Sieve trays with downcomers downcomer

Figure 8-118. Sieve tray with downcomers, tower assembly. Used by permission, Hendrick Mfg. Co.

Figure 8-124. Typical operating curve of sieve trays with downcomers. Note modes of operation used by permission, Chem. Eng., Chase, J. D., July 31 (1969), p. 105. Also see Klein [201], Figure 8-148.

Figure 8-130. Effective iiquid head for sieve trays with downcomers. Used by permission, Hughmark, G. A. and O Connell, H. E., The American Institute of Chemical Engineers, Chem. Eng. Prog. V. 53, (1957 ), p. 127M, all rights reserved.

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. 

Fig. 9.4 Sieve trays with downcomers for liquid systems with (a) high interfacial tension and dual-flow trays for liquid systems and (b) with low interfacial tension. In the case of downcomers, only the phase to be dispersed flows through the holes. The droplets are formed by jet disintegration. In dual-flow trays, both hquids flow through the same holes alternately. The larger drops spUt because of colhsion with the tray.

In systems with a low surface tension, the hole size of the sieve tray with downcomer should be smaller than 2 mm to prevent the continuous phase percolating through. For the treatment of such systems, dual flow trays without downcomers are more suitable. On a dual flow tray, the disperse and continuous phases flow in turn through the base plate holes and between the trays, producing strongly circulating convection cells. 

Fig. 6-33. a) Sieve tray with downcomer b) Dual-flow tray... 

Opening ratio 5 (sieve tray with downcomer)... 

Sieve Tray or Perforated Tray With Downcomers... 

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

The main kinds of cross flow trays with downcomers in use are sieve, valve, and bubblecap. 

Dual-Flow Trays These are sieve trays with no downcomers (Fig. 14-27b). Liquid continuously weeps through the holes, hence their low efficiency. At peak loads they are typically 5 to 10 percent less efficient than sieve or valve trays, but as the gas rate is reduced, the efficiency gap rapidly widens, giving poor turndown. The absence of downcomers gives dual-flow trays more area, and therefore greater capacity, less entrainment, and less pressure drop, than conventional trays. Their pressure drop is further reduced by their large fractional hole area (typically 18 to 30 percent of the tower area). However, this low pressure drop also renders dual-flow trays prone to gas and liquid maldistribution. 

The single-pass sieve tray with crossflow appears to be the most widely used type of tray today. In a tray of this type, the vapor passes upward through the perforations (or valves) and then through the liquid on the tray. The liquid flows down through the column and enters each plate by flowing under the downcomer weir. Then the liquid flows across the plate and over the outlet weir and into the downcomer to the plate below as shown in Fig. 12-3. In order for the liquid to flow across each plate in succession as it passes down through the column, a liquid head in each downcomer is required as depicted in Fig. 12-3. 

Figure 12-4 Height of vapor-free liquid in the downcomer of a single-pass sieve tray with crossflow.

SPECIAL SIEVE TRAYS. In some columns equipped with sieve trays no downcomers are used. There is no crossflow of the liquid the liquid and vapor pass... 

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