I let the first test fruits dry over one more night bringing the run to about 36 hours. However, I didn't notice much of a change over the last 12 hours, so the fruit was pretty much finished drying in 24 hours. Commercial units claim to work faster(nesco dehydrator manual), so I think there is room for improvement. First I'll list some observations, then some changes I'd like to try.
- The heater was probably under powered because the air coming out was just barely warmer than ambient and the inside of the box was only heated maybe one or two degrees.
- The fruit on the lower rack seemed to dry a little faster. Maybe it's because it was a little warmer close to the heater, or maybe it's because the air passed over it first. Anyway, the difference wasn't so dramatic.
- The filter was more restricting than I would like. There was a very big difference in flow with the door oven vs. closed. I think that more air flow would greatly improve drying ability, but would also make it less efficient to heat. There's probably an ideal balance that could be found with experimentation.
Here are some things I want to change for the second test.
- Double the heater power. The 150w heater is actually two 300w elements in series, so I'll just move the wires to only use one. Heat transfer won't be as good, but it might work well enough. Another idea would be to use a couple big diodes to send half the AC wave through one element and half through the other. Then I would have 300w with all of the element surface area.
- A much less restrictive filter. A piece of window screen would keep out the bugs and big stuff, but dust would go right through. If I use the same filter element(one ply of a surgical mask), but with a much larger surface area, I would get more flow and good dust filtering. Unfortunately This would require an attachment that would increase size and complexity. Ultimately I would like to go with the attachment, but temporarily I will put on a screen.
To summarize, I will rewire the heater to use 300w and temporarily replace the filter with window screen.