Liquid weir height

Column liquid loading, gal/min Downcomer backup, inches of liquid Condensing side film coefficient, Btu/hrft °F Head loss under downcomer, inches of liquid Weir height, ins. 

The value of weir height crest may be calculated from the Francis weir equation and its modifications for various weir types. For a segmental weir and for height in millimeters of clear liquid,... 

Referring to Figure 8-63, the weir height, h, must always be greater than the clearance under the downcomer, i.e., between bottom of downcomer and tray floor, hdci-Always avoid too low clearance as this can cause flooding of liquid in the downcomer. There are flow conditions... 

The bottom of the downcomer must be sealed below the operating liquid level on the tray. Due to tolerance in fabrication and tray level, it is customary to set the downcomer seal referenced to the weir height on the outlet side of the tray. Recommended seals, based on no inlet weir adjacent to the downcomer, and referenced as mentioned are given in Table 8-19. 

Intensifies as outlet weir height increases and as the liquid flow rate increases. 

Weir heights are 2 in., weir lengths about 75% of tray diameter, iiquid rate a maximum of about 8 gpm/in. of weir muitipass arrangements are used at high liquid rates. 

Ln(weir) is zero for the condition Mn< Mns. M is the mass of liquid on plate n, and Mns is the mass of liquid on the plate corresponding to the weir height or static liquid holdup on the plate. The rate of loss of liquid from the plate by weepage, however, will depend on the total mass of liquid on the plate... 

It is important to set close tolerances on the weir height, downcomer clearance, and plate flatness, to ensure an even flow of liquid across the plate. The tolerances specified will depend on the dimensions of the plate but will typically be about 3 mm. 

The height of the weir determines the volume of liquid on the plate and is an important factor in determining the plate efficiency (see Section 11.10.4). A high weir will increase the plate efficiency but at the expense of a higher plate pressure drop. For columns operating above atmospheric pressure the weir heights will normally be between 40 mm to 90 mm (1.5 to 3.5 in.) 40 to 50 mm is recommended. For vacuum operation lower weir heights are used to reduce the pressure drop 6 to 12 mm ( to in.) is recommended. 

The value of NG is expressed in terms of weir height hw, gas flow expressed as F, liquid flow Lp and the Schmidt number Scv for the vapour phase. The two key relations are ... 

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... 

The clear liquid backup is divided by the froth-density to give the froth height if this exceeds the tray spacing plus the outlet weir height, the tray is deemed to be flooded. 

The weir height on many trays is adjustable. We usually adjust the weir height to between 2 and 3 inches. This produces a reasonable depth of liquid on the tray, to promote good vapor-liquid contact. 

The sum of the crest height plus the weir height equals the depth of liquid on the tray deck. One might now ask, Is not the liquid level on the inlet side of the tray higher than the liquid level near the outlet weir While the answer is Yes, water does flow downhill, we design the tray to make this factor small enough to neglect. 

The weight of liquid on a tray is created by the weir height plus the crest height. We have defined the crest height (in inches of clear liquid) in Chap. 1, as... 

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. 

FIG. 14-32 Pressure-drop contributions for trays, hd = pressure drop through cap or sieve, equivalent height of tray liquid hw = height of weir how = weir crest hhg = hydraulic gradient hda = loss under downcomer. 

Weir Height Taller weirs raise the liquid level on the tray in the froth and emulsion regimes. This increases interfacial area and vapor contact time, which should theoretically enhance efficiency. In the spray regime, weir height affects neither liquid level nor efficiency. In distillation systems, the improvement of tray efficiency due to taller weirs is small, often marginal. 

The sieve-plate design is limited by the need for cooling coils. The tray design must ensure that the cooling coils sit easily in the layer of liquid. Therefore, the minimum weir height available was 40 mm. Despite this initial constraint a satisfactory tray design was developed. 

Weir height. This parameter sets the level of liquid on the tray in the froth and emulsion regimes (Fig. 17a,b). The higher the level, the better is the contact and the efficiency at the expense of a greater liquid backup in the downcomer. Typical absorption weir heights are 2-3 in. (50-75 mm). 

The liquid begins to flow into the top plate and collects on the plate because of the retention made by the vapour flow. When the liquid level passes the weir height (thus filling the holdup), the liquid begins to fall to the plate below and the same phenomenon is repeated until the reboiler is reached. In practice some liquid also trickles down from the plate holes when the flows are initially established. 

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

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