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Many solid pneumatic conveying applications have a requirement that the solids have to be conveyed in the dilute phase mode. In many of these applications, the conveying system consists of both vertical and horizontal pneumatic lines. The minimum velocity required to maintain solids in the dilute-phase mode in a vertical conveying line is called the choking velocity (Uch). The minimum velocity required to maintain solids in the dilute-phase mode in a horizontal conveying line is called the saltation velocity (Usalt).
Because it is more difficult to maintain solids in a dilute-phase mode in a horizontal line than in a vertical line, the saltation velocity is greater than the choking velocity. The ratio of the saltation velocity to the choking velocity ranges from approximately 3 to 6. This means that the minimum system velocity in a system composed of horizontal and vertical conveying lines in the saltation velocity. Many people try to decrease the diameter of the horizontal lines to simultaneously increase the gas velocity and to minimize the volumetric gas requirements of the system. However, decreasing the diameter of the horizontal lines also increases the solids mass flux in the lines, which means that the saltation velocity increases as well which generally results in no net gain by decreasing the line diameter.
Many conveying lines consist of a single vertical upward line followed by a single horizontal line. If the horizontal line has an appreciable length, a velocity equal to or higher than the saltation velocity is required to maintain dilute-phase flow in the line. A significant reduction in the velocity required to maintain dilute-phase flow can be achieved if the horizontal line is angled downward as in Figure 1. A downward angle of only 15 degrees is enough to significantly reduce the velocity required to prevent solids salting out in the line.