Dave Dickey Forum Moderator 286 Posts
Re: Can you suggest a suitable anchor, shaft size and motor power for my reactor?22 April 2013 at 12:52pm Last edited: 23 April 2013 at 2:12pm
An anchor impeller does not seem to be a good choice for this application. With such a low viscosity, a turbine style impeller would be much more appropriate. The density (gravity) is extremely high in the process description.
For the conditions described, a typical anchor impeller would be 2,200 mm in diameter and 3.000 mm high with two vertical arms. At 60 rpm with a 3.0 density and 100 cp fluid, the motor power would be 260 KW (not watts, kilowatts). The shaft diameter should be 165 mm.
The same anchor impeller running at 20 rpm in a 1.2 density fluid would require a 7.5 KW motor. However, with the low viscosity, the fluid would only swirl in the tank and not mix well, especially not vertically.
A better mixer design for a 100 cp fluid, even with a 3.0 density, would use a 960 mm diameter, pitched-blade turbine. The pitched-blade turbine would have 4 flat-plate blades, each 190 mm wide, mounted at 45 degree angles. The direction of rotation should be so that the flow from the impeller is downward toward the bottom of the reactor. A mixer with a 960 mm diameter pitched-blade turbine rotating at 60 rpm would require a 2.2 KW motor and will provide moderate mixing intensity. A shaft diameter of 50 mm should be adequate.
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uday uday Community Member 1 Post
Re: Can you suggest a suitable anchor, shaft size and motor power for my reactor?23 April 2013 at 1:35pmDear Sir, I am very thankful for your valuable inputs. I also went thru your many previous posts. If i am not wrong, this is a case of turbulent flow & hence you are recommending a turbine instead of anchor. Now i am trying to understand the power calulations for both the cases described by you. 1) ANCHOR WITH 260 KW OR 7.5 KW: Is there any standard configuration for anchor that needs to be considered for power calculations. Eg: D-200mm =2200mm (i.e dia of anchor-Da), Da x 1.35 ~ 3000 (i.e ht of anchor), power factor=??. 2) 45 deg Pitch blade turbine with 2.2 kw motor - Is there any standard configuration for PBT that needs to be considered for power calculations. Eg: Dia of turbine = 2400/2.5 = 960mm, W = 960/5 ~ 190mm. power factor = 1.3 ???. I also tried to calculate the power for PBT using our suggested configuration. (sheet attached). Can you pls correct me. Regards, Uday
Dave Dickey Forum Moderator 286 Posts
Re: Can you suggest a suitable anchor, shaft size and motor power for my reactor?29 April 2013 at 12:03pm
Power calculations for an anchor impeller start with the assumption that it will be used in a viscous application. Therefore, the impeller geometry is converted into an estimated viscous power number, which is constant in viscous (laminar) conditions. To estimate power in the turbulent range, a correction factor as a function of Reynolds number must be applied to the viscous power estimate. The diameter of a typical anchor impeller is 90% to 95% of the tank diameter, the vertical arms are typically about 10% of the impeller diameter, and the vertical arm height is close to the height of the tank straight-side. Of course the submerged height of the vertical arms is the only height of any significance in the power calculation. In turbulent conditions, an anchor without a baffle will swirl the tank contents, so the power never becomes a constant and will continue to become less as the Reynolds number increases.
Your power calculations are quite good. A typical pitched-blade turbine has 4 blades, each of which is 1/5 the diameter of the impeller, the blades are mounted at a 45 degree angle. With 4 baffles in the tank a good power number is 1.37 (1.3 was used in the previous answer). Reviewing your calculations, the Reynolds number appears to be low by about a factor of 100, probably in your units conversions. The Reynolds number should be dimensionless - all the units of the variables must cancel. The Reynolds number for the PBT should be 27,531, using your 3000 kg/m3 density and 100 cp viscosity. The power calculation using those properties and the 1.37 power number gives 4.58 hp. A 5 hp motor may be marginal. A 7.5 hp motor would be better.
In the previous power calculation a 1200 kg/m3 density was used. A 3000 kg/m3 liquid with a 100 cp viscosity would need to be either molten aluminum or a molten ceramic. Acquous or hydrocarbon fluids will not have such a high density. A high concentration slurry, might reach your density, but not at 100 cp. Since power is proportional to density, your power estimate may be high by at least a factor of 2.