Recuperators for small furnaces, i.e. up to 80 t/d are double shell recuperators. For melting capacities between 100 to 450 t/d tube bundle recuperators are used. The air preheating output of recuperators is far below the output of regenerators. That is why the energy consumption of a furnace with recuperators is relatively high. An advantage, however, is the higher flue gas temperature enabling energy recovery possibilities like a steam turbine generating electric power. The continuous preheating of the air and the total separation of combustion air and flue gas leading to a more clean and stable combustion. This leads to better glass qualities. The specific melting capacity is lower than that of an oxyfuel or regeneratively heated furnace due to lower flame temperature and consequent worse heat transfer.
Double shell recuperators are basically built out of two concentrical pipes, more specifically a smaller pipe is placed inside a bigger pipe. Hot flue gas is passed through the bigger pipe and cold air is passed through the smaller pipe. Hot flue gas is passed through the bigger pipe and cold air is passed through the smaller pipe. As a result the cold air gets warmer and can be used to increase efficiency of production processes. Preheating temperatures are typically around 600 °C and usually do not exceed 700 °C. In case of lead crystal glass the preheating temperature is limited to <500 °C due to corrosion.
Large recuperators are usually designed as tube bundle recuperators. Here the combustion air flows through individual tubes (tube bundle) which run in a flue gas channel lined with refractories. In general the tube bundle recuperators are more robust, but also more expensive. The maximum preheat is 750 °C. Combinations of double shell recuperators and tube bundles achieve a maximum air preheat temperature of 800 °C.
For this furnace type air casing burners specially developed by HORN® are used. These burners ensure even distribution of the combustion air around the burner lances. The furnace is usually designed as cross fired furnace. Recuperative end fired furnaces are usually very small and rare. Even more rare are recuperative end fired furnaces with ceramic recuperators, which HORN® would not recommend due to short lifetime of the heat recovery system.
In case of the end fired system the flue gas outlet opening is positioned above or next to the burners in order to achieve a vertical or horizontal horseshoe-shaped flame. In case of the cross fired system the flue gas outlet opening is positioned either on the front wall using one recuperator or on the side wall using two recuperators. The batch charging will be located at the remaining free space.
The typical features of this furnace type are the low melting surface load and the low investment costs. The glass quality can be higher at low pulls.
Also this furnace type can be equipped with a weir wall, a deep refining part and electric boosting systems in order to increase furnace capacity and flexibility.