Air preheater are liquid and air heat exchanger or exhaust and air heat exchangers. They are designed and carried out individually depending on the application. Air preheater can be performed in counterflow, co-current flow, co-current flow-countercurrent or cross-countercurrent or cross-co-current flow. The design depends on the media and the temperature conditions. For small construction, finned tubes are suited to arrange in a small space much heating surface. Is in the exhaust gas or in the air containing dust or other particles, so that deposits are expected, preferably smooth tubes are used, which are less prone to deposits and will be clean easier.