The Scientific Bulletin of VALAHIA University MATERIALS and MECHANICS –Vol. 14, No. 11 DOI 10.1515/bsmm-2016-0005 ANALYSIS OF THE COMBUSTION AIR PREHEATER FROM THE ALUMINUM MELTING FURNACES Aurel GABA, Vasile BRATU, Dorian MUSAT, Ileana Nicoleta POPESCU, Maria Cristiana ENESCU Valahia University of Targoviste, Faculty of Materials Engineering and Mechanics, Str. Aleea Sinaia, No. 13, Targoviste, Romania E-mail: aurel_gaba@yahoo.com, v_bratu22@yahoo.com, pinicoleta24@yahoo.com, cristiana_enescu@yahoo.com Abstract. This paper presents solutions and the equipment for preheating combustion air from scrap aluminum melting furnaces through flue gas heat recovery. For sizing convection pre-heaters, there has been developed a mathematical model which has been transcribed into a computer program in C + +. A constructive version of the pre-heater was drawn up and a recovery heat exchanger was manufactured and mounted on an aluminum melting furnace. Both the functional parameters values and the reasons causing the pre-heater worning out, as well as the steps taken for sizing and the achievement of a new air pre-heater able to bear the operating conditions of the aluminum melting furnace are shown. Keywords: pre-heater, combustion air, furnace, aluminum scrap 1 INTRODUCTION The preheating of the combustion air in the alumnium melting furnaces is generally made by using radiation or convection preheating heat exchangers. It was found that the preheating devices can improve the practical efficiency of aluminum melting furnaces by 4-7% for every 100 °C the combustion air is increased in temperature [1]. Also, preheating devices offer the faster return the investiments for preheating combustion air. Customers are realizing a 19-25% reduction in the fuel usage with these heat exchangers and ROIs are averaging 26 months [2]. The newest operational system of using the regenerative burners has several disadvantages: higher costs, greater space for their mantling and additional cost for their maintenance [3, 4]. Radiation air pre-heaters used in these furnaces are generally dr. Schack type. They are made of two concentric tubes where the flue gas flows through the inner tube and the combustion air flows in parallel through the section of the two tubes. The parallel flow is preferred due to the high temperature of the flue gas of over 1000 °C [1,3]. The parallel flow submits the refractory steel inner tube to lower temperatures which means a longer life for it, but this also leads to approx. 7-8%, lower temperatures of combustion air preheating, meaning a lower efficiency of recovery in comparison with the countercurrent flow [3,5,6,7]. The convection air pre-heaters used in these furnaces are usually cross flow type in countercurrent with vertical pipes between the air chambers and flue gas flowing perpendicularly on the tubes [3]. Due to the high temperatures of the flue gas of over 1000 °C, as well as of the components resulted from the flows of fluorine, chlorine based gases, etc., used in the process of scrap melting or from the reactions of the cast aluminum or magnesium oxidation and engaged by the flue gas, the life of the metallic recovery heat exchangers is extremely short [3]. As a consequence, there are studies to find solutions for improving the metallic convection recovery heat exchangers, as well as for replacing the refractory steel grades with ceramic materials (for instance the recovery heat exchangers made of silicon carbide). Such a recovery device, made of Field tubes modules, has been tested and there has been reached a combustion air preheating temperature of 815 °C, for the exhaust flue gas temperature of 1205 °C [8]. For these ceramic recovery heat exchangers there are still sealing, mechanical strength or thermal shock resistance problems. Improving the construction solution of the metallic convection pre-heaters could be made on the grounds of an exhaustive analysis of the causes leading to their worning out. In the same time, the numerous construction possibilities should be analyzed, by sizing them by means of a computer program [7]. For the convection air pre-heater sizing we use our own computer program. The mathematical model and the calculation algorithm are presented in this paper [7]. The C++ transcribed mathematical model was used for sizing of the air pre-heater required by the Contractor of the modernization of an aluminum melting furnace on the base of the following design parameters: -recovery heat exchanger type: bundle of tubes performed from refractory AISI 304 steel, outer diameter 76 mm, available; -maximum natural gas flow: 1000 m 3 N /h; -maximum temperature of the flue gas at the recovery heat exchanger entrance: 950 ºC; -maximum temperature of the hot air at the entrance of the burners: 480 ºC. In the same time, the Contractor settled that the combustion air temperature regulation is perform to exhausting into the atmosphere of certain quantity of air from the maximum available flow of 12000 [m 3 N /h]. 27