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Literature
Application Considerations for Propellers Used in Fluid Mixing
When selecting a propeller for mixing applications, there are several factors to consider:
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• The Mixing Process
• Degree of Agitation
• Shaft Speed
• Viscosity |
• Specific Gravity
• Power Requirements
• Propeller Diameter
• Tank Configuration |
Degree Of Agitation
There are three general categories of mixing or agitation levels.
Mild - A tank turnover rate of one to one and a half per minute.
Medium - Most common. Two to three tank turnovers per minute.
Violent - Over four tank turnovers per minute.
A mixer can be compared to an open centrifugal pump. At a given speed and propeller size, a fluid flow is produced and can be measured in GPM (Gallons Per Minute) as a pumping rate or in terms of tank turnovers per minute. The pumping rate increases dramatically with the speed and propeller diameter. It is important to keep in mind that changing either of these factors dramatically effect your power or motor horsepower requirements. See below.
High Speed VS. Low Speed
Mixers are typically supplied with direct drive motors running at 1750 RPM (high speed) or gear driven versions with output shaft speeds of 170-420 RPM (slow speed).
Small propellers (2" to 8" dia.) running at high speed are for light liquidsin small batches (25-300 gallons) and for dispersions where a higher degree of shear is required to break up particles, crystals or other solids.
Larger propellers (8" and up) running at slower speeds are used when higher flow is necessary to the process, or the mixture is thick (viscous). This combination is also effective when there are solids in the solution that need to be suspended. When foaming is undesirable, or colloids are present, a larger prop rotating at slow speed will limit the problem.
Effect of Specific Gravity (Density) and Viscosity
Specific gravity and viscosity do not directly effect the propeller selections, rather the power required. Power requirements increase directly with specific gravity. Viscosity is resistance to flow. It effects power draw in the laminar region only. Generally, the more viscous the material becomes, the application requires larger propellers rotating at lower speeds.
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