18 replies to this topic

[Phoenix]

I just finished building a drying cupboard this morning, and so far it seems to be working pretty well. I searched for a while to find a small dehumidifier, to save space, money and energy, since a normal 8 litre one is way overkill for the size of my cupboard.

For a long time the only type I could find that was the size I wanted was basically a box of desiccant with a fan on it, which you plug in to dry out every time it becomes saturated. Something about having to charge it for 10 hours every couple of batches of stars just put me off these. I did eventually find a miniature dehumidifier, but it cost ?90.00.

However, after a while, I came across a "Peltier" module, which I'd never heard of before. What is it? A thermocouple in reverse. Run a current through it, one end gets hot, the other gets cold. A Peltier dehumidifier is free of any compressor or refrigerant, and is therefore much smaller and cheaper than a conventional one. The one I got is rated at 65 Watts, removes 380ml per day, and would fit comfortably in a shoe box. Cost: ?40.00. Perfect.

My drying cabinet is about 45x45x50cm. Since I usually make stars in <500g batches, this is plenty big enough. The dehumidifier itself keeps this sized cupboard warm without any need for light bulbs or other additional heating. I suspect that Peltier dehumidifiers are somewhat less energy efficient than regular ones (ie convert a greater percentage of the supplied energy directly to heat, rather than using it to dry the air) which would explain why they are not more commonly used, but in the case of a drying cupboard, where heating is desired anyway, this is not a problem.

I've had it running this afternoon with a few comets in it, which seem to be making very good progress towards dryness, but I haven't really had the chance to fully evaluate whether or not it works yet. Seems to though...

[alany]

I had the same thought a few days back: I have a few Peltier devices here, a pair of heatsinks, a suitable PSU, a small fan and a box should be all that is needed.

I'd build it with the peltier device in the wall, a heatsink each side, the cold one inside the chamber, the hot one outside (maybe with fan cooling). Then a drip tray under the internal one to catch the collected moisture. A fan to circulate the air, a light bulb to warm it, some runners for screens, a door with a bit of sealing around it to improve efficiency.

The heat from the bulb would have to be tuned somewhat to keep the temperature inside warm, but not deliver more heat than the peltier could deal with, if the internal heat sink temperature rises above the dew point it won't condense water any more.

I have a peltier fridge that condensates really badly, that's where I got the idea from. In fact I could probably just use it as-is.

[Phoenix]

The dehumidifier I have uses the air that's just passed over the cold sink to cool the hot one, so warm dry air comes out. I had it running for about 3 hours yesterday, and the box got pretty warm (don't know exactly, but I'll put a thermometer in it today) and it seemed to be working fine.

I'm not sure (ie I'm just theorising about something I don't really understand) but at sensible temperatures it shouldn't stop working as it warms up, as the cold sink should be (from what I've read) about 25* colder than the hot sink, so should always be colder than the interior of the drying box (say 8* colder - total guess). So if the relative humidity in the box stays above a certain level, water will continue to condense onto the cold sink. Even if this is not the case, mine still seemed to be working with the hot sink inside the box, thus eliminating the need for a separate heat source.

Also, drying cabinets with a conventional refrigerant dehumidifier have the hot sink of that inside the cabinet, and they work, so the same should be true of Peltier dehumidifiers.

Edited by Phoenix, 05 January 2005 - 02:55 PM.

[alany]

Yes conventional driers are a closed system, they achieve some kind of equilibrium eventually with their losses through the walls. If you insulated one the temperature would rise quite high from the accumulation of the dehumidifier's waste heat.

I can't find a whole lot of data online about dehumidification, but I'd imagine the delta-T at the surface of the cold plate is pretty critical. If so it would be best if you could physically seperate the condenser from the evaporator, using the condenser to heat the airstream before it passes through the drying space. Any losses through the walls would then improve the temperature difference at the evaporator/cold plate.

I'd imagine the dehumidifier is designed to minimise radiative and conductive coupling beween the hot and cold sides already so perhaps it wouldn't make a huge improvement. The ones I've seen in the past had a squirel cage blower that sucked in air over the cold plate and then blew it over the condenser on the way out, there wasn't a lot of insulation between them just a sheet of foam on the wall in the "J" shaped air path.

[Phoenix]

I left a thermometer in the cabinet. After a few hours running it read 22*C. That is without any insulation on the 5mm hardboard walls. If I run into any convenient bits of polystyrene I might stick them to the sides of it to increase the temperature inside, and hope this doesn't cause any problems. The Passfire dehumidifier article says "...a light bulb is used to heat the air to about 100 degrees..." 22*C is about 70*F, so I've got a way to go yet, but I don't think I'll bother with any additional heat sources, I'll just try to make the best use of the heat produced by the dehumidifier. Waste not want not...

