Lint is likely to collect inside dryer vent duct of any type, and regular cleaning is good practice. Smooth walled duct is better than flex in this regard. The idea to surround the dryer vent with a larger hvac duct to act as a countercurrent heat exchanger is very simple to construct and allows duct cleaning as usual.
Good practice is to provide the dryer location with an outdoor air intake to equalize pressure. This works out, as the heat exchange duct will terminate at the dryer. In a mixed climate, we don't want to temper the incoming air in the cooling seasons. An alternate, parallel air intake without heat exchange can also be fed to the dryer location. Dampers in both intake ducts dictate whether dryer heat-exchange occurs or not. These could also be electronic dampers for automatic operation.
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