Rotary Kiln Design Based on the Hypothetical Kiln
, LEHCO, 112 Wildoak Dr., Daphne, AL 36526
A new theory for the design and analysis of Rotary Kilns is presented. The Hypothetical Kiln concept represents a departure from the traditional Rotary Kiln design. Reference to the traditional design method is provided. Basis for the theory are the new concepts recently developed for gas to solid heat transfer in pebble beds. An actual kiln is considered a small part of The Hypothetical Kiln where pebbles and gas enter from a wider temperature range. The author previously presented two Papers on the subject of pebble bed regenerative heat transfer. One at the AIChE 2004 Spring Meeting entitled Simplified Regenerative Heat Transfer and a more comprehensive Paper at the 2006 meeting of IT3 entitled Schumann’s Curves Replaced by a Simpler Derivation. This Paper shows how the principles of regenerative heat transfer in pebble beds can be applied to Rotary Kilns. In the new theory the Rotary Kiln is incorporated into the Hypothetical Kiln by discovery of the hypothetical inlet and exit temperatures. The Hypothetical Kiln contains the complete S-Curve in which pebbles enter at a much higher temperature and exit at a lower temperature than the actual kiln. The fictitious kiln is long enough so that the pebbles exit at the fictitious gas inlet temperature and the gas exits at the fictitious inlet temperature of the pebbles. Conditions in the actual kiln coincide with a portion of the Hypothetical Kiln. Important concepts and equations are developed relating the actual kiln to the hypothetical inlet and outlet temperatures of the fictitious kiln. Over twenty equations are derived based on the new theory. Application of the new equations is made to a simple case of pebble cooling using air in a Rotary Kiln. It is expected that heat transfer correlations based on the new theory will prove to be more accurate. Future work, discussed in the paper, is the development of comprehensive correlations of recast data for heat transfer in Rotary Kilns. The “Heat Transfer Constant” is divided into three parts. Part 1 is convective heat from the walls and burden. This part relates directly to kiln diameter and wall film coefficient. Part 2 is direct contact between gas and pebbles that are bouncing and rolling across the burden surface from top to bottom. This part is related to the area to volume ratio of particles, the particle film coefficient, and the fraction in direct contact with gas. The fraction in motion is related to particle properties such as shape and angle of repose, and kiln rotation and inclination. Part 3 is radiant heat flux with the gas. By the application of existing correlations of wall film coefficients, comparative analysis between kilns, input from radiation experts, laboratory measurements, and through differences it is predicted that more useful correlations of kiln data will result.