[mnementh]

Hi guys,
My work involves communications and radar kit, some of which is military usage. We need to test the kit at high and low temps. and Peltier devices were put forward as an easy hot/cold source. Our prior hot source was simply a hot plate. No hassle, dead easy. The cold side however was another thing altogether. Big chiller cabinets with holes in side and top to allow leads and waveguide to enter, to connect to the units under test.

However, it was quickly found that Peltier devices had one MAJOR drawback. Namely that the heating effect is much more efficient than the cooling.

These devices, basically, consist of a load of material specific electronic diodes in a series, parallel combination to give a trade off between current and voltage. In any case, they can dissipate between 20 and 70 watts, for the more common ones and the watts put in HAVE to go somewhere.

It, in fact, heads out the hot side, in addition to the heat pump effect of the device. So, at 20C ambient, in a device that might show a 40 degree difference between the two plates, you don't actually get one side at +40C and the other at 0C, what you get is the hot side at ~+55C and the cold side at +15C.

Still a 40C difference but water pressure at +15C is appreciable while that at 0C is nil.

Your Peltier dehumidifier will work and work well but only if you don't recycle the cooled air over the hot side of the device as the above effect will "kick" in.

What you need to do, unfortunately, is chuck away the heat from the hot side and use another method to re-heat the cooled air, light bulbs run at half power, perhaps, in the drying chamber.

This will still result in a minimilist approach and should work well.

HTH,
Sandy

[Phoenix]

The dehumidifier itself is designed to pass the newly cooled air over the hot sink. I haven't actually modified the dehumidifier at all - just boxed it up.

I admit I don't quite understand what your saying. Surely heat is heat, whether it comes from the energy wasted by the dehumidifier or a light bulb.

In the set-up I have, the damp air is first passed over the cold sink, where it is cooled and the water in it condenses out. The heat from the air is moved by the Peltier unit to the hot sink, along with quite a lot of additional heat which results from power wasted by the Peltier mechanism. The cold dry air then passes over the hot sink, picking up the heat in originally came in with, plus the extra heat produced by the Peltier unit. It then leaves the dehumidifier dryer and warmer than it was when it came in (ideal for drying stars).

Obviously, in a closed system, the extra heat produced by the dehumidifier will build up and the temperature will continue to increase. However, my drying cabinet is not a closed system - it's a wooden box. Therefore there is a point (apparently about 22*C) at which the heat lost from the box = 65 Watts - the same as the heat being put in, so the temperature remains constant.

I don't see what would be gained by dumping the heat (original and newly created) from the hot sink outside the drying cupboard, then replacing it with "new heat" from light bulbs would be. In my case, it would also mean dismantling and extensively rearranging the dehumidifier.

By the way, I just noticed that the instruction books says something to the effect of "Removes 380ml per day at 30*C and 80% relative humidity," so I doubt that it's in any danger of overheating.

Edited by Phoenix, 09 January 2005 - 05:27 PM.

[mnementh]

Hi Phoenix,
given your pretty low stabilisation point, you probably won't have a problem, as the cold side of the Peltier device will, hopefully, be near or below zero.

I should have added, in my previous post, that you can, by all means recover part of the heat from the hot side but the problem REALLY happens when the chamber is very efficient at retaining heat, so if you soak up ALL the hot sides output, the chamber will continue to get hotter untill a stable gain/loss state occurs.

If the chamber is heat efficient, that point is liable to be +70, +80 degrees C. Great for drying out your mixture/stars but the cold side of the Peltier might only be down at +20, +30 deg. C. Not very good to condense water out.

Peltiers work strictly as a "difference" engine. If the hot side temp. is X, then the cold side temp. will "normally" be X - C (where C is a constant). If C = 30, for example, then if X = 80, cold side temp. will be 50. If X = 40, then cold side temp. will be 10, etc.

Anyway, enough theory. Best way is to experiment and see what works for you. Best of luck with your project.

Sandy

[Phoenix]

Thanks, I see what you mean now. I understood that the Peltier unit is a difference engine, but I just didn't get why you wanted to use light bulbs to heat the cubpoard, but dump heat outside. However, it just clicked - if the hot sink is outside, the cold sink will be X*C colder than outside, rather than X*C colder than inside, (which it would be if the hot sink was inside the cabinet) so the temperature of the cold sink would be independant of the air temperature inside the cupboard.

