Heat exchanger for the chemical industry to cool flue gas, and to preheat a process flow and to preheat air. An air flow in a closed circular system will be preheated in one bundle, and fluor carbon gas in the production process will be preheated in another bundle. This combined preheating system (flue gas cooler) will be used in a plant that produces or blends fluoromonomers, fluorochemicals, fluoropolymers and fluoroelastomers. Our customer supplies her final products to Europe, the Middle East, and Africa. Flue gas is cooled from ca. 900 to ca. 220 degrees Celsius. Air is preheated from ca. 20 to ca. 170 degrees Celsius, and fluor carbon gas from ca. 30 to 380 degrees Celsius.