Straightening vanes

With flow in curved passages or with swirling flow, determination of a true average static pressure is, in general, impractical. In metering, straightening vanes are often placed upstream of the pressure tap to eliminate swirl. Fig. 10-2 shows various flow equalizers and straight-eners. 

Disturbances upstream of the probe can cause large errors, in part because of the turbulence generated and its effect on the static-pressure measurement. A calming section of at least 50 pipe diameters is desirable. If this is not possible, the use of straightening vanes or a honeycomb is advisable. 

Without straight- ening vanes With straightening vanes ... 

Figure 5-30. Elbow straightening vanes. Courtesy of Elliott Company...

Straighteners Vanes fitted in ductwork or air handling units before or after a change in section to produce a reduction in pressure drop. 

To establish a uniform concentration profile downstream of a 90 elbow, straightening vanes 10 cm long were inserted just downstream of the short-radius elbow. Figure 21 shows the effect of these vanes on the concentration profile. Although the concentration becomes flatter, a distinct minimum at the center of the pipe still exists. These results imply that the solids are already distributed at the exit of the elbow, and the vanes merely increase the rate of diffusion of the particles. 

Figure 21. Effect of straightening vanes on the solids concentration profile 22 pipe diameters downstream of a short-radius elbow. (Reproduced with permission from reference 55. Copyright 1987.)...

Turning vanes may be required to ensure distribution of the airflow across the catalyst. Computer modeling (CFD) or cold flow modeling may be required to ensure the proper air distribution and the proper design of the straightening vanes. 

It is especially important that enough straight pipe tffi provided both above and below the orifice to ensure a flow pattern that is normal and undisturbed by fittings, valves, or other equipment. Otherwise the velocity distribution will not be normal, and the orifice coefficient will be affected in an unpredictable manner. Data are available for the minimum length of straight pipe that should be provided upstream and downstream from the orifice to ensure normal velocity distribution. Straightening vanes in the approach line may be used if the required length of pipe is not available upstream from the orifice. 

The major requirements of the dust discharge are that upward gas flow into the cyclone body should be minimal while continuous discharge of the dust is maintained. This is often accomplished by the inclusion of baffles or straightening vanes at the discharge to suppress the vortex at this point, thus minimizing the up-flow. 

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