Free area

The pulsed-plate column is typically fitted with hori2ontal perforated plates or sieve plates which occupy the entire cross section of the column. The total free area of the plate is about 20—25%. The columns ate generally operated at frequencies of 1.5 to 4 H2 with ampHtudes 0.63 to 2.5 cm. The energy dissipated by the pulsations increases both the turbulence and the interfacial areas and greatly improves the mass-transfer efficiency compared to that of an unpulsed column. Pulsed-plate columns in diameters of up to 1.0 m or mote ate widely used in the nuclear industry (139,140). 

The tray-free area can be decreased at intervals from top to bottom as the density differential between the aqueous phase and the working solution widens. This adjustment maintains a nearly constant depth of coalesced working solution beneath each tray. For this type extractor the distance between trays (spacings) is constant from top to bottom. Alternatively, the tray area can be held constant and the height of the coalesced layer beneath the tray permitted to vary, thus providing the needed pressure drop for flow. 

D, Equivalent diameter of a cross section, usually 4 times free area divided by wetted perimeter D, for equivalent diameter of window m ft... 

Mass velocity through minimum free area between rows of tubes kg/(m -s) IV(h-ft )... 

Laminar Flow Normally, laminar flow occurs in closed ducts when Nrc < 2100 (based on equivalent diameter = 4 X free area -i-perimeter). Laminar-flow heat transfer has been subjected to extensive theoretical study. The energy equation has been solved for a variety of boundaiy conditions and geometrical configurations. However, true laminar-flow heat transfer veiy rarely occurs. Natural-convecdion effects are almost always present, so that the assumption that molecular conduction alone occurs is not vahd. Therefore, empirically derived equations are most rehable. 

For rectangular ducts Kays and Clark (Stanford Univ, Dept. Mech. Eng. Tech. Rep. 14, Aug. 6, 1953) published relationships for headng and cooling of air in rectangular ducts of various aspect rados. For most noncircular ducts Eqs. (5-39) and (5-40) may be used if the equivalent diameter (= 4 X free area/wetted perimeter) is used as the characteristic length. See also Kays and London, Compact Heat Exchangers, 3d ed., McGraw-Hill, New York, 1984. 

Noncircular Ducts Equations (5-50 ) and (5-50/ ) may be employed for noncircular ducts by using the equivalent diameter D = 4 X free area per wetted perimeter. Kays and London (Compact Heat Exchangers, 3rd ed., McGraw-HiU, New York, 1984) give charts for various noncircular duels encountered in compact heat exchangers. 

Screens The pressure drop for incompressible flow across a screen of fractionaf free area Ot may be computed from... 

The face area of the tube bundle is its length times width. The net free area for air flow through the bundle is about 50 percent of the face area of the bundle. 

Size, diameter X length, m Tubes m of free area Dryer speed, r/min Motor size, hp Shipping weight, kg Estimated price... 

The separate flow passage devices can be designed for free areas up... 

A Fractional free area (for screens, perforated plates, grids) Dimensionless Dimensionless ... 

Previous equations determine the pressure drop across the tube bundle. For the additional drop for flow through the free area above, below, or around the segmental baffles use... 

Fortunatel, our calculations indicated that we had more than twice the free area required. 

Measurement of specific chemical or biological contaminants can be very expensive. Before expending time and money to obtain measurements of indoor air pollutants, you must decide how the results will be used (e.g., comparison to standards or guidelines, comparison to levels in complaint-free areas) what substances(s) should be measured where to take, samples when to take samples what sampling and analysis method to use so that the results provide useful information. 

Measurements taken outdoors and in a control location (e.g., a complaint-free area of the building) are helpful in interpreting results from the complaint area. 

These high velocities occur at the bundle entrance and exit areas, in the baffle windows, through pass lanes and in the vicinity of tie rods, which secure the baffles in their proper position. In conjunction with this, the shell side fluid generally will take the path of least resistance and will travel at a greater velocity in the free areas or by-pass lanes, than it will through the bundle proper, where the tubes are on a closely spaced pitch. All factors considered, it appears a formidable task to accurately predict heat transfer characteristics of a shell and tube exchanger. 

Recirculation is commonly used for grinding tables due to the large exhaust airflow rates of this design. For these larger tables the free area is much larger than for the small chemical tables, resulting in a much lower pressure difference. In order to maintain an even distribution of airflow over this large surface, it is necessary to use... 

Air free area ratio (A p) The ratio of free area to the core area. 

Free area velocity The velocity in a device where the flow is not influenced by... 

Free area of an ATD The area available in an air terminal device for the discharge of air, as opposed to the actual area. 

Free area ratio The ratio of an actual opening to the obstructed portion of that opening. 

Sensing heads should be located in draft-free areas where possible, as air flowing past the sensors normally increases drift of calibration, shortens head life, and decreases sensitivity. Air deflectors are available from sensor manufacturers and should be utilized in any areas where significant air flow is anticipated (such as air conditioner plenum applicaiion.s). Additionally, sensors should be located, whenever possible, in loca[ion.s which are relatively free from vibration and easily accessed for calibration and maintenance. Obviously, this carmot always be accomplished. It usually is difficult, for example, to locate sensors in the tops of compressor buildings at locations which are accessible and which do not vibrate. 

Unwanted phases snch as liqnid sings, tramp metal, etc., can damage dry flame arrester elements so that they have to be replaced, or they can also block or ping the arrester free-area. ft is not always obvions when an arrester will be impacted by a liqnid sing or tramp metal. However, when this is snspected or has already occnrred in a process, several things can be done to avoid the problem as follows ... 

The vessel nozzle diameter (inside) or net free area for relief of vapors through a rupture disk for the usual process applications is calculated in the same manner as for a safety relief valve, except that the nozzle coefficient is 0.62 for vapors and liquids. Most applications in this category are derived from predictable situations where the flow rates, pressures and temperatures can be established with a reasonable degree of certainty. 

FA = fractional free area h, = weir height, inches... 

ACLfcAUGEMEMT URPACE FREE area. MUMDEF VElGsUT ... 

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