Pressure Drop, Sieve Tray

Please note that HOLHA is the total hole area in ft2 on a single tray deck. It is used in Eq. (3.91) to calculate sieve tray jet flood and will be used to calculate sieve tray pressure drop as well. 

Sieve tray pressure drop requires factors similar to those used for valve trays, such as weir length Lm. Please refer to Eqs. (3.53) through (3.56) for sieve tray weir length in inches. 

Before a total sieve tray pressure drop can be summed, the froth pressure in inches of clear liquid over the active area must be calculated. This froth height actually reduces the HHDS value by a factor called the aeration beta correction. This has been done by Smith, who plotted the aeration factor beta vs. FGA (see Eq. (3.120) for FGA). Equation (3.121) is a curve-fit of Smith s beta curve plot [16]. Generally a beta factor of 0.7 to 0.8 is calculated using Eq. (3.121). 

The total sieve tray pressure drop DPTRAy may now be calculated using the dry tray drop HHD from Eq. (3.114) and HHL from Eq. 

Total sieve tray pressure drop DPTRAY in inches of clear liquid is now calculated in a final equation ... 

Rem SIEVE TRAY PRESSURE DROP CALC, DPTRAY, INCHES LIQUID... 

The Bennett et al. correlation. This correlation was shown (31) to predict experimental sieve tray pressure drop data more accurately than Fair s correlation. The correlation is based on froth regime considerations and is not applicable to the spray regime. The Bennett et al. calculation of dry pressure drop is identical to Fair s, using Eqs. (6.42) and (6.43) and the Liebson et al- correlation (Fig. 6.21a). To calculate the h, term in Eq. (6.41), Bennett et al. depart from the concept of clear liquid flow corrected for aeration effects [Eq. (6.47a)]. Instead, they use Eq, (6.476) and a model of froth flow across the weir. Their residual pressure drop, hn, is a surface tension head loss term, which is important for trays with very small holes ([Pg.317]

Figure 2.25. Type C sieve tray pressure drop. [D. W. Jones and J.

The suitable control range of differential pressure controllers may be narrow, especially with valve trays (Fig. 19.126). In the valvethrottling range of a valve tray, and below the weep point of a sieve tray, pressure drop may be insensitive to vapor loads. Differential pressure control may therefore be difficult to apply imder turned-down conditions. Packed towers are not prone to this limitation (44), because pressure drop tends to be sensitive to vapor load over the entire operating range (Fig. 14.3). 

Detman, R. F., "How Weir Location Affects Sieve Tray Pressure Drop, Hydrocarbon Proc. 42(8), 1963, p. 147. 

Low dry tray pressure drop. On sieve and fixed valve trays, this means high (>11 percent) fractional hole area. On moving valve trays, this means venturi valves (smooth orifices) or long-legged valves (>15 percent slot area). On all trays, the channeling tendency and severity escalate rapidly as the dry pressure drop diminishes (e.g., as fractional hole area increases). 

Because of their proprietary nature, valve trays are usually designed by their respective vendors based on process specifications supplied by the customer. However, most fabricators publish technical manuals that make it possible to estimate some of the design parameters. The procedure for calculating valve-tray pressure drop outlined here has been adapted from the Koch Design Manual. As for the other column specifications required, they can be obtained via the same calculation procedures outlined above for the sieve-tray design. 

Pressure drop prediction follows the same approach as for sieve trays, with the primary difference being in the determination of dry tray pressure drop. Values of dry drop for fully open and fully closed valves may be estimated from relationships such as those of Klein ... 

Typical tray pressure drop for flow of vapor in a tower is from 0.3 to 1.0 kPa/tray. Pressure drop (expressed as head loss) for a sieve tray is due to friction for vapor flow through the tray perforations, holdup of the liquid on the tray, and a loss due to surface tension ... 

A splash baffle is recommended when liquid flow rate is less than 0.1 gpm per inch of outlet weir (144). One successful application of tailor-designed sieve trays with splash baffles at liquid rates ranging from 0.01 to 0.07 gpm per inch of weir has been described (374). Splash baffles should only be used at low liquid loads, because they restrict the downcomer inlet area and can lead to premature downcomer choke. A calming zone upstream of the baffle is sometimes used to minimize this problem (374). Splash baffles also increase tray pressure drop and froth regime entrainment (31). 

The greater the tray thickness, the lower the dry-tray pressure drop in sieve and valve trays (88, 226, 257, 409). It is commonly believed... 

Bolles s (1976) design procedure uses the sieve tray design procedure as a basis and modifies it as necessary. One major difference between valve and sieve trays is in their pressure drop characteristics. The dry tray pressure drop in a valve tray is shown in Figure 10-23 fBolles. 19761. As the gas velocity increases, Ap first increases and then levels off at a plateau level. In the first range of increasing Ap, all valves are closed. At the closed balance point, some of the valves open. Additional valves open in the plateau region until all valves are open at the open balance point. With all valves open, Ap increases as the gas velocity increases further. The head loss in inches of liquid for both closed and open valves can be e q)ressed in terms of the kinetic energy. 

Determine tray pressure drop terms for sieve and valve trays and design downcomers... 

Figure 19-1 shows this tower along with the tray pressure-drop data. The tower internals consisted of Linde high-capacity sieve trays. The holes... 

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