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safe materials for solar dehydrator?

 
Rob Seagrist
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posted 13 years ago
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Hi everyone, I want to build a solar dehydrator but I was wondering if OSB plywood would be safe to use for the frame? My gut tells me no since Im sure there are things in there that are toxic and shouldnt be fumed into our food. Are there any other materials that people have had good success with that are safe? I have glass for glazing, and I have window screens to put the food on for drying. I just need materials for the frame. Any help would be appreciated. Thanks!

permaculture wiki: www.permies.com/permaculture

 
                          
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Why not just use ordinary lumber?
 
Rob Seagrist
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I assume you mean just kiln dried lumber and not pressure treated? That is an option, I just don't know where to find it since most of the places around here all have pressure treated. Id like to be able to keep this unit outside as well, so I would have to seal the exterior somehow or use a tarp.

permaculture wiki: www.permies.com/permaculture

 
ronie dean
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Doesn't Paul have a video on YouTube where someone made a solar food dehydrator out of an old fridge?

Sometimes the answer is not to cross an old bridge, nor to burn it, but to build a better bridge.

 
tel jetson
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posted 13 years ago
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Rob S. aka Blitz wrote:
Id like to be able to keep this unit outside as well, so I would have to seal the exterior somehow or use a tarp.



put a little shed roof on it, too, and your sealing job should last a bit longer.

if you're going to build it out of ply-board, I suggest letting it run empty for quite a while to clear out any residual nastiness that might otherwise leak out of the wood and into your food.

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OSb in no way meets food grade lending itself to bacterial growth etc even through only incidental contact.

OSB has another trait for you to consider. It is really fantastic fighting water IF and it is a big IF, cuts must be resealed, something that very rarely happens no matter how critical it is to maintaining the product integrity.

When water enters from the edges, it causes de-lamination and the OSB falls apart quickly, you might want to use some other materials.

Professor of Thermal and Electrical Engineering, Welding/metallurgy: Licenses: PE license, Mechanical license Variety of other "certifications" from industry groups such as Refrigeration Service Engineers Society http://www.rses.org/, ASHRE http://www.ashrae.org/ Ect.

 
Anonymous
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I would not use OSB as it uses a lot of adhesives to hold the wood flakes together. OSB is particularly susceptible to absorbing water via cut edges but even the surface will absorb moisture. With common OSB that can lead to delamination.  Ask anyone who has had lots of rain while building with OSB subflooring before they completed the roof. It has happened to me. It is not waterproof.

Unfortunately most commonly available sheet type building products are man made using chemicals to bind the wood fibers, flakes or particles together, or to make a coating.  Definitely do not use any materials that are chemically treated against insects or decay like all pressure treated products.

Out of all the sheet products available plywood would likely be the best bet; it has more real wood, less glue.

How about sheet metal? The prefinished type that is used for roofing panels? It makes a suitable surface for collecting rainwater; suitable for food? 

There is a dehydrator video using an old refrigerator carcass. I don't have a link to it off hand though. I do believe the materials would be quite suitable for food handling and it's already made.

 
Rob Seagrist
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posted 13 years ago
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Thanks for the responses. I thought about the sheet metal as well in addition to plastic containers, but I am sure that they will even offgas.

permaculture wiki: www.permies.com/permaculture

 
ronie dean
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A lot of things don't require adding extra solar heat to dry. Nettles and other greens can be dried hanging in the shade (clothes line) on a dry day or even spread on newspapers in the house. Pick the nettles stem and all and hang on the line (looks a lot like hemp though so might want it out of site).

I dry mushrooms by cutting them and spreading them out on a table.

I believe, that most fruits that you want to dry, need a lot of air circulation even more than added heat.

I guess ,, if you want to make a solar heater for your drying, then find a box that will work first, (like solid wood or old kitchen range or some kinda metal tool box) then build a solar stack on top to heat and draw air in through the bottom. If you need a little solar heat then any kind of small glazing with a black surface could set low and in front of the air intake holes. You could easily overheat the fruits with too much incoming solar heat.

Sometimes the answer is not to cross an old bridge, nor to burn it, but to build a better bridge.

