Characterization of dispersed air in two newsprint paper machines

The concentration of dispersed air in two newsprint paper machines was measured as functions of time and location by a novel procedure based upon automated density measurements. Headbox pulp in one machine had about 0.2%, by volume, dispersed air. The average diameter of the air bubbles ranged between 58 and 107 μm. Density measurements showed that dispersed air could be eliminated from the headbox stock by defoamer addition. The second paper machine contained very little dispersed air in the headbox stock. A photomicrographic survey of the wet end revealed that the dispersed air was removed in the pressure screens. Continuous density measurements, in conjunction with occasional determinations of the pulp density in the absence of dispersed air, gave a continuous measurement of the amount of dispersed air.