However, in practice, the temperature seems to become stable at around 22*C, and since the dehumidifier is apparently designed to operate normally at about 30*C, then I can get away with having the hot sink inside the cabinet, and it seems to work well.

Edited by Phoenix, 08 January 2005 - 07:29 PM.

[Phoenix]

Hi,

Just an update on the performance of the dryer: I've been drying a couple of hundred grams of aluminium streamer stars. They were cut, 15mm on a side.

I've been running the dryer on and off for about three days (it's probably been on for about 1 day in total), and they're very nearly dry.

I didn't build it as a high performance dryer to get stars dry fast, I built it so it would be possible for me to make stars in the winter. I'm quite happy with the rate at which they've dried.

I'll let you know when they are fully dried, but since the dehumidifier is supposed to be able to take the humidity down to about 40% I don't foresee any problems.

[seymour]

I am planing on making myself a drying cabinet. I can have it in two places, in the house and in my shed, but I dont want live comps to be kept in the house, but nor do I want a potential ignition source in my shed!

What to do, what to do.

Does anyone have any ideas? Suggestions?

[ProfHawking]

better in the shed. less expensive to rebuild. That, or put it in its own minished. Built carefully, a drying box isnt really an ignition source.

I would go the peltier route. but it does seem pointless to use a bulb to heat it inside when the peltier heat is dissapating outside.
what i would do is employ heatpipes to move a little bit of the heat back inside the box away from the coldsink

[Phoenix]

I seem to remember that it took me a while to "get" it, but there is in fact a reason for having the hot sink (ie the side of the unit that heat is moved to) outside the box. The reason is that the cold sink will only get x degrees colder than the hot sink. If the hot sink is inside the box (where the ambient temperature is high) it is difficult for it to lose heat to the air, so it will be at a high temperature. Consequently, the cold sink will be warmer too, so moisture will not be condensed onto it as readily.

If, however, the hot sink is outside the box, where the ambient temperature is low, then the cold sink inside the box will be x degrees colder than the outside air - i.e. colder than it would have been were the hot sink inside the cupboard. For example:

Hot sink inside box:
- Outside air temperature: 15*C
- Temperature in drying box: 35*C
- Temperature of Peltier hot sink: 35 + 5 = 40*C (the 35 is the ambient air temperature around the hot sink - i.e. the air temperature inside the box, the 5 is the temperature rise caused by the heat flowing into the hot sink, from the cold)
- Temperature of Peltier cold sink: 40 - 35 = 5*C (the 40 is the temperature of the hot sink and the 35 is x, the temperature difference between the hot and cold sink).

Hot sink outside box:
- Outside air temperature: 15*C
- Temperature in drying box: 35*C
- Temperature of Peltier hot sink: 15 + 5 = 20*C (the 15 is again the ambient air temperature around the hot sink - but is this time the air temperature outside the box, the 5 is again the temperature rise caused by the heat being pumped into the hot sink)
- Temperature of Peltier cold sink: 20 - 35 = -15*C (the 20 is the temperature of the hot sink, the 35 is again x)

As you can see, the temperature in the box is the same (35*C), and temperature gradient the Peltier unit is able to produce is the same (35C* - yes, they should be that way round in this case) but the temperature of the cold sink is much lower in the second case. Obviously, having it at -15*C would not be good, as it would ice up, but this was just to illustrate my point. I know that I have over-simplified some of the numbers, but for the purpose of this explanation, that doesn't matter.

HOWEVER - despite this, I wouldn't recommend dismantling your dehumidifier in order to do this, as it would be a lot of work for a small improvement in dehumidifying power, and in fact may well cause a loss in overall drying efficiency (i.e. units of electricity used per batch of stars dried) since you would have to heat the inside of the box independently whilst dumping heat outside. I have simply built my drier in the "usual? way - with the un-modified dehumidifier fixed in the box, and the air ducted over the stars. I've been using it like this set-up for a while now, with no additional heating other than the 65 Watts supplied by the dehumidifier as waste heat, and it works fine.

BTW, I don't see why the same principles outlined above wouldn't apply to a "normal" compressor based dehumidifier too.

[JamesH]

Here are a few photos of a drying cabinet i built. I also find there is no need for an additional heat source such as a light bulb since the dehumidifier heats the air in the box up to about 40 deg C.

http://i11.photobuck...22/168af6dc.jpg
http://i11.photobuck...22/2d3ce79b.jpg
http://i11.photobuck...22/9dcf0ad7.jpg