FRICTION FACTOR

The frictional force can be expressed in terms of a friction factor. This leads to equations describing the flow of a fluid past a collection of particles. There are several approaches to treating fluid flow through packed beds. The most successful of these is the Ergun Equation, which describes flow in both the laminar and turbulent regimes. This method treats the packed column as a compact irregular bundle of tubes. Modifying the theory for straight tubes not only takes into account the irregularity of the tubes, but yields relationships similar to those derived for straight tubes as well. This analysis assumes several conditions. First, we assume that the particles are packed in random; there is no channeling in the packed bed. Channeling occurs when the fluid flowing through the packed bed finds a preferred path through the bed. We also assume that the diameter of the packing is much smaller than the diameter of the column as well. The maximum recommended particle diameter is one-fifth of the column diameter. We assume that velocity, particle diameter and void fraction behaves as a bulk behavior and hence we can use an average values. The Ergun equation is = OR = Ergun defined the friction factor; fp depends on Vs (superficial velocity), pressure drop and packed bed length. = At different flow rate Ergun determined the general form of the friction factor.Where =Presure dropL= the height of the bed =the fluid viscosity =Void space Vs= the fluid superficial velocity Dp =the particle diameter= the density of the fluid s = sphericity of the particle.

The Ergun equation tells us a number of things. It tells us the pressure drop along the length of the packed bed given some fluid velocity. It also tells us that the pressure drop depends on the packing size, length of bed, fluid viscosity and fluid density.Packing material, Berl saddles