In various technical forums, process experts as well as equipment manufacturers have opined that the design of agitators for mixing process slurry handled in Chemical industries is complicated and tricky issue. In this paper, we would like to discuss the subject in very simplified manner with systematic approach starting with brief description of involved terminology, associated design parameters and methodology with sample motor rating calculations for the slurry mixing agitator.
Basic Formula Drive Rating Calculations for Agitator:
Power Required for Agitator Drive Motor = Np X ρ x N3 x D5 watt for 1 Impeller.
Where Np = Power number (Dimensionless number dependent on type of impeller),
ρ = Density of Fluid in kg/m3,
N = Revolution per second and
D = Diameter of Impeller in m.
Indicative Power Numbers for Various Types of Impellers:
Type of Agitator Impeller
Power Number (Np)
Four bladed 45 degree Pitch Turbine
Propeller Type Impeller
Hydro-Foil Type Impeller
0.30 to 0.51
Sample Calculations to Arrive at Drive Motor Rating for Agitator:
Simplified sample calculations have been presented to arrive at the drive motor rating for the agitator of a Process tank having around 3000 m3 gross capacity with key input figures as well as realistic assumptions-
Fluid height in the tank, H = 16 m and Diameter of tank, Dt = 14 m
Slurry volume in tank = π *Dt2*H/4 = π * (14)2*16 /4 = 2463 m3
Solid consistency in Slurry = 50 % (w/w), Density of slurry, ρ = 1602 kg/m3,
Viscosity of slurry, μ = 550 cp
Agitator Impeller Diameter, Di= 33 % of tank diameter = 14 * 33% m = 4.62 m
Tip speed of Impeller = π *Di *N = 3.1416 * 4.62 *20 = 290 m/minute,
Drive motor RPM = 1500 rpm, Gear Box Reduction Ratio = 75
∴ Agitator RPM = Drive Motor RPM/Gear Box Reduction Ratio = 1500/75 = 20 rpm,
Thus revolution per second of impeller, N = 20/60 = 0.333 Revolution per second.
Flow Number Nq = 0.56 and Power Number, Np = 0.51 (assumed figures for Impeller)
∴ Pumping Capacity = Nq * N * Di3 m3/ second
= 0.56 * 0.333 * (4.62)3 = 18.41 m3/sec.
Area of Tank = π * Dt2 = π *(14)2 / 4 = 153.94 m2
Bulk fluid Velocity = pumping capacity/area of tank
= 1104.44 / 153.94= 7.18 m/min. = 23.55 ft./min.
Degree of Agitation = bulk fluid velocity / 6
(For 6 ft/min., degree of agitation =1 and Degree of agitation varies from 0 to 10)
= 23.55 / 6 = 3.93 ~ 4
Annular Area = π * (Dt2- Di2 ) /4
Where Dt = Diameter of tank and Di = Diameter of impeller in meters.
= 3.14 * (142 – 4.622) / 4 = 137.18 m2
Rising velocity of particles = pumping capacity / annular area
= 1104.44 / 137.18 = 8.051 m/min. = 0.1342 m/sec.
Tank Turnover rate = Pumping capacity / tank capacity
= 1104.44 / 2463 = 0.45 times / min.
Power Number Np = 0.51
Shaft Power, P = Np* ρ *(Di)5 * N3 /1000 kW
Where Np = Power number of impeller (Dimensionless number),
Di = Diameter of impeller in meters, Shaft RPS, N = revolutions per second,
∴ Shaft Power, P = 0.51 * 1602 * (4.402)5 * (0.333)3 /1000 kW = 49.86 kW
Taking Gear Box Efficiency = 80% and Drive Motor Efficiency = 95%,
Design margin = 1.15
∴ Drive Motor Rating = 1.15 * 49.86/(0.80 * 0.95) =73.95 kW.
Thus the drive motor of about 75 kW shall be adequate for successful operation of agitator of 3000 m3 Process tank containing fluid of considered solid content.
The developed methodology clearly reveals that motor rating calculations for any slurry mixing agitator can be carried out easily by simply replacing the associated input process conditions, operating parameters, dimensions of tanks / vessel and appropriate power number for impeller in above simplified derivation.
Trust, you will like the methodology adopted in this technical paper. We would like to have your comments.
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