Dry pressure drop

The dry pressure drop across the disperser unit [h in E3q. (6.41)] is given by a variation of the orifice equation 

The prime variables affecting the orifice coeffident, are the fractional hole area and the ratio of tray thickness to hole diameter. More than 20 published correlations are available for evaluating (12). Fair et al. (18) and Van Winkle (5) recommend the correlation by Liebson et al. (48 Fig. 6.21o). The Hughmark and O Connell correlation (66) is preferred by Ludwig (4) and Chase (30). 

K in Eq. (6,42) depends on whether the valves are fully open and also on the shape and weight of the valves. These are bek obtained from manufacturer literature (7—9), but can also be calculated from Bolles (71), Lockett s C12), or Klein s (80) methods. Klein s method was derived from and fine-tunes the Bolles model, was tested s ainst a wide data base, and gives 

For all valves fully open (above the open balance point) 

Between the closed and open balance points, the diy pressure drop is constant (Fig. 6.216), and equals the pressure drop at either the closed or open balance points. Therefore, Eq. (6.44o) can be used, with in Eq. (6.42) set equal to the velocity at the closed balance point, u cBP Alternatively, Eq. (6.446) can be used with Uf in (6.42) set equal to the velocity at the open balance point, u h.oBt-Note that between the open and closed balance points the hole velocity at the relevant balance point, and not the actual gas velodir of gas through the holes, is used as in Eq. (6.42). 

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For valve trays the effects observed only for the venturi (low dry pressure drop) valve. 

For bubble cap trays the phenomenon is believed to be induced by excessive hydraulic gradient it is recommended to keep hydraulic gradient to less than 40% of the dry pressure drop. 

Fig. 13. Breakdown into the contributing dissipation mechanisms of dry pressure drops in vessels containing Montz Bl-250.45 structured packings. Reprinted from Chemical Engineering Science, Vol. 58, Petre et al, Pressure Drop Through Structured Packings Breakdown Into the Contributing Mechanisms by CFD Modeling, pp. 163-177, Copyright (2003), with permission from Elsevier.

To take account fully of the liquid flow, reference 56 in Chapter 4 of Volume 2 provides a correction factor which depends on the liquid flowrate and the Raschig size. This factor acts as a multiplier for the dry pressure drop which in this example, is equal to 1.3, giving the pressure drop in this problem as ... 

Kc Dry pressure drop constant, mms2/m2 in-sVft2 X Mole fraction, liquid phase (note 1) -/- -/-... 

Moving valves can have a sharp or a smooth ( venturi ) orifice. The venturi valves have one-half the dry pressure drop of the sharp-orifice valves, but are far more prone to weeping and channeling than the sharp-orifice valves. Sharp orifices are almost always preferred. 

Work by Davies [Pet. Ref. 29(8), p. 93, and 29(9), p. 121 (1950)] based on bubble-cap tray studies suggests that the vapor pressure drop of the tray (the dry pressure drop) counteracts channeling. The higher the dry tray pres sure drop, the greater the tendency for vapor to spread uniformly over the bubbling area. If the dry tray pressure drop is too small compared with the channeling potential, channeling prevails. 

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). 

York and Poppele (R-17) have suggested that total pressure drop through the mesh is equal to the sum of the mesh dry pressure drop... 

Important Note Bubble cap HHD factor is equivalent to the DPntAYi dry pressure drop of valve trays. The bubble cap tray total pressure drop factor DPTRay is equivalent to the HDC2 factor of valve-type trays. You may therefore substitute these bubble cap values in the ETF efficiency equations as given for valve trays to determine bubble cap tray efficiency. 

The sieve tray dry pressure drop is calculated next, applying the following equations ... 

Using these calculated ratios, AHAA and THDIA, a hole discharge coefficient factor CFCV is calculated from a curve-fitted equation in Fig. 18-14 in the Chemical Engineer s Handbook [14], CFCV is a factor in Eq. (3.112) for calculating the sieve tray dry pressure drop. 

Counter-current gas/vapor-liquid film flows in SP above the load conditions are extremely complicated. For this reason, it appears improbable that the CFD-based virtual experiments replace real experiments entirely in the near future. However, even single-phase CFD simulations can improve predictivity of pressure drop models, since all correlations pressure drop - gas load used in practice contain some dry pressure drop correlation as a basic element. Replacing this correlation by the rigorous CFD analysis helps to avoid heuristic assumptions on possible correlation structure, which are inevitable both in conventional mechanistic models (Rocha et ah, 1993) and in more sophisticated considerations (Olujic, 1997). 

Valve trays. Figure 6.216 illustrates the dry pressure drop of a typical valve tray as a function of vapor velocity. At low vapor velocities, all valves are closed (i.e., seated on the tray deck). Vapor rises through the crevices between the valves and the tray deck, and friction losses through these crevices constitute the dry pressure drop. Once the closed balance point (CBP) is reached, there is sufficient force in the rising vapor to open some valves. A further increase in vapor velocity opens more valves. Since vapor flow area increases as valves open, pressure drop remains constant until all valves open. This occurs at the open balance point (OBP). Further increases of vapor velocity cause the dry pressure drop to escalate in a similar manner to a sieve tray. When two weights of valves are used in alternate rows on the tray, a similar behavior applies to each valve type. The result is the pressure drop-vapor velocity relationship in Fig. 619e. 

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]

Pressure drop. Pressure drop is calculated as per Secs. 6.3.1 to 6.3.3, using Fair s pressure drop correlation. Usually, it is good practice to design for a pressure drop of 3 to 5 in of liquid (approximately 0.08 to 0.12 psi) per tray. If outside this range it is best to adjust the fractional hole area (if dry pressure drop dominates) or the weir height (if wet pressure drop dominates). 

In order to allow for irrigation, Leva modified the dry pressure drop correlation to give... 

Equation (8.16) permits estimating for any packing from dry pressure drop measurements. Table 8.3 gives for various commercial packings. A more comprehensive list is elsewhere (89). The dry packing factors in Table 8.3 are those to be used in Eqs. (8.14) andi lSL... 

By inferring packing factors from dry pressure drop measurements, using Eq. (8.16). This method is only suitable for use with the Bobbins pressure drop correlation (Sec. 8.2.8). With the GPDC correlation, it biases the packing factor toward the low-liquid-rate region. 

Dry pressure drop for sieve trays should fall between 0.75 and 3 in of liquid (19 and 76 mm) to prevent excessive weeping on the one hand or jetting on the other. Since, in this case, the dry pressure drop is barely within the recommended range, it is desirable that the tray be checked for weeping using Fig. 8.12. 

FIGURE 8.14 Dry pressure drop /tdiy across valve trays. (From Chemical Engineering, McGraw-Hill, 1977.)... 

Construct the dry-pressure-drop line on log-log coordinates (optional). For turbulent flow, the gas-phase pressure drop for frictional loss, contraction and expansion loss, and directional change loss are all proportional to the square of the superficial F factor. For the dry packing the pressure drop can be calculated from the equation... 

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