Techniques for separating materials have been around since the dawn of human civilization. Some involved using thermal processes, like those used to recover salt from seawater via evaporation. Others involved extracting metals from ore, such as the use of alum for dying textiles. The earliest human farmers used a method known as winnowing, which in its simplest form involved using the wind to grade grains and remove the lighter chaff.

These separation technologies have become increasingly complex and are today used for separating solids, gases and liquids in a multitude of industrial processes. These separation processes can be thermal, mechanical or chemical in nature, and have become integral to modern civilization, which is supported by a variety of machinery. The centrifugal sifter is one of the descendants of these ancient separation methods and, like our ancestors before us, it’s important for those in industries that use them to understand how to optimize their operation.

Overview: How a Centrifugal Sifter Works

Centrifugal sifters provide highly precise screening for fine powders. They’re widely used in the manufacture of building materials, chemicals, foods, metals, paper, pharmaceuticals, rubber and other applications. Also known as a rotary sieve or rotary sifter, the centrifugal sifter uses centrifugal force to screen bulk materials in both powdered and granular form. A centrifugal sifter normally has a single inlet with two outlets. One outlet removes contaminants and oversized particles from the product, while the other lets through on-size or undersized materials for further processing or grading. 

Within the centrifugal sifter, paddles on the central driveshaft rotate rapidly, causing the material to make contact with the internal screen. Particles smaller than the screen’s mesh aperture pass through the screen before they’re funneled into the discharge hopper or are transported on for downstream processing. Oversized particles move along the screen’s length before exiting through one of the sifter’s outlets. Though there are a variety of designs, the basic dynamics of centrifugal sifter operation are essentially the same.

Enhancing Efficiency of Centrifugal Sifters in Particle Separation

Though centrifugal sifters are incredibly useful tools for the processing of bulk materials, particularly bulk powders, equipment manufacturers continue to improve upon this technology. Those who design and build processing equipment like centrifugal sifters continue to explore how they can be improved. Engineers in the private sector and scientists in academia seek solutions to the most common problems faced by industries that rely on material handling machinery, while these industrial challenges are being met with new technologies.

Equipment manufacturers continue to deal with more basic challenges as well. This includes how material properties are affected by particle size and distribution. Those who design and build material processing equipment understand that using different processes for separation will cause the same raw material to behave differently. For example, larger particles tend to dissolve more slowly and are less sensitive to heat than smaller ones. Additionally, particle size and shape can affect material flow rates, stability and viscosity.

When not correctly sized or shaped, a material’s particles can also affect product quality and clog equipment. In pharmaceutical manufacturing, for example, often the goal is to increase the surface area of particles. This enhances the efficacy of active compounds within medications, allowing manufacturers to use less expensive ingredients. This can often be done without lessening a drug’s desired effects, while also potentially reducing risks from negative side effects. Plus, it reduces costs.

Similarly, the size and shape of particles within a material can affect the production of coatings, paints and pigments. Particle size will affect these products’ coloration, durability on surfaces, sheen, strength of tint, weather resistance and other properties. A paint manufacturer can achieve greater precision in its formulas, for example, when it manufactures products to a smaller particle size. This in turn results in a better-performing product while controlling operational costs.

Properly Installing a Centrifugal Sifter

Installing a centrifugal sifter incorrectly can affect its performance dramatically, so it’s important to correctly integrate it into the overall processing system. Some manufacturers of material handling equipment even help design and integrate centrifugal sifters into a system. Even so, it’s recommended that a new sifter be inspected for damage when received, as even what looks like superficial damage may reduce the centrifugal sifter’s performance.

When positioning a centrifugal sifter in a production environment, there must be enough open space around the machine to allow access for maintenance and cleaning. Additionally, once installed it should work with no vibration. If a centrifugal sifter begins to vibrate excessively, this points to a problem that should immediately be pinpointed and remedied. Not only will these vibrations hamper performance, but they will also shorten a sifter’s lifecycle.

Conditions that may cause undue vibrations include: 

  • Base positioned unevenly
  • Broken blade on the rotor
  • Defective bearings in the motor or sieve
  • Loose fasteners on the motor
  • Vibrations from other integrated equipment
  • Excessive feed to the machine or a blinded screen

A few other tips about the operation of a newly installed centrifugal sifter should also be considered to ensure it continues to perform optimally. Controlling the feed is particularly important to protect against overfeeding the machine, for example, as it may cause finer particles to be transported into the discharge for oversized particles. Care should be taken too to ensure air doesn’t blow back into the primary discharge outlet from the hopper, as it may result in excessive carryover. Once installed, rejects should be discharged to an airtight container so that the oversize discharge outlet doesn’t allow air to flow back through the sifter, which will interfere with transporting the product.

