Thursday, September 1, 2022

From the Comments section of "Improvement...dehydrating setup"

I decided to put some of the comments into their own post because they were top-notch comments and should not be buried.

Anon contributed "Foil/metal reflectors around the pipe could also boost pretemp passively, and if designed proper be adjusted for movement of the sun for optimal capture."

ERJ responds: Yes, you are absolutely right. But I know my limitations. I can get sucked into pursuing "optimum" and risk never getting started. I banged together a system with materials on-hand. Incidentally, materials that are commonly available in most places, hence of interest to others.

Jonathan H. asked "What are your procedures for making sure your product is dried enough and packed properly to make sure it lasts?"

ERJ responds: I am still feeling my way along. I don't have a good answer. Maybe one of the other readers can give guidance or point to some websites or articles or videos they found useful.

 Anon (2) wrote "If possible, consider raising your inlet 6 - 8 ft off the ground in an open sunny area to reduce inlet humidity. When I've visited humid places, the moisture content seems much higher down low, and in shady areas.

PS - Arizona is horrible - everyone should stay away."

ERJ responds: Fabulous idea. I raided my stash of empty dog-food bags to isolate from the sod and propped the inlet on a chair. Incidentally, this drain pipe has the slits so it sucks in air along its entire length which made the portion resting on the grass a problem. Cool and humid.

Regarding Arizona, I liked the town of St Johns. It resonated with me. Otherwise I have to defer to your expertise.

Scrivener asked "Bug / dust filter on the open end of the pipe?"

ERJ responds: Not yet. Maybe never. Most bugs follow scents that flow with the wind so they are more likely to enter via the outlets on the dehydrator. It is not very dusty where I live. We had the foresight to purchase a property that was on the up-wind side of the gravel road.

Fido also voted for raising the pipe off of the grass "That culvert should at least be insulated from the lawn. Maybe just a few 2x4s."

ERJ responds: You bet. I have lots of empty dog-food bags so that is what I used.

Old NFO wrote "Interesting fix, and makes sense!"

ERJ responds: If I have one skill it is being able to see ways to repurpose common items to fill a need. Thanks for the comment.

Thanks for all of the great comments. I apologize for not always being good at recognizing them.


  1. What is the CFM requirement for the dehydrator and do you have a way of measuring static pressure?

    I'll assume the CFM is fairly low, say 10-15 CFM (none of the mfgs seem to list that spec online about their products ). The area of 4" pipe is ~12.5 sq in so at 15 CFM air speed is ~ 1.2 ft/min through the pipe so SP is, probably, nearly 0, at least not significantly different from no pipe at all. I'm also assuming non-perforated pipe because the goal has to be "maximum time duration of travel through the heated pipe to achieve maximum inlet air temperature" and perforations compromise that significantly, potentially completely. I wonder what the maximum pipe length would be for SP to increase significantly; I'm thinking a loose coil of pipe mounted on a 4X8 sheet of plywood and placed at the optimum angle for solar capture. And, while anything will get hotter in direct sunlight, rate of heat transfer through plastic is not optimum, other materials - specifically, good conductors of heat like aluminum or steel - will do much better. So....what's the length, and configuration, of highly temperature-conductive piping that would allow running just the dehydrator fan and not the heating element and would the capital expense to achieve that be more advantageous over the long term than the electricity cost of doing nothing? I can foresee circumstances in which a small solar-powered fan for the dehydrator may be more feasible than producing 120 volts at X amp at home, perhaps even the only way.

    Next, what pipe (and solar fan) arrangement would adequately support multiple dehydrators? As long as you're doing it might as well maximize the outputs.

    Random thoughts: Would some sort of rectangular flat-faced chamber placed to maximize solar heat gain supplant, or perhaps replace, some length of pipe as a way to provide a volume of heated air? And, could such a chamber also be used when heated artificially (small scrap wood fire) to allow dehydration to continue during periods of zero solar gain (eg. "night") ? And what is the maxmum heated air temperature suitable for good dehydration? Just as a baby cannot be produced in one month by getting nine women pregant, proper food dehydration is a time-fixed process.

    1. Flow rates are minimal, probably even less than you estimated. Most of the air recirculates with a little bit being burped out along the top, outer edge.

      Direction of circulation is up along the outsides, then inward,then downward at the center. If you visualize the flow lines you can imagine that the flow bends a sharp corner inward when it encounters the top. That produces a slightly higher-than-ambient pressure that pushes some air out of the unit with make-up air coming in the center hole in the top.

      If you are going to make a rectangular, solar collector then why not start with an old, upright refrigerator, remove the door and cover with clear or translucent plastic and paint the inside black. Then, add additional shelving and have a small muffin fan bringing in fresh air and a second fan creating swirl for internal air circulation.

      A reflector on the ground could add solar irradiation. Maximum solar radiation is about 1200W/M^2 so would probably do OK.

      Dead refrigerators are easy to come by since they don't seem to last as long as they used to.

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  2. "ERJ responds: If I have one skill it is being able to see ways to repurpose common items to fill a need. "

    I feel bad if I have to dispose of an item unless I have re-used it at least 3 times....


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