Plate construction downcomers

Countercurrent plates are of perforated or slottea construction and require no downcomers. The vapor and the liquid use the same openings, alternating on an intermittent basis. Layout of such plates is extremely simple. Types of such plates used commercially are Perforated (dual-flow)... 

A schematic of the two-dimensional test apparatus with three draft tubes is shown in Fig. 12. The two-dimensional bed is constructed with transparent Plexiglas plates in the front and aluminum plates at the back with a cross-section of 50.8 cm by 2.54 cm and 244 cm high. The three draft tubes have a cross-sectional area of 2.54 cm by 2.54 cm each and 91 cm high. The three draft tubes divide the bed into four separate downcomers. The two downcomers next to the side walls have a cross-section of 5.9 cm by 2.54 cm while the remaining two downcomers have a cross-section exactly two times, i.e., 11.8 cm by 2.54 cm. If all three draft tubes operate similarly, the bed should have three identical cells, each with a single draft tube. The distance between the draft tube inlet and the air distributor plate was maintained at a constant spacing of 5.1 cm throughout the experiments. 

These three types of trays have a common feature in that they all have separate downcomers for the passage of liquid from each tray to the one below. There is another class of tray which has no separate downcomers and yet it still employs a tray type of construction giving a hydrodynamic performance between that of a packed and a plate column. Two examples of this type of device are the Kittel plate and a Turbogrid tray(53). Design data for these trays are sparse in the literature and the manufacturer s recommendations should be sought. 

The stacked type of construction is used where the column diameter is too small for someone to enter to assemble the plates, say less than 1.2 m (4 ft). Each plate is fabricated complete with the downcomer, and joined to the plate above and below using screwed rods (spacers) see Figure 11.25. The plates are installed in the column... 

The segmental, or chord downcomer, shown in Figure 11.26a is the simplest and cheapest form of construction and is satisfactory for most purposes. The downcomer channel is formed by a flat plate, called an apron, which extends down from the outlet weir. The apron is usually vertical but may be sloped (see Figure 11.26b) to increase the plate area available for perforation. If a more positive seal is required at the downcomer at the outlet, an inlet weir can be fitted (see Figure 11.26c) or a recessed seal pan used (see Figure 11.26d). Circular downcomers (pipes) are sometimes used for small liquid flow rates. 

For tray colnmns, liquid flow constrictions are normally found in the tray downcomers. Example causes are lack of adequate froth collapse time, excessive pressure drop for flow under the downcomer baffle, and foreign materials being left in the downcomer by construction people. For packed columns, clogged support plates, broken packings, or growth of fouling deposits can cause liquid flow restrictions. 

Sieve plates numerous small holes of to -fe-in. diameter spaced on uniform triangular pitch to cover about 15 per cent of the plate area require liquid downcomers on each plate sieve trays are cheaper to construct and give lower pressure drop for the same plate efficiencies. 

The objective was to construct and test a half-scale acrylic model of the gas cooled reactor concept to qualitatively assess flow distribution and stability. Areas of interest for flow visualization were the inlet nozzle, the inlet plenum, the downcomer, and the lower plenum. Pressure measurements were to be taken to estimate the parasitic losses in the reactor inlet regions. The model was not a prototypical mock-up of the fueled core geometry. Instead, it included a perforated plate that simulated the pressure drop across the fuel region of the reactor. Flow fields in the outlet plenum and outlet nozzle were not to be evaluated in this test. 

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