Screw seal

A screw seal comprising, in combination,

• a housing comprising an elongate tubular conveying section and an elongate tubular sealing section, one end of each section being joined to one end of the other, the conveying section having an inlet opening longitudinally spaced from its joined end for receiving a stream of bulk solids,
• a discharge chamber connected to the other end of the sealing section,
• feed screw means within the conveying section and extending between the inlet opening and said joined ends,
• means to rotate the feed screw means to cause flow of the solids to and through the sealing section to said chamber, said flow being variable between predetermined highest and lowest operating solids flow rates,
• means for producing and maintaining a gas pressure differential between said chamber and the inlet opening during said flow, the sealing section having a cross sectional area diverging toward said other end, said divergence being greater below than above a horizontal plane defined by diametrically opposed longitudinal lines in the sealing section, said divergence and the length of the sealing section being related so that during said flow the coefficient of friction between the solids and the wall of the sealing section causes compression of the solids, said compression forming a plug having a density limiting the flow of gas between the discharge chamber and the inlet opening to a predetermined maximum leakage rate during flow of the solids between said highest and lowest rates, and
• means to break up the plug as it is discharged into said chamber.