We have been cleaning dryer vents for over ten years and a 20' air hose with a reverse ball isn't going to cut it. Just like Ram Air's extraordinarily over priced duct cleaning package (hoses & nozzles and a plastic vent cover for $1,000) doesn't do a complete job of cleaning air ducts.
To Clean air ducts you need a good reverse air unit (abatement for example total investment around $10,000 minimum).
To clean dryer vents you need a good portable air compressor, reverse and forward air nozzles ( Google heat seal dryer vent cleaning kit) a 3 motor dryer vent vac ($1100-$3,600 and available through Nikro, Air care or Rovac systems) and a professional dryer vent brush kit (rovac is my favorite on the brush kit but Nikro has the best vac).
Don't even think of getting into dryer vent cleaning with just a 20' air hose from Ram air. Sooner or latter you will burn someones home down. They will have a dryer vent fire if you don't clean the entire vent and some vents are 36 feet long. I have done dryer vents in homes that run 55' long (a very bad situation especially if you aren't properly equipped to do the job right). Also sometimes you flat out can't remove the lent with out the 400 cfm dryer vacs because the lint is wet from condensation.