Dave Dickey Forum Moderator 300 Posts
Re: What's the easiest way to do impeller power calculations?23 August 2011 at 1:29pm
The easiest way to explain and do your impeller power calculations is to convert all of your dimensions and operating characteristics to coherent metric units. These conversions will eliminate the need for conversion factors in your subsequent calculations and the definitions can be used directly.
• Tank Diameter T = 2.7 m
• Tank Height 3.0 m
Use impeller height or submerged impeller height in your power calculations H = ?? m
• Ribbon Diameter D = 2.5 m
• Pitch = 2.5 m
• Blade Width is typically D / 10 so W = 0.25 m
• Density (rho) = 1,000 kg/m3
• Viscosity (mu) = 0.100 Pa.s
• Rotational Speed N = 0.417 rps
• Power in Watts
To begin you need to calculate a viscous power number for the helical ribbon impeller [Power Number Viscous - Helix]. The expression is for 2 ribbons as specified. The viscous power number [Power Number Viscous Evaluation SI] is different from the turbulent power number normally used for turbine-style impellers. Next calculate the Reynolds number [Reynolds Number Evaluation] Because of the low viscosity specified, conditions are not viscous but rather turbulent and a correction factor [Correction Factor - Viscous Power] must be calculated to increase the viscous power number for the actual conditions. With the corrected power number, you can calculate the power [Power Calculation Metric - Viscous] required at the process conditions.
If you do the calculations correctly, you should get results like the following:
• Viscous Power Number for the Impeller Geometry = 257
• Reynolds Number = 26,000
• Viscosity Correction Factor = 92
• Corrected Power Number - 24,400
• Impeller Power 6,630 Watts (6.6 kW)
How do you calculate the power number, the Reynolds number and the hydraulic power for a tank ribbon mixer? The tank diameter is 2,700 mm and the height is 3,000 mm. The proposed ribbon diameter is 2,500 mm, pitch diameter is 2,500, number of ribbons is two. Also, what width should the ribbon be? SP.GR = 1.0; Viscosity - 100 CP; Suggested speed = 25 RPM.
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