Dave Dickey Forum Moderator 287 Posts
Re: Where should the impellers be located?22 September 2011 at 1:29pm
The answer to where the impellers should be located is simple. To be able to handle a 70 liter volume and a 275 liter volume, the lower impeller must be as near the bottom as practical and with sloped blades to roughly match the slope of the cone bottom. The upper impeller should be located approximately one-third to half way between the lower impeller and the liquid level for the 275 liter volume. Whether the tank is 30 inches in diameter, with a 10 degree cone or 20 inches in diameter, with a 30 degree cone, the liquid level for 70 liters will barely cover the lower impeller and may be in the cone for a 30 degree slope.
The real questions involve the resin viscosity, the practical mixer speeds, and the required mixing intensity. If the resin viscosity is low, less than about 200 cp, the tank will need baffles to prevent uncontrolled swirling. If the viscosity is higher, more than 1,000 cp, baffles are not necessary. Since extending baffles into the cone is impractical, any impeller in 70 liters may draw air into the liquid. With a changing viscosity, some compromise must be met, depending on the conditions at the most important point in the process.
The most practical rotational speed for a mixer in this size range is 350 rpm, which corresponds to a single 5:1 gear reduction from a 1750 rpm motor. Higher or lower speeds will depend on the available gear ratios, speed controls, and shaft design. Some lower speeds may not be allowed mechanically because of natural frequency problems. The motor will probably be about 1 hp.
Having considered two tank designs, one a 30-inch diameter tank with a 10 degree cone and a 20-inch diameter tank with a 30 degree cone, impeller diameters and rotational speeds become problematic. At 350 rpm, a lower straight-blade, radial-flow turbine about 6-inches in diameter seems reasonable for mixing intensity. An upper pitched-blade, axial flow turbine about 7.5 to 8.0 inches in diameter is probably appropriate. These impeller sizes are reasonable in a 20-inch diameter tank for a range of viscosities. The impeller sizes are extremely small for higher viscosities in the 30-inch diameter tank.
Impeller location can be reasonable, based on volume requirements, actual mixer design is more complicated.
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