Weir loading

Two primary settling basins are each 100 ft in diameter with an 8-ft side water depth. The tanks are equipped with single effluent weirs located on the peripheries. For a water flow of 10 mgd, calculate the overflow rate, gpd/ft, detention time, hr, and weir loading, gpd/ft. The overflow rate for a clarifier... 

An increase in reflux rate, assuming that the reboiler is on automatic temperature control, increases both the tray weir loading and the vapor velocity through the tray deck. This increases both the total tray pressure drop and the height of liquid in the tray s downcomer. Increasing reflux rates, with the reboiler on automatic temperature control, then will always push the tray closer to, or even beyond, the point of incipient flood. 

Weir load For trays (as distinct from downcomers), liquid load is normally defined as... 

The weir load is satisfactory for a one-pass design for the top section, but a two-pass tray is needed for the bottom section. 

Eqs. (14-16) are for one-pass trays. For two-pass trays the total downcomer area is halved before calculating the weir length. The weir length is then doubled to account for both downcomers. Two-pass tray weir loads are limited by the liquid flow into the downcomers on the tower edge (outboard downcomers). 

Swept-back weirs increase the weir length and decrease the active area of the tray (Fig. 7). They are often the lowest-cost method to decrease weir load as long as sufficient vapor handling capacity is available. 

Fig. 7.S Simplified snapshots of the front, 1.2 X 10 m /s/m. An animation of the weir, and top views of sieve tray simulations at simulation can be viewed on our web site a superficial gas velocity, Uq = 0.7 m/s weir http //ct-cr4.chem.uva.nl/sievetrayCFD. The height = 80 mm liquid weir load QJW — arrows indicate the liquid velocity vectors...

The relative weir load Vi/l should be lower than 60 m /(m h). 

Increase both sections 1 and 2 to two passes. Use section 2 downcomer clearances (both downcomers) of 0.05 m and increase weir height to 0.052 m. Run the prograrn, look at the downcomer backup, and compare your results with the items in item 9. The weir loading is still too high. 

The DC backup/Tray spacing and weir loading should be acceptable for all three runs. For a conservative design, select the worst separation (highest values of yD,c4> Bocca)- Check if this separation meets the specifications. 

The relationship of vapor and liquid under spray was observed by Sakata and Yanagi (1979) for the sieve tray As the liquid rate reduces beyond a certain amount corresponding to weir loading of 2gpm/in. (gpm is gallons per minute), vapor rate must reduce to maintain the same entrainment rate. This reducing trend of both vapor and hquid rates under very small weir loading defines the spray phenomenon. This trend is different from the tray flood phenomenon under which vapor rate increases as liquid load reduces. 

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