Sieve-plate orifice

Sieve Plates. The conventional sieve or perforated plate is inexpensive and the simplest of the devices normally used. The contacting orifices in the conventional sieve plate are holes that measure 1 to 12 mm diameter and exhibit ratios of open area to active area ranging from 1 20 to 1 7. If the open area is too small, the pressure drop across the plate is excessive if the open area is too large, the Hquid weeps or dumps through the holes. 

In the microfluid dynamics approaches the continuity and Navier-Stokes equation coupled with methodologies for tracking the disperse/continuous interface are used to describe the droplet formation in quiescent and crossflow continuous conditions. Ohta et al. [54] used a computational fluid dynamics (CFD) approach to analyze the single-droplet-formation process at an orifice under pressure pulse conditions (pulsed sieve-plate column). Abrahamse et al. [55] simulated the process of the droplet break-up in crossflow membrane emulsification using an equal computational fluid dynamics procedure. They calculated the minimum distance between two membrane pores as a function of crossflow velocity and pore size. This minimum distance is important to optimize the space between two pores on the membrane... 

Fig. 13. (a) Observed and (b) computed formation of a droplet at an orifice in a pulsed sieve-plate extraction column. (Reprinted from Chemical Engineering Science, Volume 50, Ohta M., et al. Numerical analysis of a single drop formation process under pressure pulse condition, pp. 2923-2931, copyright 1995, with permission from Elsevier Science.)... 

The dry hole pressure drop for a sieve tray may be calculated by use of the following equation for thick plate orifices... 

Orifice coefficient Cq depends on the fraction open area (the ratio of the total cross-sectional area of the holes to the column cross section) and on the ratio of tray thickness to hole diameter, as shown in Fig. 18.27. The increase in Co with open area is similar to the change in Cq for single orifices as the ratio of orifice diameter to pipe diameter increases. The coefficients vary with plate thickness, but for most sieve plates the thickness is only 0.1 to 0.3 times the hole size. For these... 

The plates may be any of several types, including sieve, bubble-cap, and valve trays. Valve trays constitute multiple self-adjusting orifices that provide nearly constant gas pressure drop over considerable ranges of variation in gas flow. The gas pressure drop that can be taken across a single plate is necessarily limited, so that units designed for high contacting power must use multiple plates. 

When vapor flows through a tray deck, the vapor velocity increases as the vapor flows through the small openings provided by the valve caps, or sieve holes. The energy to increase the vapor velocity comes from the pressure of the flowing vapor. A common example of this is the pressure drop we measure across an orifice plate. If we have a pipeline velocity of 2 ft/s and an orifice plate hole velocity of 40 ft/s, then the energy needed to accelerate the vapor as it flows through the orifice plate comes from the pressure drop of the vapor itself. 

Pictures of a typical sieve tray and valve tray are shown in Figs. 12-1 and 12-2, respectively. Sieve trays consist of metal plates with small circular perforations. The valve of a valve tray consists of a self-regulating variable orifice (Fig. 12-2) which adjusts its opening in proportion to the total flow rate of the vapor. Most of the equations for sieve trays are also applicable for valve trays. A treatment of sieve trays is presented in Sec. 12-1 and the modifications of these equations as well as additional equations needed to describe valve trays are presented in Sec. 12-2. 

The setup consists of a 0.0254 m ID and 1.52 m long glass pipe, which bears a canvas filter at its upper end, and which is sealed by an orifice plate at the bottom. This latter plate contains a single 0.4 mm ID orifice in its center. The apparatus is operated in such a way that the jet gas velocity approaches the speed of sound in the orifice. The filter keeps all material inside the system. To assess the degradation extent, one should screen the material by wet sieving through —325 mesh (44 pm). The attrition rate is defined as the ratio of the... 

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