 
Rob Seagrist
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Yeah, understood about the heating aspect. My basic problem is that I can’t get things dry enough, fast enough. This is both due to volume of produce and not enough time. Generally for cherry type tomatoes I put them out in the sun on screens and it takes up to 2 full sunny days to get them dry but not scorched. If I could control the temperature better in a covered location that would help. Right now I have to bring all the screens in overnight if its going to rain.

permaculture wiki: www.permies.com/permaculture

 
ronie dean
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OK so are you saying that the tomatoes can be dried in direct sun? I've only dried tomatoes once and used an electric food dryer.

I am guessing that you want a large box to have the tomatoes in? So say 4'x4'x8'?

Sometimes the answer is not to cross an old bridge, nor to burn it, but to build a better bridge.

 
Rob Seagrist
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I use window screens that are about 3ft x 2ft. I just stack them on top of each other which provides a cover for the bottom of the middle sheets and then I have to put a screen on the top. It works well in the sun and they are quite tasty, but not all varieties will handle it. Speed of drying is definitely what I need. Obviously, this wont work for herbs and other sun sensitive plants. So the enclosed dark box with air circulation is key.

permaculture wiki: www.permies.com/permaculture

 
ronie dean
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Do you see what i mean by putting a solar stack on top of a box? You don't heat the box but rather draw warm air through the box by the heated air rising in a solar stack of some simple design. (Like a small  black chimney.)

Then if you want to heat the inflow air, just put something black by the intake hole and the summer sun heats it...If you want hotter than that, then lean an old window or storm window over the black thing...Hotter still --- enclose and insulate.

I think an old fridge or freezer would work, unless you want it so hot that the plastic off gasses like you mentioned. Or if an appliance is taboo in your yard... Then seems like you go with whatever acceptable box you can find or go with building out of plywood.

Sometimes the answer is not to cross an old bridge, nor to burn it, but to build a better bridge.

 
Rob Seagrist
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Yes I do get it, I think . If I did a stack on top of the box it seems that the hot air containing any offgassing would go out the top and draw in clean air from the bottom of the stack. I do have materials to build the solar part and the drying box, im just concerned that it will heat up so much that the gas will soak right into the food.

permaculture wiki: www.permies.com/permaculture

 
ronie dean
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I think i would be concerned of the plastic tainting the food too. So it seems that you might look around for a large box without plastic or else build one out of plywood.

I tried to remove the plastic from a fridge once and it is a lot of work..I gave up part way through. I guess it would save money, if you wanted to put all the time in to get the plastic out and didn't mind having a fridge in the yard.

Sometimes the answer is not to cross an old bridge, nor to burn it, but to build a better bridge.

 
ronie dean
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It occurred to me today that older ice machines were made from metal inside and out. A company that sells ice and has ice machines, may have old machines that no longer work. One might pick up an old ice machine for a few bucks. They have insulation in the sides too- so might be just right for a dehydrator.

Sometimes the answer is not to cross an old bridge, nor to burn it, but to build a better bridge.

 
Rob Seagrist
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Good idea. There is an ice place right down the road, Ill have to check. Thanks!

permaculture wiki: www.permies.com/permaculture

 
Larisa Walk
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Forget the box and don't stack the trays.  Check out our dryer that has been used for over 25 years in the humid upper Midwest.  Here's a link http://www.geopathfinder.com/9473.html

We've tried the box types, including one built out of an old refrigerator and the Rodale design built in a barrel shape.  None of these designs worked as well for us. 
 
ronie dean
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Walk wrote:
Forget the box and don't stack the trays.  Check out our dryer that has been used for over 25 years in the humid upper Midwest.  Here's a link http://www.geopathfinder.com/9473.html

We've tried the box types, including one built out of an old refrigerator and the Rodale design built in a barrel shape.  None of these designs worked as well for us. 



hmmmm Walk, what is the definition of a box? They look like boxes to me... A lot of great info at that link though...thnx 4 that.

Sometimes the answer is not to cross an old bridge, nor to burn it, but to build a better bridge.

 
Larisa Walk
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By "skip the box" I meant don't stack the trays.  Spread them out so the moisture being driven off the food doesn't have to rise through multiple trays.  The design also keeps sunlight from bleaching nutrients out of the food, which is essential for everything except mushrooms.  Mushrooms actually increase their vitamin D content with sun exposure during drying, especially if the gills are facing the sun.
 
