Head Loss Under Downcomer Apron

This head loss, required for downcomer backiq) cakulaiion (Sec. 6.2.7), is calculated for segmental downcomers from (2-5,18,31,32) 

Equation (6.59) was derived from the orifice equation with an orifice coefficient of 0.6 (3), and assuming pure liquid is passii under the downcomer. Tests by Lockett and Gharani (43) showed that Eq. (6.59 gives conservative predictions, even under conditions of sig cam vapor entrainment in the downcomer underflow (Sec. 6.2.8). 

The clear liquid heigdtt, or the liquid holdup, is the he t to which the aerated mass would collapse in the absence of vapor flow. The dev liquid he t gives a measure of the liquid level on the tray, and is used in effidency, floodii, pressure drop, downcomer backup, weep- 

Sieve trays, froth regime. Most dear liquid height and froth density correlations (35,68,81-86) are based on the FVancis weir formula. A correlation by Colwell (68), based on a model of froth flow over the weir, was demonstrated to agree with experimental data better than other published correlations. Colwell s correlation is recommended by the author and by Lockett (12), and was successfully used as a build ing block in weeping correlations (56,63,69) and in froth regime entrainment correlation (40). Colwell s correlation is 

Some trial and error is required in this calculation, because the dear liquid hei t [Eq. ( .61)] and the froth density [Eqs. (6.64) to (6.66)] depend on each other. Under weeping conditions, additional trial and error is required because the weep fraction depends on the dear liquid he t (Sec. 6.2.12). 

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Head loss under downcomer apron, in clear liquid 0.70 1,02 0,55... 

Head Loss Under Downcomer Apron hoa is calculated based on BoUes (1963) ... 

Loss under Downcomer The head loss under the downcomer apron, as millimeters of hquid, may be estimated from... 

Downcomer backup. The factors that resist liquid flow from the downcomer onto the tray below are the froth height on the tray, the pressure drop on the tray, and the friction loss under the downcomer apron. These factors cause liquid to back up in the downcomer. Each of these factors can be expressed in terms of clear liquid heads. A tray pressure balance gives... 

Head loss due to liquid (low under the downcomer apron, in of liquid. 

The segments of the buildup are (a) the equivalent clear liquid bead on the tray h Ll (b) any hydraulic gradient A caused by resistance to liquid flow across the tray, which usually is not significant for sieve trays, (c) liquid head equivalent to pressure loss due to flow under the downcomer apron. A. and (d) total pressure loss across the tray above, necessarily included to maintain the dynamic pressure balance between point A (just above the floor of tray 3) and point B in the vapor space above tray 2. 

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