Tube bundles arranged for the installation into the air duct (square or rectangle shape). Both steam and hot water heating is possible. Heated air is located in the inner tube space. Using in thermal power stations, biomass plants, EfW plants etc. The tube bundle can consist of finned or unfinned (bare) tubes. Finned tubes of an air heater/air preheater are designed with round CS or SS tubes. Fins can be made from Al or CS. The dimension of the preheater can be designed according to the existing duct so as the design can be made with respect to the specific site conditions, mainly the air pollution and dust content in the air. The main purpose of the air heater is the heating of the combustion air for fired boilers but it can be used also for other application.