Open area

An interesting application of the Cassie equation (X-27) arises in the case of a screen or woven material where now /2 is the fraction of open area, ys2V is zero and ys2L is simply YLV- The relationship then becomes... 

Open areas around the operating units of a plant act as buffers to the surrounding community. Sufficient clearance should be allowed so that, if tall stmctures coUapse, other on-site buildings or equipment, or off-site properties are not affected. Adequate roadways providing entry to the plant are extremely important, and multiple entries and exits are advisable. An overcrowded plant can lead to damage or shutdown of adjacent units and may impede the movement of vehicles and materials in case of emergency (85). Another consideration is community fire-fighting assistance, first aid, and medical facihties. 

An optimized relationship is obtained between the beU jar, 60° swing-leaf valve, LN trap, baffle for the oil, and the plane of action for the diffusion pump (DP) top jet. The valve open area equals 0.38 of the cross-sectional area of the inside diameter of the furnace. The volumetric speed factor for water vapor is thus 0.38 x 0.9 crr 0.34, where 0.9 is the Clausing factor. 

Barium metal and most barium compounds are highly poisonous. A notable exception is barium sulfate which is nontoxic because of its extreme iasolubihty ia water. Barium ion acts as a muscle stimulant and can cause death through ventricular fibrillation of the heart. Therefore, care must be taken to avoid contact with open areas of the skin. Workers must wear respirators (of type approved for toxic airborne particles), goggles, gloves, and protective clothing at all times. The toxic barium aluminate residue obtained from barium production is detoxified by reaction with a solution of ferrous sulfate and converted iato nontoxic barium sulfate. According to OSHA standards, the TWA value for Ba and Ba compounds ia air is 0.5 mg/m. ... 

Sieve Plates. The conventional sieve or perforated plate is inexpensive and the simplest of the devices normally used. The contacting orifices in the conventional sieve plate are holes that measure 1 to 12 mm diameter and exhibit ratios of open area to active area ranging from 1 20 to 1 7. If the open area is too small, the pressure drop across the plate is excessive if the open area is too large, the Hquid weeps or dumps through the holes. 

The orifice coefficient deviates from its value for sharp-edged orifices when the orifice wall thickness exceeds about 75 percent of the orifice diameter. Some pressure recovery occurs within the orifice and the orifice coefficient increases. Pressure drop across segmental ori-fiees is roughly 10 percent greater than that for concentric circular orifices of the same open area. 

Most efficient performance is obtained with plates having open areas equal to 2 to 3 percent of the total heat-transfer area. The plate should be located at a distance equal to four to six hole (or equivalent) diameters from the heat-transfer surface. 

For counterflow plates, the curves of Fig. 14-25 may be used for open areas of 20 percent or greater. Plates with 15 percent open areas have about 85 percent of the cui ve values, and open areas or less than 15 percent are not recommended. For counterflow-plate columns of the segmental-baffle type, 50 percent cut, allowable values are about 15 percent greater than tnose shown in Fig. 14-25, when vertical spacings of the baffles are equal to the trav spacings shown. 

The open area for these plates ranges from 15 to 30 percent of the total cross section compared with 5 to 15 percent for sieve plates and 8 to 15 percent for bubble-cap plates. Hole sizes range from 6 to 25 mm (1/4 to 1 in), and slot widths from 6 to 12 mm (14 to V2 in). The Turbogrid and Ripple plates are proprietary devices. 

Turbogrid plate—4.6-mm (0.18-in) slot width, 14.7 percent open area... 

Ripple plate—2.85-mm in) holes, 10.8 percent open area Efficiency data from the work are summarized in Fig. 14-40. 

Pinczewski and Fell [Trans. Inst. Chem E/ig., 55, 46 (1977)] show that the velocity at which vapor jets onto the tray sets the droplet size, rather than the superficial tray velocity. A maximum superficial velocity formulation that incorporates ( ), the fractional open area, is logical since the fractional open area sets the jet velocity. Stichlmair and Mers-mauu [Int. Chem. Eng., 18(2), 223 (1978)] give such a correlation ... 

Fair s empirical correlation for sieve and bubble-cap trays shown in Fig. 14-26 is similar. Note that Fig. 14-26 incorporates a velocity dependence (velocity) above 90 percent of flood for high-density systems. The correlation implicitly considers the tray design factors such as the open area, tray spacing, and hole diameter through the impact of these factors on percent of flood. 

TABLE 14-13 Dependency of Distillation Flood Velocity on Physical Properties and Tray Open Area... 

The Karr column is particularly well suited for systems which tend to emulsify since its uniform shear characteristics tend to minimize emulsion formation. It is also particularly well suited for corrosive systems (since the plates can be constructed of non-metals) or for systems containing significant solids (due to its large open area and hole size on the plates). Slurries containing up to 30 percent solids have been successfully processed in Karr columns. 

Open Area The open area of a screen is the percentage of acdual openings versus total screen area and can be determined by the formulas given in Fig. 19-22. 

Screen Surfaces The selection of the proper screening surface is veiy important, and the opening, wire diameter, and open area should all be carefully considered. The four general types of screening surfaces are woven-wire cloth, silk bolting cloth, punched plate, and bar or rod screens. 

Square-mesh cloth is the conventional type of screen cloth, but there are many types of cloth with an oblong weave. This latter construction provides greater open area and capacity and in addition makes it possible to use stronger wire for the same size of screen opening and for the same percentage of open area. 

Cf = throughflow rate = unit capacity = open-area factor F, = slotted-area factor... 

The unit capacity can be determined from Fig. 19-21. Figure 19-22 can be used to determine the open-area fac tor and the slotted-opening factor F, for various screen types is given in Table 19-7. 

Filters for mists and droplets have more open area than those used for dry parhcles. If a filter is made of many fine, closely spaced fibers, it will become wet due to the collected liquid. Such wethng will lead to mathng of the fibers, retenhon of more liquid, and eventual blocking of the fiter. Therefore, instead of fine, closely spaced fibers, the usual wet filtrahon system is composed of either knitted wire or wire mesh packed into a pad. A looser filtrahon medium results in a filter with a lower pressure drop than that of the filters used for dry parhculates. The reported pressure drop across wire mesh mist eliminators is 1-2 cm of water at face velocihes of 5 m sec T The essenhal collechon mechanisms employed for filtrahon of droplets and mists are inertial impachon and, to a lesser extent, direct intercephon. 

Then the required opening area is determined to provide 12 changes per hour ... 

Support plate manufactured at 60% open area vs. normal 80-100-1-%. Poor separation. Taps were not available to obtain accurate pressure drop measurements. Manufacturing error. 

Support plate used as a vapor distributor with restricted open area. Reduced capacity in order to make separation. Design error. 

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