The Stamford Baffle: larger baffles for improved performance


Source: NEFCO, Inc.

A new CFD study indicates that increasing the size of the horizontal projection of a Stamford Density Current Baffle will improve the ability of the baffle to reduce clarifier effluent solids (TSS) by 5% to 10% or more.
NEFCO’s Stamford Density Current Baffle has long been recognized as “the most cost-effective improvement in clarifier performance available today.” The Stamford Baffle eliminates clarifier short-circuiting and dramatically reduces effluent total suspended solids (TSS).
NEFCO recently completed a multi-year, computational fluid dynamics (CFD) analysis of Stamford Density Current Baffle design and performance. That effort produced two new baffle systems,1) the Dual Surface Density Current Baffle, a baffle designed for larger clarifiers operating at very high flows that the company announced in 2008, and 2) Stamford Baffle 2.0 (SB 2.0), an advanced Stamford Baffle capable of reducing clarifier effluent solids by as much as 70% that was announced in 2009. Both of these baffles feature a 30-degree inclination angle rather than the original 45-degree angle, and a larger horizontal projection.
The horizontal projection of the baffle determines the extent to which the baffle can intercept and deflect the density currents in the clarifier. If the projection is too small, the baffle may not reach far enough into the path of the density currents to deflect them and they will continue rising up the clarifier wall and carry lighter solids to the effluent launder. If the baffle projects too far into the tank it may adversely affect the settling of solids in the clarifier.
Currently, the horizontal projection is described by an equation that relates the projection to the diameter of the tank,
  HP = 18” + α(Tank Diameter (ft) – 30)
The value of α was originally 0.2 inches per foot of diameter, and later increased to 0.3 inches per foot. NEFCO used this higher value in its CFD studies of the SB 2.0 design.
During 2010, NEFCO continued its CFD effort, focusing on the horizontal projection and its effect on baffle performance. Following the method of the earlier studies, TSS was computed for a series of candidate horizontal projections across a range of clarifier diameters from 70 feet to 140 feet. The basis for comparison was the TSS computed for a baffle with the horizontal projection determined by the current method.
In broad terms, the results indicate that the horizontal projection of a baffle calculated by the current method is conservative and that the performance of that baffle can be improved by making the horizontal projection larger. At a minimum, the projection produced by the current method can be increased by 10 inches to improve the solids capture performance of the baffle by an additional of 5% to 10% or more. It was also clear that larger clarifiers would benefit from even larger projections.
How large can the projection be before the baffle begins to have a negative effect on clarifier performance?  Some researchers have proposed extraordinarily large baffles. It seems obvious that there is a point at which the results turn negative, but that point was difficult to isolate in this study, especially with larger clarifiers. As the projection was increased, the results became inconsistent and ceased to reflect real world results.  It is likely that this had more to do with the analysis tools than the process under study. Modeling density driven flows, where density differences are very small, is especially difficult.
The calculation of the horizontal projection is independent of the inclination angle of the baffle, but earlier studies have shown that a baffle with a larger projection is more effective when used with a 30° inclination angle rather than the original 45° angle. An added benefit lies in the fact that the hypotenuse of any resulting baffle is significantly shorter than its 45° counterpart. This has two advantages. The baffle’s vertical footprint is minimized, an important factor in fitting the baffle into shallow tanks, and the smaller baffle is less expensive to produce and to install.
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