November 2011
Extra Onions Please!
SWECO Round Separator
MX™ Round Separator

A processing plant in South Carolina called SWECO to screen and size onion rings. They were particularly interested in the whole rings 3-4 inches (75-100 mm) in diameter.

The process starts with whole onions being hand fed into a slicer which drops them onto a conveyor feeding the SWECO MX Round 		Separator. The onions are fed so that when they are sliced they make the perfect sized onion ring just like you see on your burgers. However, the rings don't separate easily once they are sliced. This customer wanted to shake them so hard that the pieces that were stuck together would untangle and separate into proper sizes.

After the slicing, the pieces are carried by a belt conveyor that is vertical to the SWECO MX Separator and drops the rings into an approximate 4 foot freefall to the screen. The customer also installed some impingement bars for the whole slices to hit and additionally breakup up the rings. This machine has two oversized spouts 180 degrees apart on the top frame where most of the rings discharge.

 The rings are hand packed into 10 pound (4,5 kg) boxes and shipped to several fast food restaurants for inclusion on some of your favorite sandwiches.

SWECO Separators are ideal for various food applications, including onions


Selecting the Proper Screen Opening for Fines Removal Applications

Screen selection can get complicated when the objective of the screening operation is to remove fine particles. It is important to understand the specifications or requirements on the finished product. To illustrate, assume that the desired end product is coarser than a #60 US test sieve. In other words, a screening machine is used to remove the fine particles that are smaller than the opening of a #60 US test sieve. It is important to note here that due to inefficiencies inherent in all production screening machines, it is impossible to remove all fine particles. Therefore, any properly crafted product specification for a fines removal application will allow some tolerance. For example a 		reasonable specification for the allowable fines content in the screened product might be: "5% max - #60 US". What this means is that when a representative sample of the screened material is analyzed in the lab, no more than 5% of the sample should pass through the #60 US test sieve.

When presented with such a specification, one's first instinct might be to select a 60 mesh screen for use in the production screening machine. This may or may not be the proper selection. If one were to select a 60 mesh market grade screen, it is highly unlikely that the resulting product would be within tolerance. The

SWECO offers various forms of wire mesh for your separation needs

Woven Wire Screen Cloth
 reason is that the 234µ opening of the 60 mesh market grade screen is actually smaller than the 250µ opening on a #60 US test sieve. A better choice would be a screen that has an opening somewhat larger than that of the #60 US test sieve. This larger opening will encourage "nearsize" fine particles to pass through the screen and make it more likely that the screened product will meet the fines content specification. In general, the larger the screen opening, the better the fines removal performance. However, if the selected screen opening is too large, at some point a considerable amount of good product will pass through the screen and be lost to the fines stream.

It should be noted that poor fines removal performance in a production screening machine can be caused by several different factors beyond simply the screen opening selection. Examples are screen blinding, screen motion, feed rate, and material characteristics such as moisture content and static charge.


Hydrocyclone Systems

SWECO Hydrocyclone Systems provide an economical and effective method for removing solid particles in the 4 to 100+ micron range from various slurries. Hydrocyclones typically make finer separations than are practical with screening separators and at significantly higher capacities. In many applications cyclones can be used in place of decanting centrifuges, providing the desired result at lower cost.

Hydrocyclones are supplied in either an open-manifold or a Packed-Vessel configuration. Manifolds (radial or linear orientation) utilize 2-, 4-, 5-, or 10- 		inch diameter cyclones while Packed Vessels contain either 1- or 2-inch diameter cyclones. Both configurations can process feed rates from 10 to 3,000 gallons per minute, depending on the size and number of hydrocyclones installed in the system.

The use of SWECO Hydrocyclones

SPacked Vessel Hydrocyclones from SWECO
SWECO Packed Vessel Hydrocyclone
 can lower the capital, operating, and maintenance costs of a solids-separation system. Installation is straight-forward, as the hydrocyclone system is shipped assembled. Reliable operation of the system only requires a consistent feed at a steady pressure. The only moving parts are the feed pump and the slurry itself. Energy costs are much lower than those of a large centrifuge and the maintenance, when needed, is much simpler and easier to accomplish.

For demanding applications, SWECO can provide a hydrocyclone system integrated with a SWECO Centrifuge, Rectangular Separator, or Round Separator. The hydrocyclone system will concentrate the solids, reducing the volume to be handled by the centrifuge or screener. A smaller centrifuge or screener can then be selected at a considerable reduction in cost.
Principle of Operation
A hydrocyclone has no moving parts and achieves solid/liquid separation by virtue of a pressure drop across the unit. A slurry is forced under pressure into the inlet section of the liner via a tangential inlet port. This, together with narrow cyclone diameter, causes the fluid to spin at high velocity which creates a high g radial acceleration field. The solid particles, being the denser phase, are forced outwards to the hydrocyclone inner wall. Here, through internal hydrodynamic forces, the solids are ejected from the apex of the cyclone while the liquid exits counter currently via an axial port adjacent to the inlet. The separated solids can be discharged directly. Where the discharge of liquid with the solids is to be minimized, the solids can be collected in an accumulator for settling. The accumulator can then be periodically purged of concentrated solids without interrupting the operation of the hydrocyclone system.

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