Question

You have been assigned the job of designing rapid-mix and slow-mix tanks for the town of Freemont, using the groundwater aquifer as the water source. Design parameters for the rapid-mix tank:

Detention time = 2 mins No. of tanks: 2 Velocity gradient (G) = 750 /s Impeller type: radial, with Np = 3.6 Impeller diameters available = 0.25, 0.5 and 1.0 m Tank length = width B = Hwater/3 Design parameters for the slow-mix tank: Impeller: axial, Np = 0.40 Impeller diameters available: 1.0, 1.8 and 2.4 m Tapered G in 3 compartments: 90, 60, 30 /s No. of tanks: 2 Detention time = 30 min Tank length = width B = Hwater/3 For both the rapid-mix and slow-mix tanks, determine:

(i) Power input (assume 80% efficiency)

(ii) Tank dimensions

(iii) Impeller diameter

(iv) Rotational speed of the impeller

µ = 1.85 x 10-5 Pa-s

Answer 1