Weirs, flow over

Fig. 10. Stream flow over (a) a broad-crested, rectangular weir (b) a cross-current view of the rectangular and CipoUetti weirs (c) a trape2oidal-notch or CipoUetti weir (d) a sharp-crested, triangular, or V-notch weir (e) a cross-current view of the V-notch and hyperboHc-notch weirs and ( a...

For flow over a sharp-crested weir of width b and height L, from a liquid depth H, the flow rate is given approximately by... 

As noted, the weir crest is calculated on an equivalent clear-liquid basis. A more realistic approach is to recognize that in general a froth or spray flows over the outlet weir (settling can occur upstream of the weir if a large calming zone with no dispersers is used). Bennett et al. [AIChE J., 29, 434 (1983)] allowed for froth overflow in a comprehensive study of pressure drop across sieve plates their correlation for residual pressure drop /jf in Eq. (14-87) is represented by Eqs. (14-115) through (14-120) ... 

When straight or serrated segmental weirs are used in a column of circiilar cross secdion, a correction may be needed for the distorted pattern of flow at the ends of the weirs, depending on liquid flow rate. The correction factor F from Fig. 14-33 is used direcdly in Eq. (14-112) or Eq. (14-119). Even when circular downcomers are utilized, they are often fed by the overflow from a segmental weir. When the weir crest over a straight segmental weir is less than 6 mm V in), it is desirable to use a serrated (notched) weir to provide good liquid distribution. Inasmuch as fabrication standards permit the tray to be 3 mm Vh in) out of level, weir crests less than 6 mm V in) can result in maldistribution of hquid flow. 

ESPs for liquid droplets and mists are essenhally of the wetted wall type. Figure 29-9 shows a wet wall precipitator with tubular collechon electrodes (1). The upper ends of the tubes form weirs, and water flows over the tube ends to irrigate the collechon surface. 

Vapor rises up through risers or up-takes into bubble cap, out through slots as bubbles into surrounding liquid on tray. Bubbling action effects contact. Liquid flows over caps, outlet weir and downcomer to tray below. Figures 8-63-67, 79, and 81. 

Liquid throw The horizontal distance traveled by the liquid after flowing over a weir. 

Weeping A condition occurring when the vapor rate is not large enough to hold all the liquid on the tray, so that part of the liquid flows over the outlet weir while the rest falls through the holes. 

The hot well is the sump where the barometric leg is sealed. It must be designed to give adequate cross-section below the seal leg and for upward and horizontal flow over a seal dam or weir. At sea level the hot well must be a minimum of 34.0 ft below the base of the barometric condenser. For safety to avoid air in-leakage, a value of 35-36 ft is used. For an altitude corresponding to a 26-in. Hg. barometer, the theoretical seal height is 29.5 ft actual practice still uses about 34 ft. 

The notch or weir, in which the fluid flows over the weir so that its kinetic energy is measured by determining the head of the fluid flowing above the weir. This instrument is used in open-channel flow and extensively in tray towers 3 where the height of the weir is adjusted to provide the necessary liquid depth for a given flow. 

An organic liquid flows across a distillation tray and over a weir at the rate of 15 kg/s. The weir is 2 m long tmcl the liquid density is 650 kg/m3. What is the height of liquid flowing over the weir ... 

Use is made of the Francis formula (equation 6.43). where, as in the previous example, n — 0. In the context of this example the height of liquid flowing over the weir is usually designated hmv and the volumetric liquid flow by Q. Rearrangement of equation 6.43 gives ... 

In this simplified model, it is assumed that liquid may leave the plate, either by flow over the weir Ln(weir) or by weepage Ln(weep)- Both these effects can be described by simple hydraulic relations, in which the flow is proportional to the square root of the available hydrostatic liquid head. The weir flow depends on the liquid head above the weir and hence... 

With segmental downcomers the column wall constricts the liquid flow, and the weir crest will be higher than that predicted by the Francis formula for flow over an open weir. The constant in equation 11.85 has been increased to allow for this effect. 

The liquid throw is the horizontal distance travelled by the liquid stream flowing over the downcomer weir. It is only an important consideration in the design of multiple-pass plates. Bolles (1963) gives a method for estimating the liquid throw. 

Wastewater exits the lagoon and flows over a weir and into a splitter box where the flow is split to the two secondary clarifiers. Cationic polymer is added, as needed, before the secondary clarifiers, to enhance settling of the suspended solids. The settled solids consist of active biological matter and are returned via return sludge pumps to the lagoon through a return line that discharges from two pipes within 7.6m (25 ft) of the influent from the lift pumps. 

Consider a liquid flowing over a sharp crested weir as shown in Figure 8.8. Let the upstream level of the liquid be z above the level of the weir crest. As the liquid approaches the weir, the liquid level gradually drops and the flow velocity increases. Downstream from the weir, a jet is formed. This is called the nappe and it is ventilated underneath to enable it to spring free from the weir crest [Barna (1969)]. 

It is possible to operate a fluidized bed in either batch or continuous mode. Strictly, most batch applications are in tact operated in semibatch mode where the solids are treated as a batch but the fluidizing medium enters and leaves the bed continuously. In the case ot gas-solid beds used in termentation (see Chapter 6), the fluidizing gas is recirculated although reactants and products flow continuously. In true continuous operation the solids may be ted into a fluidized bed via screw conveyors, weigh teeders or pneumatic conveying lines and can be withdrawn trom the bed via standpipes or by flowing over weirs. 

An alternate configuration (Fig. 3), the "bucket and weir" design, eliminates the need for a liquid interface controller. Both oil and water flow over weirs where level control is accomplished by a simple displacer float. Oil overflows the oil weir into an oil bucket where its level is controlled by a level controller operating the oil dump valve. Water flows under the oil bucket and then over a water weir. The level downstream of this weir is controlled by a level controller operating the water dump valve. 

Downcomer operation is often described in terms of a non-flooded downcomer, where complications arise from the cascade of froth flowing over the weir onto a froth layer in the downcomer, causing entrainment into-, and gross circulation of the froth layer, in a manner analogous to a waterfall. 

In fact, the operation of a downcomer at the point of flooding can be simply illustrated froth from the active area flows over the weir onto the downcomer froth. The bulk of the vapour disengagement occurs probably in a very short time, and only the small bubbles in the froth are entrained downwards. Within the downcomer, the bubbles coalesce until they are large enough to rise out from the froth. If coalescence is not fast enough, some of the vapour is carried down to the tray below. Even with this simpler picture, the process is not easily modelled. 

Figure 2.2 shows a simple sieve tray, with a single hole. Why is it that the liquid flows over the 3-in outlet weir, rather than simply draining down through the sieve hole It is the force of the vapor (or better, the velocity of the vapor), passing through the sieve hole, which prevents... 

The tray temperatures in our preflash tower, shown in Fig. 4.4, drop as the gas flows up the tower. Most of the reduced sensible-heat content of the flowing gas is converted to latent heat of evaporation of the downflowing reflux. This means that the liquid flow, or internal reflux rate, decreases as the liquid flows down the column. The greater the temperature drop per tray, the greater the evaporation of internal reflux. It is not unusual for 80 to 90 percent of the reflux to evaporate between the top and bottom trays in the absorption section of many towers. We say that the lower trays, in the absorption section of such a tower, are drying out. The separation efficiency of trays operating with extremely low liquid flows over their weirs will be very low. This problem is commonly encountered for towers with low reflux ratios, and a multicomponent overhead product composition. 

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