How A Mixer Works

A mixer is determined by multiple parameters, and it is impossible to describe a mixer with any single parameter. Shaft power (P), paddle displacement (Q), pressure head (H), paddle diameter (D) and stirring speed (N) are five basic parameters describing a mixer. The displacement of the blade is proportional to the flow rate of the blade itself, the first power of the blade speed and the cube of the diameter of the blade. The shaft power consumed by stirring is proportional to the specific gravity of the fluid, the power standard of the paddle itself, the cube of the rotational speed and the 5th power of the paddle diameter. Under certain power and blade form, the blade displacement (Q) and pressure head (H) can be adjusted by changing the matching of the blade diameter (D) and speed (N), that is, the large diameter blade A mixer at a low speed (to keep the shaft power constant) produces a higher flow action and a lower pressure head, while a small diameter paddle with a high speed produces a higher pressure head and a lower flow action. In a stirred tank, the only way to make micelles collide with each other is to provide sufficient shear rate. From the perspective of stirring mechanism, it is precisely because of the existence of fluid velocity difference that the fluid layers are mixed with each other. Therefore, the stirring process always involves fluid shear rate. Shear stress is the force that is really responsible for things like air bubble dispersion and droplet breakup in agitation applications. It must be pointed out that the shear rate at each point of the fluid in the entire stirred tank is not uniform. The research on the shear rate distribution shows that there are at least four shear rate values in a stirred tank, which are: Experimental research shows that, as far as the blade area is concerned, no matter what type of paddle, when the blade diameter is constant , both the maximum shear rate and the average shear rate increase with the increase of rotational speed. But when the rotational speed is constant, the relationship between the maximum shear rate and the average shear rate and the blade diameter is related to the paddle type. When the speed is constant, the maximum shear rate of the radial blade increases with the increase of the diameter of the blade, while the average shear rate has nothing to do with the diameter of the blade. These concepts of shear rate in the paddle area require special care in the design of mixer scale-down and scale-up. Compared with large tanks, small tank mixers often have the characteristics of high speed (N), small blade diameter (D) and low blade tip speed (ND), while large tank mixers often have low speed (N) and large Features such as blade diameter (D) and high tip speed (ND).