regions in the vicinity of the walls of the counter flow chamber 120 might not belong to the separation region since the air flow velocities in such regions might be too low to loft the fibers 106 upwards an appropriate choice of the air flow velocities within the separation region can enhance the separation of the trash from the fibers one method for estimating the air flow velocity is next described other methods may also be used we consider a particle u2014 fiber or trash u2014 consisting of a uniform material and having a certain shape and certain dimensions in a stationary laminar homogeneous and isotropic air flow directed upwards the air flow exerts on the particle a force directed upwards the flow resistance which depends on the air flow velocity we calculate the air flow velocity v necessary for compensating the gravitational force on the particle in an air flow with this u201c threshold velocity u201d v the particle would float at the same level below the threshold velocity v the particle would fall down above the threshold velocity v it would be lofted up the threshold velocity v according to this model is k is a shape factor depending on the geometric shape of the particle u03c1 a is the mass density of air u03c1 a 1 2 kg m 3 h is a characteristic height of the particle i e a particle dimension in line with the air flow direction in a first example let us consider a cylindrical cotton fiber floating with its axis in the horizontal direction in the air flow the following values apply for this example k 1 3 u03c1 1510 kg m 3 and h diameter of the cylinder 20 u03bcm.