A chemical mixing basin is to be designed to blend chlorine and treated wastewat

Question

A chemical mixing basin is to be designed to blend chlorine and treated wastewater. The basin flow is 700 gallons per minute and the basin dimensions are 5 feet by 5 feet by 5 feet water depth. The basin will have a single mechanical blender that will transfer 100% of the mixing energy from the blades to the water. The wastewater temperature will vary from 35 F to 70 F. The selected mixing intensity in the basin shall be a minimum of 600 1/sec. What size motor (horsepower) is required for the mixer?

Explanation / Answer

mixing intensity, G = 300 s^-1 Volume of tank, V = 5*5*5 ft^3 = 125 ft^3 => V = 125*0.0283168 m^3 = 3.5396 m^3 35 degree fahrenheit = 1.67 degree celsius 70 degree fahrenheit = 21.11 degree celsius dynamic viscosity of water at 1.67 degree celsius, u1 = 1.6975 N-s/m^2 dynamic viscosity of water at 21.11 degree celsius, u2 = 0.9794 N-s/m^2 G = sqrt[P/(u*V)] where, G = mixing intensity P = power supplied to the shaft in watts V = volume of the tank in m^3 u = dynamic viscosity of water in N-s/m^2 to find minimum value of required power, u should be maximum. G = sqrt[P/(u*V)] => 300 = sqrt[P/(1.6975*3.5396)] => P = 540762.39 W => P = 540762.39/746 hp = 724.883 hp

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