Vapor flow through trays

V = total vapor flow through tray, ft /sec Nc = number of caps per tray Ns = number of slots per cap Ws = width of slot (rectangular), in. 

Va = Vapor velocity based on active area, Aa, ft/sec Total vapor flow through tray or tower, ft /sec Internal vapor flow, Ib/hr or lbs/sec. Equation 8-297... 

But what will happen to the flow of the vapor leaving tray 3, 4, or 5 I ask this question assuming that the pumparound heat duty is fixed. Will the pounds per hour of vapor flowing through trays 3, 4, or 5 ... 

At = total tower cross-sections, area, ft-Cq = vapor discharge coefficient for dry tray g = acceleration of gratdty, 32.2 ft/sec hj, = head loss due to vapor flow through perforations, in. liquid... 

Tower shells may be ferrous, non-ferrous, stainless alloys or clad (such as monel-clad-steel). The trays are usually light gage metal consistent with the corrosion and erosion problems of the system. The velocity action of vapors flowing through holes and slots accentuates the erosion-corrosion problems, and often a carbon steel tower will use... 

We have yet to discuss the most important factor in determining the height of liquid in the downcomer. This is the pressure drop of the vapor flowing through the tray deck. Typically, 50 percent of the level in the downcomer is due to the flow of vapor through the trays. 

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. 

When the vapor flow through a tray increases, the height of froth in the downcomer draining the tray will also increase. This does not affect... 

One of the most frequent causes of flooding is the use of carbon steel trays. Especially when the valve caps are also carbon steel, the valves have a tendency to stick in a partially closed position. This raises the pressure drop of the vapor flowing through the valves, which, in turn, pushes up the liquid level in the downcomer draining the tray. The liquid can then back up onto the tray deck, and promote jet flood, due to entrainment. 

The dry tray pressure drop through tray 2 decreases, due to low vapor flow through the tray deck. 

The vapor flowing between trays was at its dew point. A sudden increase in tower pressure caused a rapid condensation of this vapor and a loss in vapor velocity through the tray deck holes. The resulting loss in vapor flow caused the tray decks to dump. 

As the flow of vapor through the absorption section trays is unaffected by feed preheat, the fractionation efficiency of the trays in the upper part of the tower will not change as feed preheat is increased. On the other hand, the reduced vapor flow through the stripping section may increase or decrease fractionation efficiency—but why ... 

The pressure drop of the vapor flowing through the chimneys. The liquid on the tray has to develop enough liquid head to flow against the higher pressure below the tray. 

Finally, the pressure drop of the vapor flowing through a packed tower will be an order of magnitude less than through a trayed tower. For vacuum distillation service, this is often of critical importance. 

Increasing the tower-top reflux rate increases the rate (in ft3/s) of vapor flow through the trays, because of the combined, additive affect of factors 1 and 2. 

If an increase in the tower-top reflux rate causes the top of the tower to flood, how should the operator respond She should then increase the pumparound flow to reduce the pounds of vapor flow to tray 5, in Fig. 12.4. But suppose this causes the pumparound trays 6, 7, and 8 to flood, because of the extra liquid flow She should increase the cold liquid flow through the pumparound heat exchanger. If this cannot be done, either, then the tower pressure can be increased. This will increase the density of the flowing vapors and shrink the volume of the vapors which the trays must handle. 

And when the volume of a vapor flowing through a tray increases, so does its velocity. Any increase in vapor velocity through a tray results in higher tray pressure drop. And what is it that causes trays to flood Why, it is high tray deck pressure drop. 

Figure 1332. Internals and mode of action of trays in tray towers, (a) Some kinds of bubblecaps (Glitsch). (b) Two kinds of valves for trays, (c) Vapor directing slot on a Linde sieve tray [Jones and Jones, Chem. Eng. Prog. 71, 66 (1975)]. (d) Vapor flow through a bubblecap. (e) Sieve tray phenomena and pressure relations hh is the head in the downcomer, h, is the equivalent head of clear liquid on the tray, hf is the visible height of froth on the tray, and h, is the pressure drop across the tray (Bolles, in Smith, 1963). (f) Assembly of and action of vapor and liquid on a bubblecap tray.

Rgure 6.2 Flow-through tray vapor passages, (a) Bubble-cap tray, (b) sieve tray >c) valve tray. (M. Von Winkle, Distillation, Copyright 1967 by McGraw-Hill Inc., reprinted by permission.)... 

Many modifications of the three types of contactors just discussed have been developed in an effort to reduce costs, reduce pressure drop, equalize vapor flow through each contactor, increase plate efficiencies, or, in general, improve the operating performance of the tower. An example of this for modification of bubble-cap towers is the old Uniflux tray originally developed by Socony-Vacuum, which consisted of a series of interlocking S-shaped sections which were assembled in the form of tunnel caps with slot outlets on one side only. Segmental downcomers, similar to the downcomers in conventional bubble-cap columns, were provided. The vapors issued from the Uniflux caps in... 

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