Flow path length

For pressure drop inside tubes, d is 0.046 and F is the fluid-flow path length. Across tubes banks, a is 0.75 and F is the product of the number of tube rows and the number of fluid passes across the tube bank. The physical property term is again tabulated after being normalised so that the lowest value is approximately unity. 

Using and GPM (column liquid loading in gallons per minute), obtain approximate tower diameter for calculating flow path length. Use... 

ADM = Minimum downcomer area, fT ATM = Minimum column cross-sectional area, fr CAF = Vapor capacity factor CAFo = Flood capacity factor at zero liquid load CFS = Vapor rate, actual ftVsec DT = Tower diameter, ft DTA = Approximate tower diameter, ft FF == Flood factor or design percent of flood, fractional FPL = Tray flow path length, in. 

Figure 11 shows a t pical liquid collector plate for a column that uses one side downcomer to withdraw the liquid. The maximum diameter for such a design is about 12 ft, which is limited by the hydraulic gradient necessary for such a liquid flow-path length, For larger diameter columns, two opposite side downcomers or a center downcomer normally is used unless the total amount of liquid collected is relatively small. 

DEL = Column flooding correlating factor FPL = Tray flow path length, ins. 

A ratio of liquid flow path length to tray spacing greater than 2-2.5 1. 

Anti-Jump Baffles Yes / No / Vendor Preference Recessed Seal Pans YeS / No / Vendor Preference Specify Equal Bubbling Areas / Flow Path Lengths per pass Design Load ... 

L is the flow path length (length), and d is the flow path diameter (length). 

For pipe fittings, valves, and other flow obstructions the traditional method has been to use an equivalent pipe length Lequiv in Equation 4-30. The problem with this method is that the specified length is coupled to the friction factor. An improved approach is to use the 2-K method,s-6 which uses the actual flow path length in Equation 4-30 — equivalent lengths are not used — and provides a more detailed approach for pipe fittings, inlets, and outlets. The 2-K method defines the excess head loss in terms of two constants, the Reynolds number and the pipe internal diameter ... 

Multipass Balancing There are two balancing philosophies equal bubbling areas and equal flow path lengths. Equal bubbling areas means that all active area panels on Fig. 14-21 d are of the same area, and each panel has the same hole (or open-slot) area. In a four-pass tray, one-quarter of the gas flows through each panel. To equalize the L/G ratio on each panel, the liquid needs to be split equally to each panel. Since the center weirs are longer than the side weirs, more liquid tends to flow toward the center weir. To equalize, side weirs are often swept back (Fig. 14-22b) while center weirs often contain picket fences (Fig. 14-22c). 

The alternative philosophy (equal flow path lengths) provides more bubbling and perforation areas in the central panels of Fig. 14-21d and less in the side panels. To equalize the L/G ratio, less liquid needs to flow toward the sides, which is readily achieved, as the center weirs are naturally longer than the side weirs. Usually there is no need for swept-back weirs, and only minimal picket-fencing is required at the center weir. 

Hartman (Distillation 2001 Topical Conference Proceedings, AIChE Spring National Meeting, p. 108, Houston, Tex. (April 22-26, 2001)] reports VCFC even with conventional valve trays (14 percent slot area) at veiy high ratio (3.6 1) of flow path length to tray spacing and tray truss obstruction. 

Length of Liquid Flow Path Longer liquid flow paths enhance the liquid-vapor contact time, the significance of liquid plug flow, and therefore raise efficiency. Typically, doubling the flow path length... 

Solution Table 14-12 presents measurements by Billet (loc. cit.) for ethyl-benzene-styrene under similar pressure with sieve and valve trays. The column diameter and tray spacing in Billets tests were close to those in Example 9. Since both have single-pass trays, the flow path lengths are similar. The fractional hole area (14 percent in Example 9) is close to that in Table 14-12 (12.3 percent for the tested sieve trays, 14 to 15 percent for standard valve trays). So the values in Table 14-12 should be directly applicable, that is, 70 to 85 percent. So a conservative estimate would be 70 percent. The actual efficiency should be about 5 to 10 percent higher. 

Slope downcomers (Y/N) N Side dcmr. area (opt.), ft2 0 Active area (opt.), ft2 0 Flow path length (opt.), in 0... 

System factor 0.95 Tray spacing, in 24 Actual vapor density, lb/ft3 0.2 Gas molecular weight 21 Liquid density, lb/ft3 44 Gas rate, lb/h 8.4500e + 04 Liquid rate, lb/h 7.4500e + 05 Slope downcomers (Y/N) N Side downcomer area (opt.), ft2 0 Active area (opt.), ft2 0 Flow path length (opt.), in 0... 

Side downcomer area (optional), ft2 Active area (optional), ft2 Flow path length (optional), in... 

Side Dctw. Area (0ptn l). Ft2 Active Area (0 ptn l). Ft2. Flow Path Length (Optn l). In... 

CDCAREA = center downcomer area for 2- or 3-pass tray, ft2 CDOC = FPL factor for 3-pass tray, ft FPL = flow path length of cross-tray liquid flow from downcomer inlet to downcomer outlet, in H3 and Hs = span factors of 4-pass tray for FPL calculation... 

For configurations such as that shown in Fig. 3.5, it is helpful to input the equivalent flow path length as shown. The input units are in inches. This option, however, should seldom be used without one or both of the two preceding options, because the equivalent FPL will be calculated (in most cases) as having an accurate side downcomer equivalent dimension input. 

At high liquid rates (>6 gpm/in of outlet weir), high ratio (>2.5) of flow-path length to tray spacing, and a high fractional hole area (> 11 percent), cross flow of vapor in opposite direction to the liquid can build up froth near tray inlet and center. The froth buildup raises the... 

Check liquid flow path [Note Kis ten s guideline (1) of avoiding flow path lengths smaller than 16 to 18 in.]... 

Comment Note that the length of the flow path (LFP) will restrict any further reductions in column diameter. As stated previously (Sec. 6.5.4), flow path length smaller than about 18 in is best avoided. This limit is reached in the bottom section, center-to-side flow. 

FPL Flow-path length (distance from the inlet downcomer edge to... 

Z = liquid flow path length, m W = weir length, m... 

Liquid flow path length 0.967 m per pass 4.9 m Tray thickness 2 mm... 

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