Types of Screens

Most screens are characterized by the fact that the particles drop through the screen opening by gravity. Few designs are found where the particles are pushed through by a brush or 

There are three major types of screen designs the single stage, sequential, and tiered. Both the sequential and tiered are multistage approaches, and each of these types also varies in terms of how many parameters are measured. But these three major types can be characterized as follows  

Single Stage. A single test will be used to determine acceptance or rejection of a test material. After an activity criterion (such as X score in a righting reflex test) is established, compounds are evaluated based on being less than X (i.e., negative) or equal to or greater than X (i.e., positive). As more data are accumulated, the criterion should be reassessed. 

Tiered (or Multistage). In this procedure, materials found active in a screen are reevaluated in one or more additional screens or tests that have greater discrimination. Each subsequent screen or test is both more definitive and almost always more expensive. 

For purposes of our discussion here, we will primarily focus on the single-stage system, which is the simplest. The approaches presented here are appropriate for use in any of these screening systems, although establishment of activity criteria becomes more complicated in successive screens. Qearly, the use of multistage screens presents an opportunity to obtain increased benefits from the use of earlier (lower-order) screening data to modify subsequent screen performance and the activity criterion. 

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Tr ash removal—Removal of extraneous foreign matter from a processed material. Essentially a form of scalping operation. Type of screen employed will depend on size range of processed material—coarse, fine, or ultrafine. 

Screen Surfaces The selection of the proper screening surface is veiy important, and the opening, wire diameter, and open area should all be carefully considered. The four general types of screening surfaces are woven-wire cloth, silk bolting cloth, punched plate, and bar or rod screens. 

Square-mesh cloth is the conventional type of screen cloth, but there are many types of cloth with an oblong weave. This latter construction provides greater open area and capacity and in addition makes it possible to use stronger wire for the same size of screen opening and for the same percentage of open area. 

Vibration Amplitude and Frequency Speed and amplitude of vibration should be designed to convey the material properly and to prevent blinding of the cloth. They are somewhat dependent upon the size and weight of the material being handled and are related to the angle of installation and the type of screen surface. The object, of course, is to see that the feed is properly stratified for the most efficient separation. 

Some types of screens used in hit to lead are controversial and are used in some organizations but not in others. Examples of these are ... 

Figure 10.2, which is based on a similar chart given by Matthews (1971), can be used to select the type of screening equipment likely to be suitable for a particular size range. Equipment selection will normally be based on laboratory and pilot scale screening tests, conducted with the co-operation of the equipment vendors. The main factors to be considered, and the information that would be required by the firms supplying proprietary screening equipment, are listed below ... 

Results of the screen also show that compounds that passed four levels of the screen were from single synthetic collections, that combinatorial collections lacked the "chemical diversity" demanded by this type of screen, and that samples tested in Level 4 from natural product collections were very toxic to the liver or kidney. 

Two types of screening procedures are used to identify ES cell clones carrying a targeted integration of the construct DNA polymerase chain reaction (PCR) and Southern blot analysis (Southern, 1975). Both methods rely upon the specific juxtaposition of vector components and target locus sequences after homologous recombination. 

The decision on what type of screen to use in FBDD is affected by many different factors availability of protein for screening, compound selection, throughput, turnaround and rate of false positives and negatives. The resolution of these questions from target assessment directly impact the possibilities in this section. If there is not sufficient protein for a biophysical screen, a biochemical screen is the only choice. Protein that is not stable for... 

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