Velocity of vapor

If there is the possibility of apor and liquid rates being reduced to 50% of the indicated values, this would place the trays as selected above at the dumping point, or activation point, which is not a good operating condition. In this situation the number of holes should be reduced in order to maintain a velocity of vapor through the holes greater by at least 15% than the activation velocity. 

G = mass velocity of vapor from a bottom tube based on the (p — Do) spacing, lb/ (hr) (fP)... 

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

The absolute tower pressure (in psia) increased by 17 percent, and hence the volume (as well as the velocity of vapor through the valve tray caps) declined by 17 percent. The reduced vapor velocity reduced the dry tray pressure drop, thus reducing both the spray height above the tray deck and the liquid backup in the downcomers. 

Vg = velocity of vapor passing up through the chimneys, ft/s The orifice hole pressure drop is... 

Velocity of vapor. The slower the vertical velocity of the vapor, the faster the settling rate. 

G = mass velocity of vapor (kg/m2 sec) of tower area, dc = column diameter (m),... 

If superheating of the surface is such that the equilibrium vapor pressure is a factor 1 + 0 greater than the external pressure, the evaporation rate, expressed as the linear velocity of vapor outflow from the surface, comprises 0cx/2. A combustion rate of 1 mm/sec of the liquid EM corresponds at atmospheric pressure to a vapor outflow rate of order 50 cm/sec, to which corresponds 0 from 0.001 to 0.002 and superheating of 0.02-0.04. For an accommodation coefficient a / 1 and otherwise equal conditions, the superheating increases proportionally to 1 /a. Finally, the superheating is proportional to the combustion rate. 

The use of Fig. 13.36 requires a value of the packing factor i, which may be taken from Table 13.13 or 13.15. The abscissa term is the same as that for tray columns. It is sensible to use a value of about 80% of the maximum curve (equivalent to 1.5 in. H20/ft, or 125 mm Fl20/m) for design purposes. The corresponding ordinate term then allows deduction of the value of Cs, defined as Cs = Us[pg/(pL - Pg)]°, where Us is the allowable superficial velocity of vapor or gas through the column. Also, v = liquid kinematic viscosity = centistokes (centipoises/density, gm/ml). Example 13.16 shows how Fig. 13.36 is used. 

Hundreds of fluidized bed crackers are in operation. The vessels are large, as much as 10 m or so in diameter and perhaps twice as high. Such high linear velocities of vapors are maintained that the entire catalyst content of the vessels circulates through the cyclone collectors in an hour or so. Electrical precipitators after the cyclone collectors have been found unnecessary. 

Pressure drop through the column of catalyst pellets agrees with published data correlations (96) relating pressure drop with particle size, mass velocity of vapor, and vapor density. Variations in viscosity of the vapor can be neglected because viscosity has a very low exponent in the pressure-drop formula for the turbulent-flow conditions that exist in TCC units (239). 

Velocity of vapor in bubble formation zone (Sections 12.1 and 12.2) [m/s] Rise velocity of Zrth bubble population (Section 12.1 and 12.2) [m/s] Superficial velocity [m/s]... 

Figure 13-9 Correlation of the load factor (Fr = uvJuvsf at a fixed ratio of liquid to vapor mass flow rates, uvs = velocity of vapor based on the superficial area, ft/s, uvsf = velocity of the vapor based on the superficial area at the flood point, ft/s.) [W. L. Bolles and J. R. Fair, Third International Distillation Symposium, London, April 1979, No. 213 of E.F.C. Sec. 3.3, p. 35, by courtesy Institute of Chemical Engineers (London).]...

Eventually, the velocity of vapor in the jets becomes so large that the jets themselves become unstable near the interface as a result of Helmholtz instability (of wavelength XH) as shown in Fig. 15.63). The breakup of the jets destroys the efficient vapor-removal mechanism, increases vapor accumulation at the interface, and leads to liquid starvation at the surface and to the critical heat flux phenomenon. If jet breakup occurs at a vapor velocity UH within the jets, the critical heat flux q"m is given by... 

Vw is the absolute velocity of vapor within the porous solid (m s" )... 

V = Velocity of vapor through the holes in the tray deck, fl/sec Pv = Density of vapor Pl = Density of liquid... 

I explained that as we add more heat to the big can (more reboiler duty) and more capacity to the No. 1 coil (more condenser duty) that fractionation would get better. Better in the sense that the proof of the vodka would go up without reducing the production of vodka. Suppose, though, that the velocity of vapor leaving the big can becomes too great. We ve all seen what happens when we boil soup too quickly in a small pot. The pot foams or floods over onto the stove. We should have used a bigger pot or we should have kept the heat low on the stove. 

To summarize. I ll select a hole area for the tray, so that the velocity of vapor flowing through the holes will be big enough to keep the tray from leaking. Of course, if the tray decks are badly out of level, the above calculations are meaningless. So don t forget to inspect your tray installation for tray deck levelness (see my book. Process Equipment Malfunctions, McGraw-Hill, 2011). 

The absolute tower pressure (in psia) increased by 17 percent, and hence the volume (as well as the velocity of vapor through the valve... 

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