Power consumption in a two-roll mill

Abstract

A solution to the power consumption in a two-roll mill rotating at unequal speeds was derived based on the lubrication model analysis coupled with the boundary condition of the nonsymmetric pressure and compared with the measurement for Newtonian fluids. The experiment was carried out with the three pairs of rolls of diameter D = 8.8, 11.3, and 16.4 cm in the range of the minimum nip clearance H , = 40-610 pm, roll speed ratio Nr = 1-7.6, and fluid viscosity p = 2-1 1 Pa s. The theoretical dependence of the power on the bank width agrees well with the experimental dependence. The experimental resuits for the total power consumption and the contribution of the individual roll to it are well correlated by the dimensionless variables derived from the analysis. However, the value of the exponent of the Reynolds number is 10% smaller than the value of -1 which is expected in the theory.Roll mills are widely used in industrial dispersing processes of highly viscous suspensions, such as paints, printing inks, cosmetics, and metal pastes for microcircuits. The suspension is forced to pass through the very close clearance formed by a pair of rolls rotating at different speeds, where the shear rate in the nip amounts to an order of 103-105 s-l. Due to high shear stresses exerted on the suspension, good dispersion of the highly viscous suspension can be expected to the extent which cannot be obtained in other conventional mixers. Examples of a three-roll mill and a two-roll mill which are usually used in the dispersing industry are shown in Figure 1.The power requirement is an important variable in the design and operation of a roll mill, but it is difficult to determine exactly since the power is consumed not only by fluid motion in the nips but also at the end plates and the scraper. In order to determine the net power consumed by fluid motion in the nips, it should be accurately separated from a gross power measured in a roll mill system.