Making a Plenum Herb Dryer

We've used a number of dehydrators and dehydration methods:

open-air
solar dehydrator
old school metal box dehydrator
old school round plastic dehydrator
solar wood-kiln conversion
Tribest stainless-tray
Home-built dehydrator closet from reclaimed SIPS panesl

The last two years we used a combination of the Tribest and the SIPS close with dehumidifier, heater on thermostat, and fans.  It houses a bread rack and I built stainless mesh trays to fit.  This system has served us very well, but we need to scale up.  We were thinking of building out a large box (room) with a bunch of racks.  We may still do that, but we are also wanting to incorporate a plenum dryer for bulk herbs that don't require such delicate handling.  

Has anyone build a plenum herb dyhdrator before?  Not a massive commercial scale one - I'm thinking more like the size of a bath tub.  

Attaching pictures from The Organic Medicinal Herb Farmer, revised edition.

This is roughly what I'm aiming for, except perhaps somewhat smaller.

I'm looking for a design plan or schematic of sorts, or at least someone with experience weighing in on my emerging plans.



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Editing to add a little more context:



So a plenum dryer (to the extent of my understanding) is a box, entirely filled with plant matter, with airflow (and maybe heat?) introduced from underneath the heap.  So what you have is moisture egressing from the vegetative material, then being forced up and out of the surface of the pile.  It has to be turned/mixed up a couple times to make sure it's evenly dried.  It has to be totally full to work, as that is what creates air pressure moving up through the matter from beneath, rather than just finding a channel of flow through the least densely populated area.  I'm not sure, but it seems clear to me that these will work best in an area with low ambient humidity.

The reasons we're looking at implementing a plenum dryer:
1) It's a low-cost, efficient way to dry a larger quantity of herbs at once, with low real-estate and less labor
2) It seems like a way to use more of the harvest

I have a few questions:
1) Are there any mold considerations in "low-flow zones" of the box?
2) Is it common to finish drying in a more exacting method?

Beau, I'm interested in following your progress. We are moving in a different direction this year at our farm, focusing more on drying flowers than herbs. We've done a lot of the same methods as you, and I think we're going to build out a drying room.
I wonder if the bread racks could be adapted to the plenum method by using the available plastic covers for them and/or trays with sides, and a box fan at the base? Placed in a heat/humidity controlled room? It might be flexible use between either trays or plenum.

I'm reminded also of Alton Brown's "Good Eats" episode about dying herbs, where he used furnace filters as trays and one more as a lid, strapped to a box fan with bungees. Halfway through, he flipped the filter stack over and blew air through the other direction.

I grew hops last year for the first and while researching how best to handle and dry them I came across this video. The filter style that Kenneth mentions is at about 13 min mark and the style that may help you is at about 19 min. At least this is what your description sounds like to me... Hope this helps!

So, that sounds like a grain drying bin from any big ag farm.  The heat is to reduce the relative humidity on a LARGE scale. On your scale, I think a dehumidifier is still a better option.

I have a friend that dabbled in selling seeds. He built one for seeds, it was a room with a plywood plenum fed with an old furnace fan. The top of the plenum had holes to set bins of seeds in. I think they were grey busboy totes with holes drilled in the bottom and lined with muslin. Then he had a couple dehumidifiers and a window ac to dry the room as the air circulated through the seeds.

Kenneth Elwell wrote:I wonder if the bread racks could be adapted to the plenum method by using the available plastic covers for them and/or trays with sides, and a box fan at the base? Placed in a heat/humidity controlled room? It might be flexible use between either trays or plenum.

Part of the deal with the plenum is that there has to be enough mass of matter to slow down the air and create positive pressure beneath the vegetation.  So I'm having trouble imagining a sensible way to convert the bread rack to a box that can achieve a partial "air seal," while retaining its multi-use characteristics.

I'm reminded also of Alton Brown's "Good Eats" episode about dying herbs, where he used furnace filters as trays and one more as a lid, strapped to a box fan with bungees. Halfway through, he flipped the filter stack over and blew air through the other direction.

I'd like to see a picture.  It the vegetative matter enclosed on the sides?

I wrote:

Cheryl Loomans wrote:I grew hops last year for the first and while researching how best to handle and dry them I came across this video. The filter style that Kenneth mentions is at about 13 min mark and the style that may help you is at about 19 min. At least this is what your description sounds like to me... Hope this helps!

Neat overview.  The closest thing to what I'm going for the large- and semi-large scale commercial systems.  His box system is similar in some regards, but it doesn't achieve positive pressure due to the reversed air flow.  But it does give me some materials ideas.  Thanks!

A screenshot of the one of interest:


I checked out https://hopsharvester.com/hops-drying-floors, referenced in the video, and requested some info from them.

I'm curious about the function and manipulation of the gates.  Might not be operable for herby materials.  

I do know from the Carpenters' book that their box requires raking to stir/mix contents and achieve even dehydration.  My guess is that the yellow gates achieve this function with hops in the hopsharvester system, but that it is limited to hops and similar, and might not work with leafy stuff.

