This tek took about 1 month to perfect. A lot of trial and error. I also used Solidworks to work out the wind turbulents. Unfortunately, I could not get it to work right with a single fan, the only way I was able to get good steady vertical air flow was with 2 fans.
Here is the finished product in action:

Inside are three trays of mushrooms with air blowing across the sides of the fruits with air blowing up from the bottom as well. This will dry 3 shelves of shrooms in about a day tops.
To get started, lets discuss getting your 2 fans together and testing your power supplies you have laying around. If you need a power supply, you want a 12V DC, 1A or more supply. You can google these and usually they are under $10. You'll also need a voltmeter to make sure you have the best powersupply/fan combination to have adequate air flow. Also, fan noise was taken into account with this design, so I recommend you choose at least 90mm fans. You want to use CPU fans for this, meaning they should be at least capable of 2000-2500 rpm. For your power supply, before you go and cut the ends off a supply, realize that very rarely do supplies actually output what they say they will. You'll need a supply thats 12VDC, at least 1A output. I recommend using your existing voltmeter, or buying one for $3-$6 online, or at a science surplus store. I bought mine for $3 from American Science and Surplus. You can pick up CPU fans from frozencpu.com for about $5-$10, or just use 2 that you have lying around that are identical. To test, look at the fan and look at it's voltage and amperage rating. If it is a onboard CPU fan, it'll be rated at 12V and have an amperage around 250-500ma. To test the draw, plug in your supply, and remember that on the fan the black wire is ground, and the yellow wire is positive. If the fan is a multispeed, it'll have 3 or 4 wires, these extra wires can simply be cut off.
First lets look at the amperage draw of your fan. If the power supply is center-positive, put your red lead from the voltmeter into the center, and connect the black lead to the yellow wire. Take the black wire from the fan and touch that to the outside of the power supply connector and look at the draw in amps from your meter. You'll be surprised when you see it'll only draw about half what the fan is rated for. What you are looking for here is speed. You want the supply that provides enough power for the fan so that it'll operate around 2000 rpm. Also, you can simply test the power supplies voltage by connecting your red lead to the center hole on the connector if it is a center-positive supply and touching the black lead to the outer shell of the connector and switching your meter to volts, also don't forget to move the positive lead from the amperage hole on the meter to the voltage hole (they usually are separate, with COM/black, being the one lead that always stays the same). If you read more than 12V, like 16V or higher, no worries, a lot of supplies are designed to put out a higher voltage only to supply 12V when a drop occurs by connecting something to it. Generally, the supplies with the higher voltages will give you better fan speed. Once you have found the right power supply for your fans, which creates good speed, it's time to begin. First off, remember that we need to connect the fans in parallel. This means that you first tie the two black wires together and the 2 yellow wires together in bundles, then take the black wire from the power supply with the white strip on it and connect that to the bundled yellow wires, and the solid black wire from the supply and tie that to the bundled black wires from the fans. Also important to remember, we will be using a tapered sterilite tub and attaching the fans to the underside of the lid, cutting holes into the top, so the air is blowing out of the box.
Ok, lets look at what we'll need to get started:


What you see here is some galvanized rabbit screen from the hardware store with some wire snips to cut it and a box of 100 5/8" screw in hooks. Those are about $4 a box. I used a 56qt/53L sterilite tub which has really nice tapered sides, this makes getting the trays in and out very easy. Make sure your roll of screen is longer than the tub, this way you only have to use a little bit of screen for each tray. You'll notice that the end of the roll is folded underneath so that there aren't any pointy ends of wire hanging off the roll. This will come in handy later since this also adds strength to the screen when you make your shelves. They just drop in and pull out. What else is nice is that all the verm/perlite/etc particles just drop off the fruits and land on the bottom of the tub. I just clean mine out once a month or so.
The first step is to fasten the fans to the underside of the lid. This is made harder by the fact that we're dealing with PolyPropylene tubs, for more information, take a look at the different aspects of fastening things to PP using adhesives in my TEK on Using and Maintaining Sterilite and Rubbermaid tubs: Using/Maintaining Sterilite and Rubbermaid Tubs
For my fans, I chose the oxidation method along with both regular plastic epoxy and some stuffit foam since my fans were stock socket 775 .45A fans that were round with notched handles on the sides. So, I had to cut out circles for the fans, and notches for the extra plastic on the fans, and I filled in those notches with stuffit. I chose the Dremel method of cutting holes from the TEK. So, to get started, whether you have round or square fans, you want to place the center points half way from the center point on the lid (look at the top of the lid, there is a plastic molding point in the exact center), to the edge of the lid the long way like in the final product picture above. Using a permanent marker, go ahead and mark the circles on the bottom of the lid where the fans will go. So, for a 90mm fan, your circle will be 90mm in diameter. Once the circles are made, go ahead and cut out the holes:

Now you are ready to attach the fans and fix up the cabling so we can do a basic air pressure test. Using one of the adhesive methods in my tek above, fasten the fans to the bottom side of the tub's lid:

Now, if you have notches from stock Intel fans, etc, go ahead and apply stuffit to seal any edges which are showing, so the seal is perfect:

You'll also notice in the above picture, I've already bundled and soldered the wires as described above, don't solder just yet. Just remember, the striped lead from the power supply is positive and the solid black lead is negative:

