Francis weir equation

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,... 

The value of the weir crest how in Equation (12.60) employs the classic Francis weir equation ... 

As shown in Figure 12.4, the liquid on tray 1 flows over a weir in order to reach tray (i-l), and we may use the Francis weir equation in order to relate the liquid flow rate in kmol/s to the height of the liquid on the tray ... 

The weir height, is the actual height of the weir. The minimum weir height is 0.5 inch with 2 to 4 inches more common. The weir must be high enough that the opposite downcomer remains sealed and always retains liquid. The height of the liquid crest over the weir, h(,j.est can be calculated from the Francis weir equation. 

For large straight weirs when the downspouts are not running full, the Francis weir equation may be used. This weir formula is frequently employed neglecting the approach velocity, and in some cases this may not be a satisfactory approximation. The equation for zero approach velocity is... 

For the liquid height above the weir how, usually a modified Francis weir equation is used. This classical equation stemming from 1883 is restricted to conditions where the weir height is very large compared to the crest height. This equation ignores the influence of the vapor load, however. 

Open-loop behavior of multicomponent distillation may be studied by solving modifications of the multicomponent equations of Distefano [Am. Inst. Chem. Eng. J., 14, 190 (1968)] as presented in the subsection Batch Distillation. One frequent modification is to include an equation, such as the Francis weir formula, to relate liquid holdup on a tray to liquid flow rate leaving the tray. Applications to azeotropic-distillation towers are particularly interesting because, as discussed by and ihustrated in the Following example from Prokopalds and Seider... 

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 ... 

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. 

Explain in detail whether or not the following equation for flow through a rectangular weir is dimensionally consistent. (This is the modified Francis formula.)... 

The height over the weir is calculated from a form of the Francis equation, which for a straight segmental weir is... 

As shown from Table 1, in Francis formula, the approach velocity is not considered, which will cause obviously calculated flow errors, and the flow value is very close as calculate by the formula of Rehbock and this article. In fact, if the sectional velocity distribution of weir head under different HIP can be accurate measured, the value tzcan be determined, and the flow rate can be precise calculated by Equation (9), when for preliminary calculation can also use Equation (12). 

Minimum Weir Loading A very low weir loading is manifested when the liquid height over outlet weir, is less than the limit of 0.25-0.5 in. (Chase, 1967 Davis and Gordon, 1961 Kister, 1992). A sufficient liquid height over the weir above the limit provides a stable liquid distribution. We can apply the Francis correlation in equation (12.27) to determine the minimum weir loading ... 

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