of oxygen. So it rapidly depletes the oxygen in the air. But (much more so than in the case of hydrocarbon fuels) this just changes the combustion reaction to produce carbon monoxide, instead of putting the fire out. And carbon monoxide is poisonous (large quantities of carbon dioxide aren't much fun either but CO is much worse). This explains most of the warnings about not burning stuff n tents (which by the way aren't as ventilated as those I was brought up cooking in).

If you burnt charcoal to dry your clothes, it would work well, but you wouldn't be in a fit state to worry about at clothes if you were breathing the same air.

3

Yes.

It's a known fact that warm air can hold more moisture than cool air. Put a wet garment in a warm dry room and it will dry faster than in a cool dry room. It's called relative humidity, the warmer the air, the more moisture it will hold.

In an enclosed area, a warmer room will dry out a garment more, but only until the air reaches 100% humidity, or equilibrium is reached (the air is as moist as the garment).

What's interesting is after you warm the space you're in with a small flame, and wait for the air to get humid. You can subsequently cool the space and the moisture the air will condense on any cool surface area, such as the walls of your shelter (I'm imagining drying clothes out in a quinzee). When you heat the room again, the relative humidity will once again be low, and you can dry your clothes some more (so long as you take measures to prevent the moisture in the room from condensing on your wet clothes in between). Note that this is not a quick or efficient way to dry your clothes. It's more effective to produce a lot of heat, and have a vent in the space to allow air to circulate and water vapor to escape.

3

Where others have said correctly that warmer air can contain more water vapor than cold air, a very important detail is that you need to move cold air to the wet clothes and warm the air there to create a large evaporation of water from the clothes that will dry it.

Very much like blowing cold air on a misty windscreen in a car to get the mist off it. The cold air is warmed by the windscreen and then it is absorbing the moisture off the windscreen. It takes longer to remove the mist from the windscreen with hot air as that already contains a lot of water vapor.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.