A heat-transfer model for the rotary kiln: Part II. Development of the cross-section model
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I.
INTRODUCTION
T H E importance of the rotary kiln in the metallurgical and chemical process industries was discussed in Part I m of this series. It was pointed out that although versatility has ensured its widespread application in the past, there exists a pressing need for improved understanding of all aspects of kiln performance in order for it to remain competitive against specialized, but less versatile, process equipment. Heat transfer has been identified as the rate-limiting process in many kiln applications. A new model for rotary kiln heat transfer, which allows both an increased understanding of the factors determining kiln thermal performance and enhanced predictive capabilities, has been developed in this study. As an integral part of this work, detailed measurements of heat transfer were made on a 0.4 m I.D. by 5.5 m pilot kiln. Both the pilot kiln and the notable results from the trials were discussed in Part I. The present paper outlines the development of a unified model for heat transfer at any specific kiln axial position, i.e., over a cross-section perpendicular to the axis of kiln rotation. The cross-section model will be shown to be capable of reproducing, in detail, the observed behavior of the pilot kiln. In addition, results will be presented which illustrate the interaction and the relative importance of the various heat transfer paths which occur at a cross-section. The rationale for reporting the cross-section model and results, rather than only an overall model to be developed subsequently for the entire kiln, is twofold. First, it has been shown [21 that heat transfer at any axial kiln
P.V. BARR, Assistant Professor, Centre for Metallurgical Process Engineering; J.K. BRIMACOMBE, Stelco/NSERC Professor for Process Metallurgy and Director of the Centre for Metallurgical Process Engineering; and A.P. WATKINSON, Professor, Department of Chemical Engineering, are with The University of British Columbia, Vancouver, BC V6T lW5, Canada. Manuscript submitted May 12, 1987. METALLURGICAL TRANSACTIONS B
position is essentially a local phenomenon; therefore, a cross-section model can be constructed using only local conditions, such as temperatures and emitting gas concentrations, and ignoring conditions farther afield, i.e., axial gradients of temperature and gas concentration. Therefore, a cross-section model can be marched over the length of the kiln as the basis for an overall heattransfer simulation. Since the cross-section model provides the key element for further modeling efforts, it is essential to establish its adequacy. Second, there is the need to improve our fundamental uiaderstanding of kiln behavior. The various heat-transfer paths occurring at a cross-section of a kiln are shown in Figure 1. It can be seen that the rate of heat transfer to the bed, for example, is the net result of heat transfer from the freeboard gas, the exposed wall, and the covered wall. Understanding the complex, dynamic (due to the rotation of the kiln) interaction of these various heat-transfer paths and p
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