Dave Dickey Forum Moderator 286 Posts
Re: Follow-up to previously answered impeller diameter question25 June 2007 at 1:29pmMy understanding of the question and the process is:
The tank is 2 m in diameter and 1.5 m tall. The tank does not have baffles, but should have them for the density and viscosity described. I have assumed that the tank will contain about 3.75 cu m of liquid, with a viscosity of 1.8 centipoise and a density of 1,450 kilograms/cu m. The volume and physical properties are not critical within plus or minus 10% or 20%.
The previous question asked about a three-blade, pitched-blade turbine, 880 mm in diameter, with a 122 mm diameter hub and blades 122 mm wide at the hub and 80 mm wide at the tip. The blades are mounted at 60 degrees with respect to the shaft. Blade angle is usually measured with respect to the horizontal, so I have used information for an impeller with 30-degree angle (from horizontal) blades. Running this 880-mm diameter impeller at 180 or 360 rpm gave impossible results.
For the new question, asking about a smaller impeller, I have assumed that the smaller diameter impeller will be the original impeller with the blades carefully cut shorter. The blade lengths need to be cut to within about 2 or 3 mm radius of each other to keep forces balanced and the power draw even.
Without baffles the mixing intensity is quite limited. At 180 rpm, I would recommend an impeller diameter of 500 mm, which puts the blade width at the tip at about 106 mm. At those conditions the impeller will draw about 240 watts and should have about a 1 kW motor. At 360 rpm, the impeller should be only 350 mm in diameter, with a 113 mm blade width at the tip. The impeller will draw about 500 watts and again a 1 kW motor is reasonable. This impeller size at 360 rpm is rather small for the tank. However, the speed is high enough to give adequate mixing.
A much better design for this application would be for a tank with baffles. Typically, a tank has four baffles at 90-degree locations and about 200 mm wide. The mixing possible in a baffled tank is much better for this low-viscosity liquid and much less likely to splash out of the tank. At 180 rpm, a 650-mm diameter impeller would have a 99-mm blade width at the tip. The power input would be about 2 kW and provide three to four times as much mixing intensity as in the unbaffled tank. A 3.7-kW motor is suggested. At 360 rpm, a 450-mm diameter impeller with a 108-mm blade width would require about 4.2 kW. A 5.6-kW motor would be appropriate.
I believe that these recommendations should answer your questions and give you sufficient information to design and install a workable mixer.