DESIGN OF SETTLING CHAMBERS

Abstract

Various industries release dust and smoke to the atmosphere causing air pollution problems. Settling chambers are the devices which are introduced to the industrial exhaust system to remove solid particles from the emission. The particles while passing through the chamber, settle over the settling trays under the action of gravity, thus cleaning the gas. These chambers can collect reasonably small particles with excellent reliability. Depending upon the number of settling trays and their inclination, there can be various types of such settling chambers. These devices are usually metallic structures which withstand large temperature variation. Design of such chambers involves their fluid dynamic and structural behaviours. As an usual practice, the cost of such device, used in the industries, is passed on to the consumers. Any reduction of cost of the devices would mean benefiting the consumers. Present status of study and research on the design of settling chambers is not adequate. The present design method is based on the fluid dynamic considerations only with little attention paid to the structural safety aspect and minimization of cost. Thus, there is a necessity to develop a method for the design of these devices considering all these aspects. Design of settling chambers involves: (i) fluid dynamic behaviour to maintain a desired collection efficiency, (ii) structural behaviour of load bearing members to ensure safety, and (iii) minimization of cost to achieve economy. In the present study, the cost functions were formulated for the following configurations: (i) multiple inclined trays; (ii) double entry multiple inclined trays; (iii) single inclined tray; (iv) multiple horizontal trays; and (v) single horizontal tray settling chambers. These chambers were considered to be made of (ii) strip steel supported by light gauge beams and columns. It was found that similar smaller units in parallel is economical than a single large unit. The fluid dynamic constraints consist of nonturbulent flow in the settling chamber; achieving the design efficiency; and avoiding reentrainment of the deposited particles. The structural constraints include safety criteria for the load bearing members viz. strength, serviceability, shear for tray sheets and beams; and slenderncss ratio, overall buckling etc. for columns. The cost functions and constraints constitute nonlinear combination of a large number of design variables including chamber dimensions (length, width and height); number of smaller units (settling chambers); number of tray sheets (for multiple trays devices); number of spans (in case of a continuous beam); and the dimensions of tray sheet, beam and columns. The design problem, thus, boils down to nonlinear constrained optimization. The random search method has been found successful in obtaining the minima. The optimization method, though based on a sequence of random number, was found to converge to the same minima. Using the algorithm, a parametric study was conducted by varying the input variables such as the design efficiency; flow rate; particle mass density; cleaning interval; and the dust loading. The functional dependence of the input variables on the optimal cost provided an interesting study.