As many know, most heat pump (ventless) clothes dryers use R22, a HCFC that is about to be phased out. Once that happens, they will most likely have to switch to a HFC. However, it turns out that a HFC suitable for use in a clothes dryer is hard to find. R22 already has operating pressures in excess of 500PSI in a clothes dryer, so anything with a higher operating pressure will be impractical. That is, unless you use a new idea that can reduce operating temperatures without increasing drying time too much.
Why not pull a partial vacuum (25" Hg or so should be easy to achieve with a very simple vacuum pump) on the chamber before drying so water boils at a lower temperature? That corresponds to lower refrigerant pressures.