Perforated Plates and Screens

Perforated Plates and Screens A nonuniform velocity profile in turbulent flow through channels or process equipment can be smoothed out to any desired degree by adding sufficient uniform resistance, such as perforated plates or screens across the flow channel, as shown in Fig. 6-38. Stoker (Ind. Eng. Chem., 38, 622-624 [1946]) provides the following equation for the effect of a uniform resistance on velocity profile  

V is the area average velocity, K is the number of velocity heads of pressure drop provided by the uniform resistance, Ap = KpV2/2, and a is the velocity profile factor used in the mechanical energy balance, Eq. (6-13). It is the ratio of the area average of the cube of the velocity, to the cube of the area average velocity V. The shape of the exit velocity profile appears twice in Eq. (6-154), in V%mIV and a2. Typically, K is on the order of 10, and the desired exit velocity profile 

Screens and other flow restrictions may also be used to suppress stream swirl and turbulence (Loehrke and Nagib, J. Fluids Eng., 98, 342-353 [1976]). Contraction of the channel, as in a venturi, provides further reduction in turbulence level and flow nonuniformity. 

---

The A-A extruders are equipped with a flat perforated plate or screen, combined with two flat extrusion heads. The wet mass is pressed axially through the screen parallel to the feed screws. The extrusion forces are the highest, compared to other low pressure extruders, resulting in a hard and dense extrudate. It is commonly used in the extrusion of food, thermoplastics, and other industries, where a large pellet diameter and thermosetting properties are desired. 

Batch fluid beds may appear in several forms. The process chamber has a perforated plate or screen in the bottom and a drying gas outlet at the top, usually fitted with an internal filter. The drying gas enters the flnid bed through a plenum chamber below the perforated plate and leaves through the filter arrangement. The batch of material is enclosed in the process chamber for the duration of the process. 

Most of the structure of the product is formed in a (tubular) B-unit. The residence time should be related to the rate of crystallization of the fat blend so that at the end of the tube, crystallization is practically complete. In general, a residence time of some minutes is sufficient. To obtain a smooth, plastic and packable product, some working is usually carried out by placing a metal perforated plate or screen at the end of the unit. Without working there, a brittle and too hard structure may result. 

Industrial screens - Woven wire cloth, perforated plate, and elect-roformed sheet - Designation and nominal sizes of openings Test sieves and test sieving -Vocabulary... 

Apertured non-woven fabric n. A non-woven fabric having many small through-holes made by laying the fabric on a perforated plate or screen and applying fluid pressure. 

---