Glass for industrial purposes is usually manufactured in continuously operative furnaces of the tank type. Such glass furnaces include a long main chamber in which a suitable number of burners are mounted in order to melt the glass-forming raw material that is charged at one end of the chamber, the molten glass being discharged at the opposite end. Behind the main combustion chamber there is provided a secondary chamber through which the fumes generated by the burners pass before they are blown out in the open through a chimney or the like. For the burners fossil fuels, such as fuel oil or natural gas, are most often used. These fuels are burned while generating flames in the area above the bath of liquid glass that is continuously formed in the main combustion chamber or melting chamber. The fumes leaving this melting chamber have a very high temperature, e.g. within the range of 1400-1700.degree. C. In order to recover the thermal energy of these hot fumes as far as possible a recuperator is provided in the secondary chamber. By means of this recuperator part of the thermal energy can be transferred to the combustion gases which are necessary for the burning of the fuel in the burners and which are led to the burners from the outside through one or several feeding conduits. Previously known recuperators for this purpose are in the form of so called radiation baskets which are mounted in the upper part of a vertical shaft included in the secondary chamber. These baskets comprise a great number of rather slender tubes which are held together in a basket-like configuration and which are hung up in the vertical shaft by means of special means of attachment in the upper part of the shaft. Through a collecting tube the combustion air or combustion gas is led through the tubes of the basket, said air being preheated before it reaches the burners in the melting chamber. In practice said radiation baskets are, however, associated with several disadvantages. One disadvantage is that the attachment means of the collector tube and the individual radiation tubes have to be carefully protected by means of a refractory mass in order not to be attacked by the fumes. Furthermore, the installation of the basket calls for relatively expensive arrangements in order to minimize the thermal stresses in the structure due to thermal expansion. Another severe disadvantage is that the basket in its entirety has to be disassembled and removed from the secondary chamber in case anyone of the tube members of the basket structure would fail, e.g. due to leakage. This means that the furnace, which usually should operate continuously during periods in the range of 5 to 8 years, has to be put out of operation during the time necessary for repairing the basket. Such breaks in the continuous production are extremely costly.
A problem in connection with the use of air as a combustion gas is that nitrogen oxide is formed during the combustion, such nitrogen oxide being environmentally noxious. In order to eliminate this problem the use of oxygen instead of air for the combustion has started in recent years. Also in connection with the use of oxygen as a combustion gas there is a need of preheating the gas by utilizing the inherent heat of the hot fumes. However, a change-over to preheated oxygen cannot be carried out quite easily. Thus oxygen is a reactive and agressive gas and at the same time the material in the recuperator tubes are subjected to very high temperatures e.g. 1000.degree. C. or more. In previously known recuperator tubes either a ferritic stainless steel having a high content of chromium of the type 27 Cr, the remainder Fe, or an austenitic steel of the type 21 Cr, 11 Ni, Si, REM, the remainder Fe, have been used. The advantage of the first mentioned steel type is that is has a good resistance against attacks from the fumes, even upon a rich presence of sulphur compounds leading to sulphuration due to the formation of sulphate on the tube walls. A disadvantage of this steel type is, however, that it has a low creeping strength. On the other the austenitic steel has good creeping properties, but this type of steel does not, however, withstand the fumes in a satisfactory manner. This is true for all alloys having a high content of nickel. In case oxygen would be fed through tubes with such a high temperature as 1000.degree. C. or more there would in both cases be an obvious risk of shell formation in the surface layer of the tube material due to oxidation. The use of preheated oxygen as a combustion gas also involves additional security requirements owing to the face that the gas is very reactive and can easily be ignited with an explosive progress upon possible leakages.
USPTO Backgrounds
USPTO Backgrounds