An aspirating-type volumetric feeder (Fig. 3) is based on a continuous circumferential groove in a powder-feed wheel that fills with powder by gravity. Powder is packed into the groove by a counter-rotating, inverted conical stirrer. The conical stirrer helps to maintain a more consistent powder bed density above the feed slots, ensuring uniform filling of powder in the groove.
Powder is picked up in an aspirating venturi, through which the carrier gas flows, creating a vacuum that sucks the powder out of the wheel. The uniformity of volumetric feed for this type of feeder becomes a strong function of the packing efficiency and packing uniformity of the powder particles into the metering wheel.
Powder pulsing can occur in volumetric feeders, related to the holes, slots, and screw pitches. These devices all deliver packets of powder into the carburetor. The periodic interruption in powder flow is sometimes carried through to the powder-injection port at the gun. One method used to mitigate pulsing in screw feeders is a cyclone mixer, a tangentially injected conical chamber with an axial exit.
This information is from ASM Handbooks Online, Vol. 5A: Thermal Spray Technology, Coating Structures, Properties, and Materials. To find this information (subscription required), click on the link below and scroll down to Figure 3.
