I have been working with ECHO East Africa to develop a passive dryer that can dehydrate food safely and effectively in our climate in coastal Tanga, Tanzania, which is hot and very humid. If we can make it work here, the dryers will probably work anywhere with sunny days and a tropical climate.
We have had good results with direct dryers where the food is exposed to the sun. But knowing that direct exposure to the sun may damage nutrients in the food, we have also been trying to improve our indirect dryer in which the food is not exposed to the sun.
The indirect dryer is based on plans developed at Appalachian State University. Our most recent attempt can be seen below.
This dryer has at times gone to 55 C in testing without food in it, even though the collector area does not have two features that Appalachian State work said increased performance. Our collector does not have the tin foil lining and multiple layers of expanded metal for the absorber. To keep the cost down, this model just uses two layers of fiberglass screen for the absorber.
Despite the good temperatures when empty, this dryer has a difficult time drying wet foods like mangoes in the humid conditions here. This leads to questions for anyone who might have experience with this kind of dryer:
On this dryer and others like it, I have observed that the natural draft is very sensitive to wind speed and direction. The upper air vents are at the top of the dryer under the edge of the roof and the openings can be adjusted. Does anyone have any experience with “tuning” these vents based on wind direction?
In previous trials with dryers like this, I experimented with different chimney configurations, but did not see a dramatic improvement in performance.Do you have any ideas on how to modify the dryer to make it more adaptable to minor changes in weather patterns such as wind direction?
I think some of the variability I saw with the other indirect dryers came because airflow could bypass the trays. That bypass is now limited in this new model, but I am wondering if the screen size on the tray is too restrictive. We are using a lightweight expanded metal screen with a layer of lightweight window screen on top. If you have experience with this kind of food dryer, what kind of and size of screen are you using in the trays?