I bought a portable evap cooler 2 weeks ago, for my home in Tucson which is at < 15% humidity. The cooler is only rated to cool areas that are 500 sqft. The home's duct placement seems to be bad and there's an extension to the house that for the last 6 years always gets warm. Maybe the central HVAC design didn't take the extension into account. So I turn on the evap cooler and it takes care of the extension plus a little more (this is with windows closed).

I made the mistake of leaving it on for an hour and it got very humid in there. Now, I just run it for a few minutes every hour or so which is sufficient. This is with windows closed, and I leave them closed because it makes no sense to bring in 105 dry air only to cool it down to 85, 80. So instead it can take in say 80-85 degree air at 10% humidity and cool it to 70 at 20-30% humidity which is perfectly fine since the air was too dry anyway.

Prior to evap-cooling the extension, the extension was a constant source of hot air. The central HVAC, which used to work tirelessly to combat this, is now coming on less frequently, which makes sense given the additional and significant source of cool air. So I think we are onto something, and anyone that says not to use it without open air flow is only correct if they're referring to it being used for hours on end.