Terminology Used in Design of Slurry Mixing Agitators

Hi Friends,
In present post, we will discuss about the various terminology used while carrying out design calculations pertaining to drive motor rating for slurry mixing agitators in Alumina refinery. It is prudent to mention here that without understanding these terminology, it will be very difficult to understand the sample design calculations which will be published shortly.

Major terminology with their physical significance and importance have been clearly elaborated in subsequent paragraphs.

Tip Speed:

Tip speed refers to the speed of a point on the outermost arm of the impeller. It is used for expressing and comparing the speed of various diameter rotating bodies including agitator elements.

Impellor Power Number:

The impeller power number, N_p represents an important parameter used for calculation of impeller power requirements for mixing of liquids or suspensions in the turbulent region. Power number is dependent on the design of impeller. As published by various reputed manufacturers, the indicative power number for different types of impellers are given below-
Power no. for pitched blade turbine = 1.2
Power no. for hydro foil type impellers = 0.70 and
Power number for Aero foil type impellers = 0.40.

Impellor Flow Number:

Impeller discharge flow number, N_q, indicates the pumping capacity of the impeller in a given geometry. The impeller power number to impeller discharge flow number ratio gives information about pumping efficiency of the impeller in a given geometry.

Three factors are important in design of mixing impellers-

(i)           Geometry (No., width, angle of blade etc.)

(ii)          Speed (Rotational or peripheral) and

(iii)         Diameter relative to that of vessel.

Tank Turn over Rate:

Tank turnover is the measure of how effective a mixer’s pumping rate is relative to turning a batch over a particular number of times a minute.  In order to make this calculation, the pumping rate of the mixer is divided by the volume of the batch.

Bulk Fluid Velocity (Intensity) :

The pumping capacity of the specific mixing head is a measure of how vigorous the mixing is within a volume of fluid concluded on the basis of following parameters-

• ·   Low to moderate mixing   - 7.2 to 10.7 m/min.
• ·   Fairly vigorous mixing       - 10.7 to 14.3 m/min.
• ·   Vigorous mixing               - 14.3 to 18.0 m/min.
• ·   Extremely vigorous mixing - 18.0 to 27.0 m/min.

   Hope, the relevant terminology have been clearly explained here. In case, you have better and simpler information on the subject, please feel feel free to communicate us in the interest of friends of alumina fraternity.

   Regards.

  Kunwar Rajendra.