Maintenance & Other Tips for Centrifugal Sifters

Most centrifugal sifters require very little maintenance. However, to maintain peak performance it’s important to routinely inspect for broken or worn components. Breakdowns, along with the inevitable unplanned downtime, are often just the result of haphazard inspections and a lack of regular maintenance. If a centrifugal sifter is properly maintained, only the parts that experience the most wear will require maintenance, such as lip seals and screens. As long as a centrifugal sifter doesn’t vibrate too much and the screen frame is kept clear of contaminants, the machine will likely last a long time.

Even with regular inspections and maintenance, it's important to track a centrifugal sifter’s output over time. Changes in a sifter’s output help determine if there’s a problem, though this may just be an indication that the screen requires changing. Most models are relatively easy to clean and maintain, but having an inventory of spare parts is prudent. This is especially true of those sifter components that are most likely to wear, which will in turn keep downtime to a minimum. Since screens control a product’s particle size, it’s recommended to keep replacements on hand. Some manufacturers even have preventive maintenance programs to ensure their equipment maintains sufficient throughput and lasts as long as it should.

Routine inspections should concentrate on the following centrifugal sifter components:

  • Screens: Must be frequently inspected to maintain the requisite output and replaced should they show too much wear.
  • Rotor: As the main moving component within a centrifugal sifter, it controls how the product is fed and then thrust against the screen; before cleaning, inspecting or performing maintenance work on the sifter’s rotor, the machine should be completely disconnected from power.
  • Motor: Once disconnected from the power source, rings and seals should be replaced should they show wear, though it’s also important to secure them afterward to ensure no unwanted vibrations.
  • Bearings: When a centrifugal sifter vibrates excessively, overheats or begins to make grinding noises, it may mean there’s a problem with one of the bearings; some types of sifters feature fewer bearings and/or seal them to keep these issues to a minimum.

Other components that should be inspected include seals and rings that help protect the motor and bearings from contamination. Generally, these only require replacement when damaged or show considerable wear.

Centrifugal Sifter Trouble Shooting 

Should a centrifugal sifter fail to start properly once installed and there’s no obvious exterior damage, it may be because of internal components misaligned during shipping or installation. The foundation should also be level so as to keep it from vibrating. Checks should also be performed to confirm material flows properly through and from the sifter. To avoid electrical issues, a certified electrician should verify the equipment is properly connected and that the wiring is sound. Any manufacturer of a centrifugal sifter should also provide help with troubleshooting problems that arise with their equipment.

Tips on Optimizing Centrifugal Sifter Operations

Basic maintenance is important to keep a centrifugal sifter operating properly like any industrial equipment. This means keeping it clean and ensuring it’s properly lubricated to prevent corrosion and optimize performance. This includes the controls, which should be kept free of dust or other contaminants to prevent short circuits or other electrical failures.

A few other tips to ensure optimal centrifugal sifter performance include: 

  • Depending on the application, it’s often recommended to overhaul a centrifugal sifter’s motor on an annual basis.
  • Regular checks of the wiring should be made to ensure the centrifugal sifter is properly powered and to prevent electrical issues.
  • Regular inspections of the centrifugal sifter’s internal components, which include lubrication and cleaning dust from the motor.
  • The sifter’s screen should have an even tension along its surface when installed or replaced; if this isn’t the case, the screen should be adjusted until tension is even over the screen’s surface.

While these general tips should be followed to keep a centrifugal sifter in good working order, it’s also important to pay attention to any other specifications from the equipment manufacturer regarding maintenance.

Prater Industries: Providing Solutions for Particle Separation Challenges

Prater Industries is a leading manufacturer of centrifugal sifters and other separation equipment used in the material handling sector. Our seasoned engineers have the capability to customize our centrifugal sifters to optimize them for specific applications. For example, our engineering team developed a solution for a client in the powder coating industry that resulted in an augmentation used to improve all of our centrifugal sifters’ screen designs. In another instance with one of the largest cocoa processing companies in North America, Prater’s engineers installed custom upgrades that allowed them to optimize their operations.

The Rota-Sieve® is Prater’s patented design for a centrifugal sifter that can handle free-flowing materials that include agglomerates, granulates and powders. Able to provide high-capacity screening, our Rota-Sieve® has an easy-to-clean design that enables them to better classify, scalp and sift material. Further, our engineers can customize our centrifugal sifters to provide specific features for your application. To learn more about our patented centrifugal sifters and other material-handling equipment, contact the experts at Prater Industries today.