Ruh

shoot of over twice the head. The effect for straight weirs is probably less, and Edminster (Ref. 7) has suggested neglecting the portion of the weir that is closer to the wall than the value of Acr. This would appear to be a reasonable assumption.

For large downspouts running full, it is recommended that Eq. (16-11) be used, employing for D four times the hydraulic radius, which is equal to the cross-sectional area of the downspout divided by the perimeter.

When the liquid head over the weir is low, the discharge from one side to the other will vary if the top edge is not level. This variation can be reduced by using a V-notched top which reduces the effective weir length at low rates of flow. In general, it is desirable to keep the head over the weir low because this reduces the variation of liquid depth on the plate for different operating rates. Values of her of 1.0 in. are common, but they seldom exceed 3 in.

In some cases, under- and overweirs are used at the outlet to handle

two liquid layers on a plate. Such an arrangement can also be used to hold back foam from the down pipe. The clearance between the underweir and the plate must be adequate to handle the liquid load.

The bottoms of downflow pipes must have enough clearance to allow the liquid to flow easily, but they should not allow vapor to by-pass up through them. This vapor by-passing is usually prevented by a positive seal which holds liquid above the bottom of the down pipe (see Fig. 16-1). In large columns, this seal is often made an inlet weir which serves to distribute the liquid as well as seal the down pipe.

For the reversal loss at the bottom of the down pipe, it is recommended that a loss equal to one kinetic head be used with a coefficient of 0.6.

= 0.57|, where hD = loss in head at bottom of down pipe, in.

Vd = maximum velocity at bottom of down pipe, f.p.s.

Liquid Gradient. One of the important factors that must be considered in the design of a bubble plate is the liquid gradient across the plate. It is obvious that the liquid level will normally be higher at the liquid inlet than at the outlet. In small towers, this difference in level offers no serious difficulties, but in towers of moderate and large diameters it can become so great that the vapor distribution is poor and the overflow may by-pass the plate by dumping through the caps.

The gradient is due to the resistance to liquid flow across the plate and results from (1) friction with the caps and the plate, (2) eddy losses in the liquid due to repeated acceleration and deceleration, and (3) resistance due to the effects of vapor flow.

Experimental data on this gradient have been published by several investigators (Refs. 10, 11, 12, 13, 18, 19,29). Gonzales and Roberts (Ref. 12) studied a plate with 4%-in.-diameter caps which were in. tall using air and water. Their column was rectangular in shape with 12 rows of caps in the direction of liquid flow. Based on liquid flow around staggered pipes, they developed the following equation for the gradient as a function of the number of rows of caps:

where h liquid depth on plate A,B *= constants

Their data were taken at liquid levels less than the top of the caps and agreed well with the equation for both aerated and unaerated conditions. A and B were functions of both the water and air rates.

Bijawat (Ref. 2) reviewed the data obtained by Gonzales and Roberts and, on the basis of orifice-type flow of the liquid between the caps, suggested h* = A' - B'n (16-15)

This equation correlated the data about as well as Eq. (16-14). Seuren (Ref. 29), Ghormley (Ref. 11), and Kesler (Ref. 19) obtained data on a rectangular plate with 10 rows of 4-in. caps in the direction of liquid flow. Good, Hutchinson, and Rousseau (Ref. 13) investigated the liquid gradient on a rectangular plate having 12 rows of 3-in. caps for a number of operating conditions.

Klein (Ref. 20a) has investigated the factors that cause the loss in head of the liquid flowing across the plate. His data indicate that with no vapor flow there is essentially no hydraulic gradient even at liquid rates considerably greater than those normally employed. It was concluded that the loss in head was due to high frictional losses for the flow of the vapor-liquid mixture. The drag per unit area was determined by measuring the force on a plate suspended in the aerated liquid, and values ten times as large as for unaerated liquid flowing at the same linear velocity were found. Abnormally high frictional losses have also been reported for the flow of mixtures of vapor and liquids in pipes.