Begging your pardon, but there ain't nothin' between the burner at the top and the load going round and round but the perforated cylinder in commercial gas dryers. If there were, you would need one exhaust for the burner and another for the dryer. If you can observe a gas commercial dryer in operation, you can see the flame pulled down by the blower. Hamilton's first gas dryer for home use used a burner to heat a radiant plate positioned where the electric element would be in an electric dryer. This model, and we have seen two, had a little round flue in the top of the machine, near the back corner with a little cone-shaped draft diverter-like cap on it to vent the burner and the used the regular dryer air flow system to circulate air through the drying chamber. It was tried and abandoned. There is currently no domestic gas clothes dryer that uses heat exchanger technology.
As for dry cleaning, the modern cleaner-extractor-dryer is one machine like a giant combo for PERC or CO2 and operates in a closed condensing system to recover the solvent. Most are totally electric in operation, some using heat pumps, while others can be connected to steam to supply the heat. You really don't want to use PERC around a gas flame. While PERC is not flammable, when it is in the atmosphere around burning gas, compounds can form that have been known to eat holes in fabrics.
