Sieve trays pressure drop over

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 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). 

The plates may be any of several types, including sieve, bubble-cap, and valve trays. Valve trays constitute multiple self-adjusting orifices that provide nearly constant gas pressure drop over considerable ranges of variation in gas flow. The gas pressure drop that can be taken across a single plate is necessarily limited, so that units designed for high contacting power must use multiple plates. 

Sieve trays, 651-652, 662, 681-686 allowable velocities in, 656-661 cost of 709 efficiency of 661-667 pressure drop over, 667-679 Silicon rubber, 435 Silver, 432... 

The pressure drop over sieve and valve trays may be estimated using correlations presented by Lockett (1986) and Kister (1992). For bubble cap trays the procedures described by Bolles (1963) can be adapted for computer based calculation. Kister (1992) reviews the methods available for estimating the pressure drop in dumped packed columns. The pressure drop in structured packed columns may be estimated using the method of Bravo et al. (1986). 

The pressure drop over a sieve tray is 0.3-0.5 kPa, while the pressure drop in a packing element corresponding to one theoretical tray is only about 0.08 kPa. The small pressure drop of a packed low-pressure column significantly reduces the energy needed for the separation. 

As noted, the weir crest 4, 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 trays their correlation for residual pressure drop h L in Eq. (14-100) is presented in detail in the previous (seventh) edition of this handbook, including a worked example. Although more difficult to use, the method of Bennett et al. was recommended when determination of pressure drop is of critical importance. 

A sieve-tray tower has an ID of 5 ft, and the combined cross-sectional area of the holes on one tray is 10 percent of the total cross-sectional area of the tower. The height of the weir is 1.5 in. The head of liquid over the top of the weir is 1 in. Liquid gradient is negligible. The diameter of the perforations is in., and the superficial vapor velocity (based on the cross-sectional area of the empty tower) is 3.4 ft/s. The pressure drop due to passage of gas through the holes may be assumed to be equivalent to 1.4 kinetic heads (based on gas velocity through holes). (Tray thickness = hole diameter and active area = 90 percent of total area-see Fig. 16-12). If the liquid density is 50 lb/ft3 and the gas density is 0.10 lb/ft3, estimate the pressure drop per tray as pounds force per square inch. 

The tray hydraulics supplies equations to calculate the liquid molar hold-up from geometry, as well as for coupling the pressure drop with vapour and liquid tray flows. The following relations hold for sieve trays. The liquid flow rate over a wire crest of length W and height is given by the formula (Francis) ... 

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