SHAPE
Bulk density
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What is the capacity of my screener?
The most difficult screening application, grading removes both fines and overs from the feed material. At its most complex, grading might separate the product into several different size fractions within a single screening machine. For exmple, in salt applications, the user endeavors to create two or three final products from the same material. The coarsest particles might become salt for deicing roads, while the middle fraction may end up being an ingredient for animal feed, and the finer particles serve as granular table salt. Accuracy and efficiency are critical components for optimal return on investment in grading. Depending on the amount of concentrated particles near the separation points, it usually requires the lowest capacity among screening objectives.
Three:
Grading
Fines removal separates the finer particles from a product, usually totaling less than 10% of the feed. In cereal, for example, the fines often range in size from broken bits to fine dust, all much smaller than the desired product size. With fines removal, most of the feed material moves over the screen with very little needing to pass through the mesh. This increases bed depth which requires lower capacity to achieve optimal efficiency when compared to overs removal.
Two:
Fines Removal
"Overs" refers to any particle larger than the desired size of the final product, such as separating clumps of flour from the fine particles to generate the desired final product. Generally, this screening application removes less than 5% of the total feed. Because there is only a small percent of overs and product fraction, the process lends itself to the highest capacity rate.
One:
Overs Removal
Separation Requirements
Product Quality
The quality requirements for the final product can add intricacies to the screening process that directly impact the capacity of the screener. Quality assessment can range from visual inspection to full-scale particle sie distribution analysis of the final product. For instance, in agriculture, some end users simply look at the soybean and corn to assess the quality of separation from unwanted organic pieces, like pods and sticks. In other cases, the user needs more measurable and quantifiable quality assurance and often within very specific parameters. The sieve analysis used for particle size distribution may set the maximum allowable limit of oversize and/or undersize particles in their final product. For example, in producing granular table salt, quality requirements may only allow a maximum of 5% fines and a maximum of 10% overs in the product fraction. When a prodcut is screened using quality specifications, the capacity becomes critical to the accuracy of the process, making this form of screening the lowest capacity of any application.
Background
- Material Properties
- Particle Size Distribution (PSD)
- Screening Objectives
- Separation Requirements
Bulk Density
First, the material itself requires an assessment, beginning with bulk density. Screening is volumetric process limited by the screener's set area of wire mesh and the preset diameters for its inlet and outlets. The greater the volume of the material, the deeper the bed depth becomes, reducing the percentage of material that will reach the opening in the screen mesh and separate appropriately. The material bulk density defines the volumetric flow of material within the screener and thus the resulting bed depth. For example, imagine screening similarly sized fractions of ground barley with a bulk density of 25 lbs/ft3 (400 kg/m3) versus foundry sand with a bulk density of 100 lbs/ft3 (1602 kg/m3). Assuming all other variables are the same, the less-dense barley will run at a volume and bed depth four times greater than the sand. Therefore, the barley requires a lower feed rate than the sand to achieve the same separation performance in the same unit.
Utilizing a screener effectively relies on the appropriate settings for the screened material and the desired screening objective. Changing either the feed material or the purpose for using
the screener directly impacts its recommended capacity. This
is why manufacturers do not assign a specific fixed capacity
rating to a machine based solely on the model number.
Each screener is initially customized for the original end
user’s application. If the machine is repurposed for another
application, many of the process variables that drive screener
capacity recommendations will also change. That means the
same machine originally rated for a certain capacity now needs
to operate at a different rate to screen effectively for the new
material or objective. Therefore, screener manufacturers need
precise details regarding these four key application parameters
to determine capacity rate:
material properites
SCREENING OBJECTIVES
PARTICLE SIZE DISTRIBUTION (PSD)
MATERIAL PROPERTIES
4 Key Application parameters
MOISTURE
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4,500
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4,500
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4,500
SEPARATION REQUIREMENTS
Shape
The shape of the particles impacts material flow through the screener, which impacts the screening capacity. Most materials come in jagged, granular or round shapes and typically the separation occurs on a square wire mesh. The more irregular or jagged the shape of the particles, the slower the material flow. The more uniform the shape, like granular and crystalline particles, the easier the separation, resulting in faster material flow. As the material flow rate increases through the screener, the bed depth decreases, resulting in a higher screening capacity. For example, if two plastic products are screened on the same machine, but one has a uniform shape as seen in image 1 and the other has an irregular shape as seen in image 2, the capacity rate for the uniform material will likely be higher due to the increased material flow and shallower bed depth.
Moisture
Moisture within a product, whather from water, oil or fat, hinders separation and slows capacity. More specifically, the type of moisture and how densely it packs the material together makes a difference in that outcome. For instance, some agricultural products have an inherent 5% to 10% moisture to them that has less impact on capacity rate than the .2% or .3% moisture found in industrial or frac sand. A moist material that sticks to itself requires processing through a dryer prior to screening, because the self-adherence makes efficient separation highly unlikely. In the field, conducting a quick "snowball" test of squeezing a handful of the material together can determine if the material is too moist for an efficient separation. If the squeezed material clumps together, it needs to be dried first.
Ascertaining the particle size distribution is achieved by running the material through a stack of increasingly finer sieves (typically a Ro-Tap machine). The amount of product retained on each sieve profiles the feed by determining the concentration of particles at each separation point. Understanding this distribution greatly influences the overall capacity of the machine in relation to the screening objective.
Material Properites
Particle Size Distribution (PSD)
Screening Objectives
The screening objective defines the core purpose of what the equipment is trying to achieve through separation. The complexity or simplicity of that goal drives the recommended capacity for that machine. The objective can be as simple as removing dust or large clumps from the product, or as complex as separating the material into five different fractions with each fraction having quality specifications. Following are the three commonly used screening objectives.
Conclusion
Investing in a screener and not setting the correct capacity rate can eat away profits, impact product quality, and potentially damage the equipment. But because all screeners are customized somewhat to their initial purpose, only an accurate understanding of the material properties, screening objectives and separation requirements can establish the machine's capacity.
The best course of action, therefore, is to talk to an industrial screening expert, like Rotex Global. Rotex has been around for over 175 years, innovating and leading the bulk material separation industry. Our experts can walk end users through the numerous criteria that impact screening and performance. The outcome, then, is a determination of exactly what capacity the used, repurposed or new machine can achieve for optimal effectiveness.
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One:
Overs Removal
Two:
Fines Removal
Three:
Grading
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