Sieve or Perforated Trays

Perhaps the simplest of cross-flow column tray designs is the sieve tray or perforated tray. The tray is a flat metal plate and the vapor openings are holes drilled in the plate. The holes are usually round, ranging from 1/8- to 1/2-inch diameter. Sieve trays have no liquid seals to prevent liquid from flowing down the holes. Liquid flow down the holes is prevented only by the upward flow of the vapor. 

---

Figure 8-67A. Sieve or perforated tray with downcomers.

Sieve or perforated trays. These are much simpler in construction, with small holes in the tray. The liquid flows across the tray and down the segmental downcomer. Figure 11.52 indicates the general form of tray layout. 

In sieve or perforated tray towers, the continuous phase runs across each tray and proceeds to the next one through a downcomer or riser. The dispersed phase is trapped as a coalesced layer at each tray and redispersed. The designs for light phase or heavy phase dispersion are shown in Figure 14.12(d). Either phase may be the dispersed one, but usually it is the raffinate. Both the reduced axial mixing because of the presence of the trays and the repeated dispersion tend to improve the efficiency over the other kinds of unagitated towers. 

The sieve miy will he selected as the most common crossflow device, and detailed attention to its performance characteristics will be given. The other crossflow devices will then be considered as modifications of the sieve tray. A schematic diagram of this besic sieve, or perforated, tray is showa in Fig. 5.7-3. Allocations of cross-sectional area are shown in Fig. 5.7-4,... 

Sieve Tray or Perforated Tray With Downcomers... 

In Section 2.5, bubble cap trays, sieve trays, and valve trays are cited as the most commonly used devices for contacting continuous flows of vapor and liquid phases however, almost all new fabrication is with sieve or valve trays. For all three devices, as shown in Fig. 13.1, vapor, while flowing vertically upward, contacts liquid in crossflow on each tray. When trays are properly designed and operated, vapor flows only through perforated or open regions of the trays, while liquid flows downward from tray to tray only by means of... 

There are two different types of energy input in sieve tray or perforated tray columns (Fig. 6-39) in a pulsed sieve tray extractor (PSE) the liquid column is pulsed in a swing tray extractor (STE) fixed sieve trays are mounted on a swinging axis which is driven by an infinitely variable, directly coupled geared motor, including crank shaft and connecting rod. The top and bottom section are expanded to allow the phases to rest and then separate. 

For perforated tray decks (sieve or grid trays), I calculate the pressure drop of the vapor flowing through the holes (inches). The pressure drop I want is the weight of liquid on the trays that I calculated above in step c. The idea is to keep the tray from leaking through the tray deck perforations. 

This is the case with diameter determination. The relation of Equation 8-250 for the perforated tray or sieve tray with downcomers can be used for the plate without downcomers. Generally, the liquid level and foam-froth height will be higher on this tray, hence the ralue of h., clear liquid on the tray, may range from 1-in. to 6-in. depending on the service. 

The extent of entrainment of the liquid by the vapour rising over a plate has been studied by many workers. The entrainment has been found to vary with the vapour velocity in the slot or perforation, and the spacing used. Strang 60-1, using an air-water system, found that entrainment was small until a critical vapour velocity was reached, above which it increased rapidly. Similar results from Peavy and Baker 6 11 and Colburn 62 have shown the effect on tray efficiency, which is not seriously affected until the entrainment exceeds 0.1 kmol of liquid per kmol of vapour. The entrainment on sieve trays is discussed in Section 11.10.4. 

For slot (bubble cap), perforation (sieve), or full valve opening (valve tray) area, the ratio of total hole area AH to Aa is 0.1 or greater. 

FIGURE 8.11 Discharge coefficient for sieve-tray performance. Note Most perforated trays are fabricated from 14-gauge stainless steel or 12-gauge carbon-steel plates. 

In the development of the above series of equations, Zuiderweg has used the work of many prior investigators and relied henvjly on the data recently released by Fractionation Research, Inc, (FRl), as reported by Sakata and YanagP 26 on the performance of two types or commercial perforated tray. Zui-derweg also presents correlations that define the flow regime transitions, pressure drop, entrainment, columa capacity, aad other operating parameters of sieve trays. 

Sieve plate A, tray or plate B, perforations C, downcomer to plate below D downcomer from plate above. 

---