Smart Mixing

While working in his laboratory to dissolve powder in water, Mark Saunders realized he needed more mixing. He did what many people in his situation would do: he bought a bigger impeller. After all, it’s usually cheaper and easier to install a new impeller rather than buy a stronger mixer. That worked okay for a while, at least until the motor burned out. He replaced it, but before too long, the motor failed again. What Mr. Saunders didn’t realize at first was that the motor failures were primarily caused by the additional horsepower draw of the larger replacement impeller. And while he gained greater mixing, we’ll see that is not the whole story. The new impeller was larger not only in overall diameter, but in the design of the blade (greater pitch), too. His dilemma: he needed thorough mixing, but due to physical constraints of the application–no room or ability to mount a bigger and stronger mixer–he had to stay with his current mixer.

"Generally when people say they need a larger impeller, they really mean they need more mixing." says Bob McDermott, longtime LIGHTNIN® mixer sales representative. Mixers are essentially pumps, processing fluids usually within open vessels. And like a centrifugal pump, mixers use an impeller to move the fluid. But mixer operators need to keep in mind that a fixed amount of horsepower is available for each mixer model.

Mixing impellers are classified in essentially two general categories: axial and radial flow impellers. Each type moves the fluid through the vessel in a distinct flow pattern. In a typical overhead mixing application, axial impellers push the fluid down and away from the impeller, eventually flowing back up the tank walls and being simultaneously drawn back through the top of the impeller. Radial flow impellers move fluid outward to the sides, some of the fluid being forced down, some of the fluid being forced upward along the tank wall. While there are many variations of these impellers, for the scope of this article we’ll consider the axial flow impeller in water-like viscosities.

Before installing a different impeller, there are a couple other remedies Mr. Saunders could have tried. First, he should install baffles if possible. Using four baffles –flat plates approximately 1/12 the tank diameter attached equidistantly on the wall of a vertical cylindrical tank –will ensure a consistent power draw and help provide a more effective flow pattern for mixing. Note: baffles are not required when mixing highly viscous fluids, since the viscous drag of the fluid acts as a dynamic baffle. Second, he should make sure the impeller is in the optimum position, usually placed in the center when used with baffles and 2/3 to 1 impeller diameter off the tank bottom for efficient mixing.

The real key to successful mixing is using the right impeller for the application in regard to both the fluid characteristics and the mixer motor horsepower. Impeller manufacturers, such as LIGHTNIN, through careful evaluation, assign each impeller model a power number and a flow number. The power number is used to measure the horsepower draw, and the flow number is used to figure the primary flow in gallons per minute. Understanding a couple of mixing equations, the user can compare different impeller designs for his or her application:

Shaft horsepower (SHP) = (impeller power number x impeller diameter5 x rpm3 x specific gravity)/1.525 x 1013

Primary flow (in gallons per minute) = [(impeller flow number) x (impeller diameter)3 x (RPM)] / 231

Compare two popular Lightnin impellers: the high-efficiency A-310 and the A-320 (a high-efficiency but also a high-viscosity impeller recommended for fluids greater than 3000 centipoise). Suppose each impeller measures 5" diameter and is spinning at 350 rpms in water (specific gravity = 1). When you perform the calculations:

A-320

A-310

Power number

0.65

0.30

Flow number

0.68

0.56

SHP

0.0057

0.0026

Flow (gpm)

129

106

As you can see in the table above, the A-320 horsepower draw is more than twice the A-310, yet the flow is only 22% greater. To try and improve the mixing of his water-like solution, Mr. Saunders replaced his standard marine impeller with LIGHTNIN’s A-320 impeller. This put too much stress on the motor, and ultimately led to failure. Had he chosen the A-310 impeller, he would have increased his mixing (flow) to an adequate level, but wouldn’t have overloaded his motor. To make matters easier for the end user, LIGHTNIN includes correctly sized impellers that are designed slightly below the mixer’s maximum motor horsepower. Furthermore, their digital-display LabMaster™ mixers feature all the essential parameters of the mixing process such as power, torque, flow, and speed. The user can simply toggle between the fields to view the pertinent values and monitor or record them with a computer.

Mixing is a dynamic process, and often requires careful evaluation and attention to process details. <%=Session("company_name")%> and LIGHTNIN aim to improve process results by providing information and the necessary tools. Each mixing application is unique, and setting up the process correctly is imperative for success. When faced with choices about mixing, don’t hesitate to contact our Applications department. We’ll ensure you get the answers you need.

Bob McDermott, LIGHTNIN® mixer representative contributed to this article.

Ben Wilbert, Mixers Product Manager, <%=session("company_name")%>