This question is lacking at least two critical pieces of information:  1) the type and size of the impellers if that selection has been made and 2) the process application or mixing intensity required.  So assuming that three 4-blade, 45-degree pitched-blade turbines are used with the given fluid properties and the rotational speed for the mixer, the following are three possible options.  For low intensity mixing, three 600 mm diameter turbines will require a 0.25 KW motor.  For moderate intensity mixing, three 700 mm diameter turbines will require a 0.75 KW motor.  If intense mixing is required, then three 900 mm diameter impellers will require a 1.5 KW motor.

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