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re: fruit dehydrator
4 nov 1997
lap hoang   wrote:

>i would like to know if a greenhouse can be used as a commercial fruit
>dehydrator.

i suspect it can. i dried some tomatoes in a greenhouse this summer by
slicing them in about 1/4" slices, squeezing the juice out of the slices
by hand, and laying them on a screen door. it took about 3 days, but
fl has more sun than pa, and there was no overnight heat storage...

>i am drying fruit using a small ($60) dehydrator.  at 12o degree fareinheit,
>it took 24 hours.

it seems to me that 120 f is about the most you can expect in a plastic film
greenhouse, because polyethylene film has little "greenhouse effect," ie
it is almost transparent to longwave reradiation. if drying takes more than
8 hours, it seems helpful to store some daytime heat for the night, so drying
can continue during the night. i put 200 55 gallon drums full of water under
the benches in a 20x100' greenhouse, reducing the first winter's propane
heating bill from $8,000 to $1,500. one way to store heat for continuous
drying would be to build an insulated box inside the greenhouse containing
some sealed containers of water, and circulate greenhouse air or air from
a separate glazed cavity on the south side of the box through the box during
the day with a fan when the air outside the box is warmer than the air
in the box, and turn off the fan at night. the fruit to be dried would be
above the containers of water in the box.  

>how can i keep the greenhouse atm. low in humidity.

put some plastic film on the ground, under the groundcloth, to prevent
evaporation from the ground, and vent the greenhouse to the outside by
turning on a fan with a humidistat when the air becomes too humid, perhaps
through a duct with two concentric poly film tubes to make a condensing
counterflow air-air heat exchanger, with outside air entering through the
inner tube and inside air leaving via the outer ring of the duct.

nick



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