Rob Seagrist
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Thanks for the info Walk, Ive been slowly reading it.

permaculture wiki: www.permies.com/permaculture

 
ronie dean
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Walk wrote:
By "skip the box" I meant don't stack the trays.  Spread them out so the moisture being driven off the food doesn't have to rise through multiple trays.  The design also keeps sunlight from bleaching nutrients out of the food, which is essential for everything except mushrooms.  Mushrooms actually increase their vitamin D content with sun exposure during drying, especially if the gills are facing the sun.



Excellent info...So, then, no plants benefit from direct sunlight when drying and fungi produce vitamin D from solar exposure..?   Then, I imagine, that one must watch the fungi closely when drying in the sun to prevent them from drying too much...?

And what about meats? Some sun OK?

Sometimes the answer is not to cross an old bridge, nor to burn it, but to build a better bridge.

 
Larisa Walk
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Veggies only need to be crispy dry and fruits will end up somewhat leathery in texture.  Leaving anything in the dryer longer than it needs can mean loss of quality.  Most vitamins are negatively affected by heat, light and oxygen.  In the dryer, you're cutting out the light but you still have heat and oxygen happening.  Those 2 factors are unavoidable in a dryer.

As for meat, we're vegan and have never dried any, either solar or otherwise.  But I tell workshop attendees that solar drying animal products is unwise as you would need dehydrating conditions for longer than a solar day can provide.  Spoilage organisms on meat and eggs can be deadly, so nights in the solar dryer could be a nightmare scenario.
 
Rob Seagrist
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Walk...
If I read that article correctly, the heat gets driven downwards through the material instead of bottom to top(apart from the reflective stuff below)?

Also, that page mentions a dehydrator could be as simple as a glass jar, do you have more info on that? Cheers.

permaculture wiki: www.permies.com/permaculture

 
ronie dean
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If you click on the link that takes you to the Virginia folk's pics, you can see that the 'dryer' gets hot enough to burn foods. 

I think with a little thought, it would be fairly easy to design the unit to be used as a solar cooker, for occasions when you are not using it as a food dryer.

----------------------------------------------------------------------------

I know you asked Walk the question, Rob, i re-read the part about the glass jars.

Quote GeoPathfinder:
Next in our nutrient-saving, energy-saving hierarchy is food drying, or dehydration. Solar-dried foods don't require fossil-fueled energy inputs to preserve them or store them, unlike refrigerators and freezers. Just a tightly sealed glass jar will do! No electricity required, just sunlight from a safe nuclear power source about 93 million miles away.


I think it means that they are storing the dried foods in the glass jars. Comparing the jar as a free storeage device compared to a fridge or freezer.

Sometimes the answer is not to cross an old bridge, nor to burn it, but to build a better bridge.

 
Rob Seagrist
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Ah yes, you are correct

This design makes me think that I could use the OSB I have for the solar collection, and then put a barrier between it and the food so all the gases go up and away. At least it wouldnt be direct or contained in the same space to permiate. Do you think that will work?

permaculture wiki: www.permies.com/permaculture

 
ronie dean
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Rob, I wouldn't use that OSB wood for anything - ever. I have Black Locust that i could use for legs for the benches and I think that old oak pallets could be recycled to use for some other parts. Right now I work so many hours that, time, to make things on the cheap is not priority for me. If i couldn't find a decent wood I would invest in the cedar. They mention that the stainless screens will last for generations - so skimping on the construction would make more work later to rebuild.

Sometimes the answer is not to cross an old bridge, nor to burn it, but to build a better bridge.

 
Rob Seagrist
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Alrighty, cedar it is

permaculture wiki: www.permies.com/permaculture

 
jason herrick
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Couple things i think could help....
Clay: it can be used to fill all kinds of gaps and sealed with glaze...also glaze is a paint...clay is in most paint...also ....
Indian Ink/black paint....dye: made from ash and water with a little oil....added to glaze makes black paint...work with it to get proper proportions to make a substance usable for your application.
Alternatives to paint and caulking.
 
jason herrick
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Linseed oil has been used to seal wood since beginning of time...cedar keeps buga away and weathers well. If you want your food to dry faster make a bigger solar collector and work with the exit for the air to keep a constant movement and increase are velocity....the more the air moves the better.
 
jason herrick
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Screens from windows have bpas and chemicals...stainless screens are preffered though metals are no better for u...put wax paper between racks/screwns and fruit....
 
Abe Connally
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Go Metal. It lasts longer, and it's easy to work with.

Here's my Solar Dryer: http://www.velacreations.com/food/preservation/item/36-solar-food-dryer.html

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