Sieve holes

High capacity/high efficiency trays have valves or sieve holes oi bodi They typically achieve higher efficiencies and capacities by taking advantage of the active area under the downcomer. At this time, each ot the major vendors has its own version of these trays, and the designs arc proprietary. 

Trays may have damage to caps, valves, distributors, sieve holes, or packing for packed towers. 

Dissolve PVP-lodine in water, mix the solution with the fragrance and the syndet base. Pass the blend 4 x through a three-roller mill. Give the blend 3 times through a plodder with a narrow sieve hole disk. 

Pass the blended material through a wide sieve hole disk combined with a mouth hole disk. Pleat the area of the 2 disks is to 50 °C by a heating collar. 

Vapor bubbles up through the sieve holes, or valve caps, on the tray deck, where the vapor comes into intimate contact with the liquid. More precisely, the fluid on the tray is a froth or foam—that is, a mix-... 

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. 

Figure 2.2 shows a simple sieve tray, with a single hole. Why is it that the liquid flows over the 3-in outlet weir, rather than simply draining down through the sieve hole It is the force of the vapor (or better, the velocity of the vapor), passing through the sieve hole, which prevents... 

Vg = velocity of vapor or gas flowing through the sieve hole, ft/s... 

From the designer s point of view, the top tray of the stripper must have a several times greater number of sieve holes or valve caps on its tray deck than the bottom tray. If, however, all the trays in the stripper are identical, then either the bottom tray will leak (see Chap. 2), or the top tray will flood. Either way, stripping efficiency will suffer. 

Hole Sizes Small holes slightly enhance tray capacity when limited by entrainment flood. Reducing sieve hole diameters from 13 to 5 mm ( to in) at a fixed hole area typically enhances capacity by 3 to 8 percent, more at low liquid loads. Small holes are effective for reducing entrainment and enhancing capacity in the spray regime (Ql < 20 m3/hm of weir). Hole diameter has only a small effect on pressure drop, tray efficiency, and turndown. 

Not all trays are fouling-resistant. Floats on moving valve trays tend to stick to deposits on the tray deck. Fouling-resistant trays have large sieve holes or large fixed valves, and these should be used when plugging and fouling are the primary considerations. 

Fig. 3.1. Also note that this nonconventional design has the downcomer outlet area as additional active tray area. This additional active area is the tray deck area under the downcomer having valves, bubble caps, or sieve holes that allow the gas to pass through under the liquid downcomer area of the next tray up. ICPD tray programs dealing with the design and rating of sieve, bubble cap, and valve-type trays allow this active area input. This is an option shown in Table 3.1, which is offered in the three tray design/rating computer programs given in this book.

PRESSURE DROP THROUGH THE CONTACTOR ASSEMBLY. Causes for pressure drop through the contactor assembly are shown in Fig. 16-10 for bubble caps as (1) contraction, (2) friction in riser, (3) reversal of flow direction, and (4) friction in annular space. Similarly, Fig. 16-11 shows for sieve trays that this cause for pressure drop is (1) contraction and (2) friction in the sieve hole. The total pressure drop due to the preceding causes is primarily a function of the kinetic head. The pressure drop as feet of liquid equivalent to one kinetic head is... 

For sieve trays, the number of kinetic heads equivalent to the total pressure drop through the plate itself is a function of the ratio of the sieve-hole diameter to the tray thickness and the ratio of the hole area per tray to the active area per tray as shown in Fig. 16-5. This pressure drop for a reasonable sieve-tray design is generally in the range of 1 to 3 kinetic heads, and Fig. 16-12 can be used to choose the most reasonable number to use in preliminary designs Designating the number of kinetic heads obtained from Fig. 16-12 as K.H., the pressure drop due to gas flow through the holes for a sieve tray expressed as liquid head is... 

PRESSURE DROP DUE TO LIQUID HEAD ABOVE SLOTS, SIEVE HOLES, OR VALVE OPENINGS. Reference to Fig. 16-10 shows that the total head above bubble-cap slots for an average cap is the sum of static submergence Sm, height of liquid crest above weir ha, and average liquid gradient 0.5hg. The same... 

Active area of plate is 88 percent of the total column cross-sectional area The total area of sieve holes is 5 percent of the active area of the plate Weir height = 2.0 in. 

Estimate the gas pressure drop across the tray, the percent of the pressure drop due to liquid head above the sieve holes, and the liquid head in the... 

K.H. from Fig. 16-12 (with area of sieve hole/active area of plate = 0.05 and tray thickness/hole diameter = 1.0) = 1.5... 

Percent of A pT due to liquid head above the sieve holes... 

The major factors that determine the amount of entrainment are plate spacing, depth of liquid on the tray, and vapor velocity in the space between the plates. Slot or sieve-hole vapor velocity and liquid flow rate have some effect on the entrainment, but they are not of major importance. 

Vc = linear velocity of gas in riser, reversal area, or annulus of bubble cap (maximum value) or in sieve hole, ft/s = maximum allowable superficial linear velocity of gas (based on net cross-sectional area of tower for vapor flow), ft/s, see Eq. (3)... 

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