Eric Reed

I decided to calculate if a change in temperature would be enough to counter the change in humidity in relation to air density. Playing around with the numbers it turns out that it is mostly a wash. less than a 1% difference in density. It most likely makes little difference if you use a up or down draft design. I would need to measure exact temperatures and humidity levels to determine if my estimates were close.

I used:
20% humidity increased to 30%
115 deg F cooled to 100 deg F

Same pressure both cases

While it is true that moist air is less dense than dry air at the same temperature, the phase change from liquid to gas removes some energy from the air. This loss of energy reduces the air temperature, thus increasing its density. The cooler/humid air will then sink. My downdraft dehydrator heats up to about 130 deg. F in the winter, and can dry cherry tomatoes cut in half in 2 days. I have yet to try it out in the summer months. I do have it sealed very tight with latches that pull the door shut. I get less air movement when it is empty or when the food has dried out.

Another Simulation

See attachments for some Quick CFD work I did on RMH. This could be a easy way to test out modifications without building a prototype. A limitation I have with this tool is I have to estimate the volume flow rate after the initial combustion and I can't simulate the added gasses from the secondary combustion in the vorticities.