What is Screening Efficiency?

What is screening efficiency? In the bulk solids industry this is the one question that is often asked, but rarely understood. Nine times out of ten it is related to the performance of a vibrating screen, and in many of these cases a lack of performance is the issue.

To help us unlock the secrets of screening efficiency, we’ve enlisted the services of Tony Webber, managing director of iBulk Solutions, one of Australia’s leading separation technology companies.

“The key to lifting the veil that shrouds the question of screening efficiency is to correctly identify the application requirements,” said Tony. “We help our clients with this task right from the first instance and then ensure that the specifications are supported with correct and clear documentation so there is no doubt regarding the exact level of performance a vibrating screen needs to achieve.”

Technically, screening efficiency is described as the comparison of the undersize material carried over the screening media and discharged with the oversize product in contrast to the input amount of undersize that was initially fed into the screen.

This result is expressed as a percentage and given the term screening efficiency.

“Simply put, to increase screening efficiency you want to reduce the amount of undersized material that gets thrown away with the oversize material that you are trying to extract it from, the more undersize material that  gets thrown away the less effective the screen is,” explained Tony.

Experience has shown that often the performance of vibrating screens falls below operational expectations. Much of this can be attributed to the fact that the data provided to machine designers and manufacturers is often incorrect.

Over the years Tony and the team at iBulk have faced some significant challenges in identifying the exact screening needs for their clients.

“Sometimes it’s hard to convey just how important it is to ensure users of vibrating screens have documented their requirements accurately. It never ceases to amaze me that professional engineering consultants offer screening specifications that are ambiguous and unachievable,” remarked Tony. “This is where our team of highly experienced engineers can assist to ensure our client’s screening efficiency requirements are not only met, but often exceeded.”

Tony went on to explain the simple things he would include in the basic documentation if he was developing a set of specifications for the design of a screening machine:

• What is the product that needs to be screened?
• What is the products bulk density?
• Is the product dry and free flowing, or is it moist?
• If moist, what percentage is moisture?
• Is the product abrasive?
• What is its continuous temperature?
• What is the feed rate to the screener?
• What is the cut size that needs to be removed?

“In addition, the provision of a full Particle Size Distribution (PSD) of the feed material needs to be presented with the enquiry as well as a Material Safety Data Sheet (MSDS),” said Tony. “The above list is in no way exhaustive and can go on for several pages, but even if answers were only given for these questions, the machine selection would become more accurate and application difficulties would be all but eliminated.”

This brings us back to screening efficiency. After installation it is essential to determine if the equipment is fit for purpose and if it’s achieving the client’s objectives.

Tony favours the use of the Taggart (1966:7-04) formula for this purpose. It is a simple formula and easy to apply:

“At iBulk, we endeavour to achieve the highest screening efficiency required in our customer’s applications. We understand that our success is linked to that of our customers, hence a mutual understanding of the key terms used in our industry is fundamental,” Tony said in closing.

For further information contact iBulk® Solutions on (03) 9768 3955 or visit www.ibulk.com.au.