The heating method of the nitrogen electric heater and the failure of the air duct electric heater

There are really many types of electric heaters in the market, some of which we have not touched at all, so we can say that we don't know anything about them. Nitrogen electric heaters and air duct electric heaters belong to this category. What I want to learn here is the heating method of the nitrogen electric heater and the common problems of the air duct electric heater.