Dry trays

This problem, as with flooding, also impairs product quality. No fractionation occurs in the dry section, so the temperature difference decreases. However, unlike flooding, the pressure drop decreases and stays very steady at the ultimate minimum value. This problem is usually easier to handle than flooding. The problem is caused by either insufficient liquid entering the section or too much liquid boiling away. The problem is solved by reversing the action that caused the dry trays. 

For stable tray operation, the hydraulic gradient should be less than one half the dry tray pressure drop. For conditions of high weir height and high Vq (Pv) the greater... 

This metliod calculates the dry tray pressure drop and allows for correcting the two-phase flow effects at various entrainment ratios. 

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... 

Fv = valve tray F-factor, ft- /min/valve Fvm = valve tray F-factor at the beginning of the valve open region, fr /min/valve g = gravitational constant, ft/s he = clear liquid height, in. ho = dry tray pressure drop, in. 

Compare calculated h j with value of dry tray pressure drop as given ... 

This is based on the correlation of Mayfield [45] where hjt (weep) = dry tray pressure drop at tray weep point, in. liquid. 

Set minimum design dry tray pressure drop 30% above the value of hdt (weep). 

Orifice coefficient. Figure 8-129, read at 0.41 tray/hole gives Cq orifice coefficient = 0.75 Hole velocity = 347/8.66 = 40.06 fps Dry Tray pressure drop... 

Closed and Open Loss Coefficients for Dry Tray Pressure Drop Equations 8-314 and 8-315... 

Beyond point B on the diagram, the pressure drop for the tray increases as the vapor rate increases. Use Equation 8-314 or 8-315 to determine the dry tray pressure drop, AP, in. liquid, Bolles [205] per Klein [201] ... 

Dry tray efficiency, fraction Wet tray efficiency, fraction... 

Total number of actual trays in tower Number of caps per tray Number of slots per bubble cap Valve density, number of valves per ft or Number of valve units on a valve tray Depth of notches in weir, in or Exponent defined by Equations 8-288 and 327 Dry tray pressure drop for 50% cut baffles, in. liquid per baffle or... 

The chloroimide decomposed violently on a drying tray at 60°C [1], and a bottle accidentally heated to 50°C exploded [2], Decomposition of the heated solid occurs after a temperature-dependent induction period. Unheated material eventually exploded after storage at ambient temperature [3],... 

APdry Pressure drop over dry tray expressed as head m L... 

When the dry tray pressure drop is significantly less than the hydraulic tray pressure drop, then the tray will start to leak or weep, and tray efficiency will be adversely affected. 

For a tray to function reasonably close to its best efficiency point, the dry tray pressure drop must be roughly equal ( 50 percent) to the hydraulic tray pressure drop ... 

As illustrated, liquid accumulates on the low side of this tray. Vapor, taking the path of least resistance, preferentially bubbles up through the high side of the tray deck. To prevent liquid from leaking through the low side of the tray, the dry tray pressure drop must equal or exceed the sum of the weight of the aerated liquid retained on the tray by the weir plus the crest height of liquid over the weir plus the 2-in out-of-levelness of the tray deck. 

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

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. 

Bottom tray in tower leaking, due to a low dry tray pressure drop. 

Work by Davies [Pet. Ref. 29(8), p. 93, and 29(9), p. 121 (1950)] based on bubble-cap tray studies suggests that the vapor pressure drop of the tray (the dry pressure drop) counteracts channeling. The higher the dry tray pres sure drop, the greater the tendency for vapor to spread uniformly over the bubbling area. If the dry tray pressure drop is too small compared with the channeling potential, channeling prevails. 

Low dry tray pressure drop. On sieve and fixed valve trays, this means high (>11 percent) fractional hole area. On moving valve trays, this means venturi valves (smooth orifices) or long-legged valves (>15 percent slot area). On all trays, the channeling tendency and severity escalate rapidly as the dry pressure drop diminishes (e.g., as fractional hole area increases). 

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