portable boot dryer

Systems that use fans are not using pure heat to evaporate water The heat already in the air blown into the boots does some of the work for free. Even so if you could achieve 300% efficiency you still have 600g of battery per drying cycle. I think I dont mind wet boots

wow @Ian_H glad I've got you thinking! even if it doesn't work I'm not too fussed it's all part of it. I'm just gonna enjoy making, experimenting and having some fun tinkering with electronics. If it does end up working that a bonus and if not oh well. But like I said thanks for actually thinking about this problem and expressing your thoughts because something is better than nothing. I'll let you know how I get on. thanks a lot

I can't say I can add to any of the scientific/technical details that have been contributed here, but, I will say I think people are probably being a bit too negative on the idea. @jonah Keep in mind most of the regular posters here are experienced trampers, who often do multi-day (or longer trips). Minimising weight and unnecessary items is second nature. Putting up with some hardships such as wet boots are part and parcel of heading bush. However, most of these people (us) are at the pointy end of the tramping pyramid. There is a massive, broad base below of more casual trampers whose behaviour does not necessarily mirror that of the membership here. I have watched (and tramped) with enough people who will take their boots off to cross a river, put them back on to walk 50m then take them off again to cross the same river. Rinse — so to speak — and repeat all day. I've seen people pull glass casserole dishes out of their pack so they could carry their lasagne. I think these people would not mind carry some weight to keep their boots dry (provided the thermodynamics work). They are usually only doing overnighters, so, getting multiple uses out of the battery would not be such a big deal. Say what you want about Kathmandu but they didn't become the giant in the NZ outdoor market that they are by catering to the pointy end of the pyramid. Having said that, I would be more interested in the chemical-based alternative. I have no idea behind the science (and have no idea what it even is) of it but is there simply a way it could be "wrung out" to remove excess moisture? It probably does not need to loose all the water weight straight away as you could put up with a slight increase in weight for the trade off of dry boots! I don't even know if it would be necessary for it to even fully dry the boot, sometimes it is just so much nicer to put on a slightly damp boot than one that is soaking wet and cold. I must say this is not something I have thought about as I have just accepted that boots get wet and they are hard to dry (without destroying them up close to a fire). You then deal with that day after day after day... But, if you could design the right product that may not be the case. Worth experimenting with I think.

Though the chemical idea sounds attractive, my big question is how does it work? If it works like silica gel packets, by absorbing the water. Then that means you have to carry out 200g of water-laden pellets with you. Not ideal! So as pointed out above - it would need to be disposable or 'wringable out'. If it performs some reaction which results in water&chemical being converted into a gas, then what reaction & what gas? Long time since I did 3rd-form chemistry ... == I'd counter Ian's maths on the fan-based approach. As geeves says, the energy for the evapouration of water is _not_ supplied by the battery. It comes from the cooling of the air that passes over the wet boots. It is effectively an inside-out kalgoolie fridge. As with the kalgoolie fridge, no external input of energy is required for this solution. Thermal energy in the air is converted into the kinetic energy (latent heat) required for the change of state from liquid to gas. Your fan just needs to ensure that saturated air is removed and replaced by dry air - which instinct tells me will require much less energy that that suggested. The maths for that bear no resembelance to those supplied by Ian. Not saying they'll be any better!

Just out of interest has anyone ever weighed their boots wet and dry? Is 200g of water a realistic number? Thats 4/5 of a metric cup

>As geeves says, the energy for the evapouration of water is not supplied by the battery. It comes from the cooling of the air that passes over the wet boots. It is effectively an inside-out kalgoolie fridge. I agree that can be the case, but it depends on the temperature and relative humidity of the air you're working with. Say you have good drying conditions, 25° or more and low relative humidity. You just have to blow air through, the water evaporates, the evaporation cools the air to say 15° and your boots dry out. The energy to evaporate the water comes from the air going from 25° to 15° and the only thing on your device energy budget is the fan to move the air. But, say we're down at 15°, higher relative humidity. Blowing air through will evaporate some water if we're not at 100% humidity, but the cooling will increase the relative humidity and we'll quickly be at 100% relative humidity and get very little drying. We'll have to put some heat in, reducing the relative humidity and so the warmer air will absorb more water, cool down as it does so and exit the boot. If we heated the 15° air to say 25°, then it cooled back to 15° as it absorbed water, the air exiting the boot is the same temperature as the air entering, and the air hasn't supplied any heat at all. In this case, all the heat energy to evaporate the water must have come from our battery. The second scenario (cooler overnight, high relative humidity) was the one I was thinking of when calculating energy needs. I do agree energy needs could be a lot less if there is an external source of heat - warm dryish air, sunshine, hut wood stove, car heater while the car is running. On the other hand, colder and 100% humidity and the moist air exiting at above ambient temperature and your energy needs are even higher. Jonah - I'm not trying to discourage you, but I would encourage you to do some early and realistic experiments to see if you can achieve worthwhile drying at a reasonable energy budget. (And I'd be keen to hear the results!)

Cheers @Ian_H doing some tests now 😀👍