# Topic: What is the power number for 3-blade segment impeller?

posed by
Can anybody please tell me what is the power number for 3 blade segment impeller?
• Dave Dickey Forum Moderator 324 Posts

#### Re: What is the power number for 3-blade segment impeller?

Last edited: 23 April 2013 at 2:15pm

This question is lacking several pieces of critical information, since power number is a function of all blade features. In an effort to provide as much information as possible, the following three-blade power numbers are offered for different blade angles as measured from horizontal:

3-blade, 30-degree, pitch Np = 0.550

3-blade, 45-degree, pitch Np = 1.19

3-blade, 60-degree, pitch Np = 1.73

All of these power numbers are for flat blades with actual blade widths 1/5 of the impeller diameter. Other blade widths will affect the power number.

Cambered blades, as used for hydrofoil impellers result in different power numbers:

Those values should cover most of the three-blade impeller options.

The answers by this expert are based on the best available interpretation of the information provided. The consequences of the application of this information are the responsibility of the user. If clarification is needed, please submit a further question.

• Anonymous CP Reader Community Member 147 Posts

#### Re: What is the power number for 3-blade segment impeller?

We would like to know the Power number for the 2 x 3 blade segment configuration of impellers with the following details: Impeller outer diameter: 470 mm Impeller Inner Diameter: 210 mm, Operating Speed : 30 RPM Vessel Volume : 1000L, Vessel Diameter: 900 mm We use Water For Injection as reference fluid, so you can take water density into consideration for the calculations. We are operating this bioreactor as a gassed system where the inflow of gases is used to control the Dissolved oxygen at a particular set point.
• Dave Dickey Forum Moderator 324 Posts

#### Re: What is the power number for 3-blade segment impeller?

Last edited: 23 April 2013 at 2:14pm

This response only helps confirm that we are not communicating on the same level. A power number is not the power that is required to operate a mixer. A power number is a characteristic of an impeller geometry that can be used to calculate the impeller power. Knowing the impeller power number, fluid density, rotational speed, and impeller diameter, the impeller power can be calculated for any size impeller with the same geometry: number of blades, blade width, etc.

This additional information would imply that a value for the impeller power for these conditions is being requested. However, several new communication problems arise. The impeller description of a "2 x 3 blade segment configuration" is not clear. Does the impeller have 2 blades or 3 blades? Are there 2 impellers each with 3 blades? What are segments? Segments is not a term commonly used to describe a mixing device. The description of the vessel means that the liquid level for 1,000 liters in a 900 mm diameter vessel must be about 1,600 mm, so 2 impellers may be a reasonable assumption.

The reference to an outer diameter and an inner diameter for the impeller seems to imply something about a blade width or shape, although that description is still unclear. The operating speed of 30 rpm is low for a 470 mm diameter impeller of any blade width or number of blades. Using an extreme value for the power number, the impeller power must be less than 50 Watts, even with baffles (not specified) in the vessel. The addition of gas to the system further complicates an estimate for impeller power. The power with gas may be less than 40% of the power without gas.

A wild guess about the actual impeller power is that it is less than 10 Watts. Hopefully, this information satisfies what is wanted and needed.

• Erik Hasenfus Community Member 1 Post

#### Re: What is the power number for 3-blade segment impeller?

Last edited: 23 January 2015 at 4:39am
Hello Mr. Dickey, I'm referring to your first answer to the original question. I wondered if you could tell me how the power numbers were determined. What experiments were performed? and how? or were they calculated, maybe with CFD? Thanks in advance...
• Dave Dickey Forum Moderator 324 Posts

#### Re: What is the power number for 3-blade segment impeller?

Power numbers are usually determined by experimental measurement with small-scale, geometrically similar impellers.  The most common methods involve some combination of a torque and speed measurement.  Torque can be measured with a torque arm and force measurement or a shaft torque sensor, usually based on strain gages.  All dimensions, speed, and force measurements must be accurate, especially with small impellers.  Power readings from laboratory mixers are usually unreliable.

CFD calculations can be done, but only if the models have been validated and blade loads can be accurately evaluated.

Large scale measurements of impeller power are possible with watt meters and compensation for drive losses.

You can find published articles on power number determination.