Post Mixing Optimization and Solutions

Impeller: RT

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rt4.jpg (28730 bytes)
rt6.jpg (27535 bytes)

Images courtesy of P. Csiszar

There are probably more mixing studies done and articles written using the Rushton Turbine (we name it RT) than any other impeller. The casual mixing investigator will think that mixing is only about RTs. Well, with the huge amount of work done on the RT, there is also a wide disparity of data on the RTs.

J.H. Rushton invented an impeller he called a flat bladed turbine. Everyone else knows about it as the Rushton Turbine or a flat bladed, disk turbine (or disc turbine). The Rushton Turbine is considered a generic impeller today. The data presented here is quoted from his very famous work back in 1950. Mixing theory was in its infancy back then. The 5th exponent on impeller diameter was still being questioned. Rushton studied in his report many different RT and other impeller configurations, many that aren't used any more today.

  Np wB/D= 0.1 Np wB/D= 1/12 Np(rel) wB/D= 0.1 Np(rel) wB/D= 1/12
RT3 3.4 3.3 0.550 0.567
RT4 4.4 4.3 0.717 0.733
RT5 5.4 5.4 0.900 0.900
RT6 6.0 6.0 1.000 1.000
RT8 7.8 7.8 1.300 1.300
RT10 8.2 8.7 1.367 1.450
RT12 10.0 9.9 1.650 1.667

Today, the 4- and 6-bladed versions of the impeller are most common.  His power numbers seem high, compared to what we are used to seeing with RTs today.  It is possible that his equipment was reading a bit high, but the values on the Impellers Page and in the first two blue column s of this table are directly quoted from his tables. Here we offer a relative scale of power numbers (Np(rel)), relative to the most common Rushton Turbine, the RT-6. Remember that placement of the RT in the tank does affect the Np, so before you think everyone is wrong, check the conditions under which their power study was done. If you are convinced that your NP(RT6) is correct, you can use this handy table to estimate what a different type of Rushton Turbine might do at the same location.

For this table, the impeller off bottom distance, OB, is equal to the impeller diameter, OB/D=1.0. The liquid level is equal to the tank diameter, Z/T=1.0. The impeller diameter, D, is about 1/3 of the tank diameter, D/T=1/3. The fully baffled condition is assumed. Rushton studied the impact of the different baffle standard widths, 1/10th for the metric system and 1/12th for the American/British method. See the Baffle Section for more information on this.

If you are using a 5-bladed RT in a gassed application, contact us to learn why you are probably having problems. If you are too shy to do that, you can wait and it will eventually be posted here at Post Mixing, but you may be wasting precious resources in the meantime.

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Last modified: February, 2013
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