Ian, as Bob Salomon and George says a little fan would be necessary, at 20ºC 1000L of 100% water saturated air only contains around 17mg of water (17gr/m3), this is half a drop per L. If original air is at 50% humidity you need some 5 L of air for each drop. So some air forced circulation is interesting to not depend on the natural circulation across the HEPA filter.
https://www.engineeringtoolbox.com/m...ir-d_1403.html
Evaporation speed depends on temperature, air speed and air humidity. The most pratical way is that you simply mantain internal humidity low by injecting some fresh air. I'd use a fan from an old PC box.
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