A new theoretical air leakage model was developed for both conventional and highpressure rotary valves. Different sizes of rotary valves (D = 250 and 300 mm) and a single-slit apparatus were tested over a wide range of conditions (N = 0 to 30 rpm; Delta p(v) = 0 to 300 kPa; c = 0.10 to 0.25 mm). All the model predictions were found to be within +/- 6% of the experimental data. This was a dramatic improvement compared with all previous models and even suppliers' curves, where errors were found to be between -48% and +39%. A new centrifugal vent hopper also was developed to maximize the removal of air leakage and, hence, feed rate capacity. Based on data from three products (130 mu m Poly Powder, fibrous perlite, and 10 mu m ground powder), the new vent hopper was found to increase feed rate and conveying capacity by at least a factor of two.