In the case of a one-stage heat recovery system for machine supply air heating generally only around 5-8% of the energy content of the hood exhaust air of the dryer section is used. The rest just disappears through the roof. Voith retrofittable multi-stage heat recovery systems allow the energy from the hood exhaust air, but also that of other waste heat flows e.g. vacuum pump waste air, to be used efficiently. As well as the machine supply air the water for the process (fresh water, spray water, white water) and/or for heating the machine hall ventilation can therefore also be heated.