You want to drill the smallest hole possible in the corner if the tub to let in your power supply wires. Then you can pull it a little tight and fasten the wires together. Remember, yellow bundle to black stripped wire, black wire to black bundle of wires from fans. Here is what the underside of the lid should look like once you've soldered and ran you wires through the little hole and taped up the soldered wires:


Now, lets do a quick air pressure test, lets put the lid on top of the box and make about 2" of room or so at one of the narrow ends, so air will be sucked into that area, and take a piece of regular paper and see if the box will hold it to the opened area by the very end of the paper, this will show that the box has a great vacuum created by the fans when the power supply is plugged in:

Above, the entire sheet of paper is held upright just by the end of the box, so we have a good vacuum going.
Now it's time to go ahead and build our shelves and drill the holes. To keep from cracking the box when drilling your holed, please refer to my tek above. This is important, or you'll have cracks everywhere when you drill your air holes. This is also where all the math comes into play. Before we get into the math, lets start by creating our shelves and shelf holders. First thing to do is take your 5/8" hooks and straighten them out a bit so if you visualize screwing the hook into the inner side of the tub, and dropping a screen on top of the hook, we want the hook opened enough to allow the screen holes to go over the hook. Now, you can leave a little hook left in the eye of the loop so that you can squeeze the sides of the tub, slide in the screen, let the hooks grab onto the screen, and un-squeeze letting the hooks pull the screen taunt. This works really really well. So, to prepare our hooks, use needle nose plyers and another set to hold the hooks by the screw part:

The first few will be hard, but then they only take a couple seconds per hook once you get the hang of it. Now, we want to make horizontal lines along the outside of the tub where we will drill our tiny holes to act as guides for the hooks to screw into. Starting at the bottom, use a tape measure and measure up 3 inches from the platform the tub is sitting on and have the tape flat against the tapered side of the tub and make a mark:

You can see the black dots I'm making here every couple of inches around the tub. Then take a ruler, angle iron, etc and make a straight line connecting the dots:


And measure up 6 inches and 9 inches and do the same:

Now we have three lines going all the way around the tub. Let's start with the first line on the bottom only, go ahead and take a drill bit just smaller than the diameter of the screw part of the hook and drill holes for your hooks along the line spacing them about 4-5" apart, then screw in your hooks with the open side of the hook facing up.
Now it's time to fashion your first shelf. Go ahead and take your measure tape and measure the distance across from one side of the tub to the other on the first line three inches above from the bottom. Add 2 inches to this number and cut that much off the roll of screen. Now, since the screen is longer than the tub, go ahead and measure the inside length of the tub and cut off as much as necessary from the end of the screen so that it'll fit all the way down to the first line length wise. You want to snip as close as possible to the horizontal strand of galvanized steel so that you don't have pointy spikes of steel hanging off the long end of the shelf. Now, since you cut an extra 2" from the roll, and the first couple inches of the roll are already folded under the screen, you are now going to fold the 2" under the screen on the same side as the other side of the screen already is. I did this with a steel angle iron, you can also use a metal or wood yard stick. The trick is to place the screen on a flat surface and fold the screen against the stick or iron:

Once it it folded, take a hammer and hammer down the screen so that the fold is perfectly flat. Now you have your first re-inforced shelf. Go ahead and drop it in the tub attaching it to the hooks:


You should have a good flat snug fit. Now it's time to start drilling holes and doing the basic math. What don't want is to put too much negative pressure onto the fans so that they wear out quickly. We want this tub to last for years. Since the fans are brushless, they will, and in order to make sure, we want to deliver the same approximate air volume through the holes in square inches per second as the fans themselves. Or slightly more. We first calculate the surface area of the fans. This is:
2 * 3.1415926 * (1/2D)^2 Where D=Diameter. So for a 90mm fan, this would be: 12723.45 mm^2. So, since a 1/2" hole is 126.68 mm^2 we would need to drill exactly 100 holes to perfectly match the surface area of the fans. If we use a 3/8" bit, that would be 178 holes, so you can quickly see how big a difference the drill bit size makes. From trial and error and some Solidworks work, the best bit to use is either 1/2" for fans greater than or equal to 100mm and a 3/8" bit for fans less than or equal to 100mm. Now, you want to drill approximately 75% of the holes needed to get really good air flow. So, for a 90mm fan, we'll drill 133 3/8" holes. To get started, stagger 25 or so holes below the 3" line. Stagger them between the bottom and just below the line itself. Next, we are going to take the remainder number of holes and divide that by 3 and drill them evenly spaced right along the lines so that the bottom of the holes line up with the lines drawn. This will mean that air will blow directly across the top of our screen shelf across the fruits in the chamber. So, for our 90mm fans, lets go ahead and drill about 35 holes evenly spaced around the top of our line with the screen out. Once the screen is back in, it will look something like this:


You can see how the air will now be blowing directly in from the sides and the staggered holes on the bottom will bring the air in the bottom and create a inward vertical airstream floating up through the chamber and out the top of the unit. Once all the shelves are filled with fruits and the unit is on, here is what it will look like in action:

Typically, it takes a day or so to dry out fruits. The unit is relatively quiet and doesn't use much energy at all.
Enjoy!
-------------------- .... Always curious, always looking for better ways, mind always wonders .... TRUE HEPA Filtered Air Pumps TEK Everything you need to know about tubs TEK Fast Tub Dehydrator TEK
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