Portable mixers are used throughout the chemical industry and many other manufacturing industries to blend liquids and suspend solids. Success depends on a basic understanding of agitation. In particular, the user should know how a mixers orientation affects fluid flow.
Flow patterns control how liquids are blended and solids are suspended or incorporated into liquids. Other mixing characteristics can be important, such as reducing the air entrainment. Ultimately, the product must not only meet the customer's requirements but also the processor's need for economical production. Understanding the fundamentals of fluid agitation can help meet both objectives.
Figure 1. Hydrofoil impellers like the A-310 above provide the most flow for the power.
The typical portable mixer has a motor that is 2 hp or less. Its usually clamp-mounted on the side or cup-mounted near the side of an open tank. Generally, the tank doesnt have baffles. Low cost, and readily available, these mixers are useful for a variety of applications.
Even though theyve been around for years, portable mixers are often not used to their full potential. Understanding how a portable mixer works doesnt require an engineering degree. However, you do need to know some basics of mixing and flow patterns. Typical applications include blending liquids and mixing solids and liquids to make solutions or dispersions. The goal of blending is to create or maintain uniformity in miscible or nonmiscible liquids. Uniformity is a measure of the normalized decrease in concentration differences against a blend time.
The addition of powders is usually more complicated than simply blending liquids. Some solids dissolve to form solutions, while others form dispersions or suspensions. Solutions form best when the added particles or agglomerates can be suspended off the bottom of the tank. What works best during one phase of the mixing process may not work well later on. Dissolving powders may change the mixing properties of liquids, e.g., increase the viscosity. Poorly dispersed powders can form lumps or large agglomerates. Rapid initial mixing often minimizes the formation of lumps, provided the solid is added in a controlled way, but also increases entrainment of air. Viscous fluids create other mixing problems, for which portable mixers have limited capabilities.
While portable mixers can do many different tasks, selecting the best mixing characteristics can make the difference between success and failure of a batch or product.
A common misunderstanding about mixing is that vigorous motion on the surface isnt necessarily a sign of good mixing. Often the first sign of poor mixing is a strong vortex. In other cases, the surface motion may be helpful for certain process steps, such as powder addition. Learning what happens below the liquid surface and when certain mixing characteristics are required will go a long way to understanding how to make the best use a portable mixer.
The portable mixer
Portable mixer sizes typically start at 1/4 hp and go to 2 hp. Some larger mixers have 3-hp motors, but those usually require a rigid mount, different from the portable-style adjustable mount. Simply knowing the motor horsepower isnt enough to gauge the mixing capabilities of a mixer; in the past, rules of thumb often referred to horsepower per 1,000 gallons.
Portable mixers fall in two general categories direct-drive (high speed) and gear-drive (low speed) mixers. The direct-drive mixers have a shaft and impeller that turn at the motor speed, usually 1,750 rpm. The impeller, also known as a prop, hydrofoil, or turbine, on direct-drive mixers is typically from three (3) to seven (7) in. in diameter. Gear-drive mixers often run at 350 rpm, although other speeds are available. Gear-drive mixers will have impellers from eight to 17 in. in diameter. A variable speed drive may offer a range of speeds.
It should be no surprise that a small impeller operating at a high speed or a large impeller operating at a low speed produce the same mixing intensity.
P = 6.566 X 10-14(NpN3D5ρ)
P: Agitator power, hp
Np: Impeller power number, vendor
N: Shaft speed, rpm
D: Impeller diameter, inches
ρ: Fluid specific gravity
The difference is in how much horsepower is required. A 1/4-hp gear-drive mixer will usually provide more agitation intensity than a 1-hp direct-drive mixer. The reason is that agitation intensity is more closely related to how much torque is applied to the fluid than how much horsepower. Torque is proportional to power divided by speed, so the same power at a lower speed can create more intense agitation.
Most portables use either a marine-type propeller or a vendor-specific hydrofoil impeller. The hydrofoil impeller is an improvement over the marine propeller because it weighs less and can provide more flow at a given power draw. Both types of impellers provide strong axial flow, a characteristic that plays an important role in deciding how best to position the mixer in the tank to get maximum advantage for your application. Other types of impellers can be used for mixing applications, including pitched-blade turbines, straight-blade turbines, disperser disks, or other special application designs.
Typically, small impellers operating at high speed are more cost-effective for blending small batches of liquid or creating dispersions and emulsions. Large impellers operating at low speeds are better for bulk mixing or in high viscosity fluids.
Positioning the mixer
Proper positioning of a portable mixer is an effective in-plant way for process improvement. A good portable mixer mount should allow the mixer to be positioned with respect to both the vertical axis and horizontal orientation. A well-positioned portable mixer starts with the shaft angled between 10° and 15° from vertical. This position should place the impeller on the same side of the center of the tank as the mixer mounting. The objective is to have the axial flow from the impeller sweep down and across the bottom of the tank. In such a configuration a vortex may form.