R Scott wrote:So, that sounds like a grain drying bin from any big ag farm.  The heat is to reduce the relative humidity on a LARGE scale. On your scale, I think a dehumidifier is still a better option.

Yes, exactly - a plenum dryer like big grain bins.  Although I am unclear as of yet whether there are differences and considerations for drying herbs rather than grain.  Regarding heat, that would be applied with a controlled thermostat - you certainly don't want to cook it, but staying near 90-100*F is optimal.  Doesn't take much heat, but some - yes.  I think if I set it up in a climate- and humidity-controlled space, I cant let the room treatment take care of the dehumidification.

I have a friend that dabbled in selling seeds. He built one for seeds, it was a room with a plywood plenum fed with an old furnace fan. The top of the plenum had holes to set bins of seeds in. I think they were grey busboy totes with holes drilled in the bottom and lined with muslin. Then he had a couple dehumidifiers and a window ac to dry the room as the air circulated through the seeds.

This is close to what I'm going for, except that I want to go at a slightly larger scale.  I want to be able to haul in 3-4 wheelbarrows of material, dump it all into the same bin, and let 'er rip.

It seems like most comparable systems until recently were indeed built as "hop oasts."  I'm ravenously looking for other instances of folks using this kind of thing for leafy herbs.

Here's another comparable hop oast at roughly the scale I'm considering, sourced from https://jhawkins54.typepad.com/files/hop-harvesting-and-processing-gorst-valley-hops.pdf

I wrote:


I checked out https://hopsharvester.com/hops-drying-floors, referenced in the video, and requested some info from them.

I'm curious about the function and manipulation of the gates.  Might not be operable for herby materials.  

I do know from the Carpenters' book that their box requires raking to stir/mix contents and achieve even dehydration.  My guess is that the yellow gates achieve this function with hops in the hopsharvester system, but that it is limited to hops and similar, and might not work with leafy stuff.

Found a picture of the guts of the hopsharvester gates.

I need confirmation, but it looks like maybe you dump a load into the top, then after some hours, open the top gate dropping/mixing the contents to the lower tier, then doing so once again, until it finally drops to a collection bin near the bottom, above the airflow entry.

Here's a smaller scale hops oast from instructables that's kind of like what the video guy above made, except that it's all in a vertical column.  It still won't produce positive pressure, as air can more easily escape the lower holes than the top vent, so I would duct air input directly into a hole at the bottom.

I question whether leafy stuff would work on trays like this.  It was my impression that leafy stuff needs to be all in one big gob, rather than on separate levels.  

The furnace filter/box fan method is indeed mentioned in the video Cheryl posted. It's really a budget hack of the round plastic dehydrator units, but also way easier for leafy stems when loading it up because of the form-factor. (a step towards the plenum dryer)
My take-aways from it are: 1) scalability by adding more filters/trays up to the limiting power of the fan 2) airflow is focused in the axial direction past the herbs, not leaking out the sides 3) the filter material is diffusing and restricting the airflow some, reducing the short-circuit effect of partially loaded screens 4) airflow is the key element when compared to static trays in a dehumidified enclosure/room. 5) material closest to the "dry" input air dries faster/more completely than the "exhaust" adjacent, so "stirring", flipping the stack, or running some of the trays longer are good ideas.

The Gorst hops oast picture you posted, as well as the next one in their presentation showing it "in a tent" with dehumidifier and heater seems scalable and adaptable to varying harvest amounts. Theirs was in 6, 12, 18, etc... harvest bin increments (since the base and lid fit a 2x3 bin matrix) but no reason you couldn't do 1x2 or 2x2 and stack it higher.
The instructable is basically the same idea with wood bins rather than harvest totes or filters, and is probably hobbled by the box fan's power... (The centrifugal fan seems better and they are easy to source)
The "hops floors" systems do seem suited to hops or something else "flowable" like maybe chamomile, or loose leaves, but not whole stems. The hops floors seem to be a "flow-through" system where a fresh batch of material is repeatedly added at the top while at the same time dry material exits from the bottom. (see my comment #5 above)
However, the "floors" do suggest that the mass need not be one unified mass, merely spread out evenly on each "floor" to prevent the air from channeling. So the bin stack should work fine.

The things I like about the harvest bin idea is handling. You can harvest directly into it, bring it in, and set it to drying, no other touches. You can pretty quickly unload/reload the dryer, and even store the dried herbs in the bins while waiting to garble them. That way your harvest, drying, and garbling processes aren't dependent on such exact scheduling, just the capacity of the dryer and enough bins. The bins are lightweight, stackable, and easily washed between uses, more can purchased ready to use if you need more, maybe even have a slot for a label?
The bin stacks in the "matrix" (1x2, 2x2, 2x3...) might also allow you to dry 2, 4, or 6 different herbs at once where each stack would be one herb/variety since the airflows are separated by the bin walls... though probably best if exhausting the moist air, rather than recirculating in a "tent" and making everything "minty" or "lemony-balmy". This exhausted, ambient air version could also be a good first step to get the bulk moisture out, before moving into a dehumidified space, to not swamp the system.