Closed circuit pneumatic conveying

Claim 1

In a closed circuit pneumatic conveying system for transporting solids from a hopper at a pick up location to a delivery location by suspending the solids in air at the pick up location and transferring the suspension to the delivery location and thereat separating the solids from the air, and returning the air to the pick up location, comprising: a. a pick up conduit for being placed in open communication with the hopper for receiving solids by gravity flow, b. separator means for receiving air-solids suspension at the delivery location and thereat separating the solids from the air, and a conduit communicating the pick up conduit with the separator for conveyance of air-solids suspension from the pick up location to the separator, c. an air pump for circulating air through the circuit, and a conduit communicating the separator with the pump inlet, d. an air return conduit communicating the pick up conduit with the outlet of the air pump, said air return conduit having a restriction for maintaining the air in the circuit between the pump discharge and the restriction at a positive value, and e. a vent means in the air return conduit between said restriction and the pump discharge for venting some air from the systems to the atmosphere and maintaining a negative pressure downstream of said restriction to the pump inlet, the improvement which comprises: f. a throttle valve interposed between the air return line and the vent, and g. means for controlling the throttle valve in dependence on demand for solids at the separator, for moving the throttle valve toward the open position when the demand is large and moving the throttle valve toward the closed position when the demand is low.