This design is a hybrid configuration of both open and closed cycle dryers and it is not gas tight. There are many variations on this design, with the most important being the "direct heated" or "self-inertizing" system. In the self-inertizing design, a direct-fired heater is used and the air entering the system is limited to that required for combustion. An amount of air equal to the combustion air is bled from the system at the other end of the process. The gas (mainly products of combustion) is recycled through the dryer. The recycled gas has a very low oxygen content, making it suitable for materials that cannot be exposed to oxygen, due to explosive hazard or product degradation.