Weirs height liquid over

The clear liquid depth is equal to the height of the weir hw plus the depth of the crest of liquid over the weir how this is discussed in the next section. 

Finally we can now calculate the vapor flow rate through the tray from the pressure drop through the tray (P i - P ) and the liquid height on the tray, which we can get from the weir height fi and the height of liquid over the weir... 

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. 

Bubble-Cap Trays (Fig. 14-27a) These are flat perforated plates with risers (chimneylike pipes) around the holes, and caps in the form of inverted cups over the risers. The caps are usually (but not always) equipped with slots through which some of the gas comes out, and may be round or rectangular. Liquid and froth are trapped on the tray to a depth at least equal to the riser or weir height, giving the bubble-cap tray a unique ability to operate at very low gas and liquid rates. 

The clear liquid height is equal to the sum of the weir height, the height over the weir, and hah1 the hydraulic gradient, giving... 

A sieve-tray tower has an ID of 5 ft, and the combined cross-sectional area of the holes on one tray is 10 percent of the total cross-sectional area of the tower. The height of the weir is 1.5 in. The head of liquid over the top of the weir is 1 in. Liquid gradient is negligible. The diameter of the perforations is in., and the superficial vapor velocity (based on the cross-sectional area of the empty tower) is 3.4 ft/s. The pressure drop due to passage of gas through the holes may be assumed to be equivalent to 1.4 kinetic heads (based on gas velocity through holes). (Tray thickness = hole diameter and active area = 90 percent of total area-see Fig. 16-12). If the liquid density is 50 lb/ft3 and the gas density is 0.10 lb/ft3, estimate the pressure drop per tray as pounds force per square inch. 

Fv = Flow in ftVsec L = Width of weir, ft H = Height of liquid over weir, ft... 

Picket fence weirs are used in low-liquid-rate applications (Fig. 8). Picket fence weirs can serve two purposes at low liquid rates. First, they reduce the effective length of the weir for liquid flow increases the liquid height over the weir. This makes tray operation less sensitive to out-of-level installation. Second, pickets can prevent liquid loss (blowing) into the downcomer by spraying. This occurs at low liquid rates when the vapor is the continuous phase on the tray deck. Picket fence weirs should be considered if the liquid load is less than 1 gpm per inch of weir (0.0267 ft /sec/ft, 0.00248 m /sec/m). At liquid rates lower than 0.25 gpm per inch of weir (0.00668 ft / sec/ft, 0.000620 m /sec/m) even picket fence weirs and splash baffles have a mixed record in improving tray efficiency. Operation at liquid rates this low strongly favors the selection of structured packing. 

For a so-called "wall-behaved" sieve tray, Zone A comprises a froth (bubbly, or aerated, mixture of vapor and liquid) with observable height. Liquid droplets are projected or carried into Zone B, and some of them may be italmined from that zone to the tray above. There is also droplet movement into Zone C. in addition to nomial movement of froth over the outlet weir. For many designs an attempt is made to have Zone A predominete in the mass transfer process the well-behaved sieve tray operates in the froth contacting mode if at all possible. 

The equivalent height of vapor-free liquid over the weir may be calculated for circular columns by use of a modified version of the Francis weir formula which was proposed by Bolles.1 For a straight segmental weir... 

The amount of liquid on the plate increases with the weir height and with the flow rate of liquid, but it decreases slightly with increasing vapor flow rate, because this decreases the density of the froth. The liquid holdup also depends on the physical properties of liquid and vapor, and only approximate methods of predicting the holdup are available. A simple method of estimating h, uses the weir height h, the calculated height of clear liquid over the weir and an empirical correlation factor /3 ... 

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