OCR Text |
Show exchanger, such as recuperator. The typical regenerative heat exchanger for preheating of combustion air w a s a bed in a container packed with ceramic balls with alternating flow (Fig.2). Its size varies depending on the cycle time between 30 seconds and several minutes, that is, the amount of heat storage, and a typical regenerator on a nominal rating of 586 k W burner can produce preheated air exceeding 1273 K, generally within 300 K of the furnace temperature [28]. If nonuniform temperature distribution in a cross-section of the bed occurs, the efficiency of heat exchanger decreases. Therefore, uniform temperature distribution in a cross-section perpendicular to the flow direction is required for higher performance of the regenerator. In contrast, the volume of a honeycomb type regenerator for the necessary heat capacity can be minimized because of its large surface to volume ratio, and thereby direct installation of regenerator into a burner becomes possible. Temperature distribution in a ceramic honeycomb is quasi-one-dimensional, because ceramic honeycombs assure the uniformity of temperature in a cross-section. Accordingly, combustion air of 1570 K was actually obtained by this regenerative system for the mean furnace gas temperature 1623 K, that is, approximately 50 K difference, and almost constant temperature of combustion air, less than 50 K variation during a cycle, w a s realized by the following procedure even with the small heat capacity of regenerator [17]. In Fig.3 temperature profiles in ceramic honeycombs along the flow direction at both start and end of discharge mode of the regenerator are schematically illustrated. The amount of heat between two limiting profiles in the figure is available for heat-recirculation during a cycle. One can anticipate that the temperature of preheated air can be kept almost constant during a cycle from the fact that the temperature change on furnace side is small. Besides, the length of the ceramic honeycombs should be selected so that the temperature at the ambient side does not rise so much in order to keep the heat loss rejected with exhaust gases be minimized. |