Construction and Operation
The Double Planetary Mixer includes two blades that rotate on their own axes, while they orbit the mix vessel (also known as bowl) on a common axis. The material to be mixed is loaded into a cylindrical vessel with a nearly flat bottom. The blades continuously advance along the periphery removing material from the vessel wall and transporting it to the interior. After one revolution the blades have passed through the entire vessel, after three revolutions most materials have been mixed, and after only 36 revolutions, the blades have contacted virtually the entire batch (see figure 1).
The drive system of a double planetary mixer consists of a motor and a gearbox that drives the planetary head. Each planetary blade is generally driven by gears that rotate due to the motion of the planetary head. Variable-speed drives can be provided to operate the mixer over a wide range of speeds, ensuring high torques even at lower speeds. Located below the drive system is a cylindrical cover for the bowl. This cover may be provided with nozzles, liquid spray arrangement, or viewing ports. A major advantage of this vertical type mixer is the shaft seals do not directly come in contact with the product material.
Material may be charged into the mixer either through the nozzles on the top cover or directly loaded into the mixer bowl. Depending on the size of the mixer, either the bowl or the drive system of the mixer is lifted or lowered using hydraulic arrangement. The mixer bowl may be jacketed for circulation of heating or cooling media. The mixer can be designed for operation under pressure or vacuum.
After mixing, the material is discharged through a bottom valve, or by manual scooping of the material from the bowl. For extremely viscous materials, hydraulically operated automatic discharge systems are available that push the material out through the discharge valve. Figure 2 is the photograph of a 750 litre working capacity, double planetary mixer with a hydraulic arrangement for lifting and lowering of the bowl and a pneumatically actuated flush bottom discharge valve.
Depending on the properties of the material to be mixed and the blade type, the specific power for the double planetary mixer ranges from 30 to 50 kW/m3. Because of the vertical configuration of the mixer, the material fill levels in the double planetary mixer can be as low as 20% to a maximum of 85% of the total bowl volume.