Wednesday, August 31, 2022

"Improvements" to the dehydrating set-up


Is there an engineer alive who cannot look at a simple, elegant system and think "I can make that better"?

Guilty as charged.

I moved the food dehydrator off of the porch which is on the north side of the house and put it on the picnic table in the center of the yard.

After scratching my head, I decided it really NEEDED an solar air pre-heater.

I had 16' of 4" diameter (5" OD), black, corrugated drain pipe. Black is good for collecting solar energy.

I did a little bit of measuring:

The opening for the air-intake in the top of the dehydrator is approximately 1-5/8". This is an inside-diameter

I looked through my scrap pile and found some steel tubing that used to support a soft-sided swimming pool. It has an outside diameter of approximately 1-7/16"

I cut a 5-1/2" long piece with the end on the slant like the pointy end of a hypodermic needle. That was so when I assembled the device I could not bottom out the edge of the short, white pipe to the inside wall of the black drain pipe.

Mediocre picture of the 5-1/2" long piece

Assembled black drain pipe and white adaptor. The open part of the slant cut is facing the long way.

Yes, I know there is crud on the outside of the drain pipe. There is also one other problem. It will suck air from the short end and not pull it from the solar heated section.

That was easy enough to fix with a plastic grocery bag and some crumpled up newspaper. Not much in the way of premium materials were used in this system and it was laid-up loose so I can fiddle with how it sits.

The next improvement will be to move the picnic table about 8' to the south so it is exposed to sunlight for a longer period during the day. You can see from the first picture that it is in the shade.

Incidental notes: 

The cooling for the electric motor is in the bottom of the unit and does not draw its cooling air from the top of the unit.

The unit draws 300W and the motor probably draws about 40W. So most of the energy draw is to heat the air swirled through the unit.

If the grid gets shaky, then it would be stupid to use solar generated electricity to make heat when you can get it directly from cheap, dumb plastic pipe and/or plastic film. 300W can drive 7, 40W fans but only one dehydrator.


  1. What is the resultant pre-heating benefit, as measured?

    1. Current ambient temperature, mid-afternoon is 79F. Temperature at inlet is 133 for a gain of about 50F.

      If you look at steam partial-pressures, 133F has a PP of about 16kPa and 79 has a PP of about 3kPa. So 133F air has the ability to hold about 5 times as much water, as vapor, than 79F air.

      These are first-order, back-of-envelop type approximations and you should not bet the rent on them.

  2. 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.

  3. That pre-heated air sounds like a good idea.

    I've been wanting to make a go at food dehydration myself, but our outside is hot (good !) and humid (not good). Still, I saw a link to dehydration unit from a box fan and it sounds just crazy enough to work.

  4. I'm and engineer too so we use a similar system on our dehydrators here in Arizona. Much simpler design; they are located in Arizona.

    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.

  5. Bug / dust filter on the open end of the pipe?

  6. guilty: That culvert should at least be insulated from the lawn. Maybe just a few 2x4s.

  7. Interesting fix, and makes sense!


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