Procuring representative samples of particulates suspended in gas streams demands that the velocity at the entrance to the sampling probe be equal to the stream velocity at that point. The environmental engineer can equalize the velocities by regulating the rate of sample withdrawal so that the static pressure within the probe is equal to the static pressure in the fluid stream at the point of sampling. A specially designed pitot tube with means for measuring the pertinent pressures is used for such purposes. The engineer can maintain the pressure difference at zero by automatically controlling the sample drawoff rate.
Figure 5.11.2 shows the pitot tube manometer assembly for measuring stack gas velocity. The type S (Stauscheibe or reverse) pitot tube consists of two opposing openings, one facing upstream and the other downstream during the measurement. The difference between the impact pressure (measured against the gas flow) and the static pressure gives the velocity head.
FIG. 5.11.2 Type S pitot tube manometer assembly.
Figure 5.11.3 illustrates the construction of the type S pitot tube. The external tubing diameter is normally between aEh and Ek in (4.8 and 9.5 mm). As shown in the figure, the distance is equal from the base of each leg of the tube to its respective face-opening planes. This distance (PA and PB) is between 1.05 and 1.50 times the external tube diameter. The face openings of the pitot tube should be aligned as shown.
Figure 5.11.4 shows the pitot tube combined with the sampling probe. The relative placement of these components eliminates the major aerodynamic interference effects. The probe nozzle has a bottom-hook or elbow design. It is made of seamless 316 stainless steel or glass with a sharp, tapered leading edge. The angle of taper should be less than 30°, and the taper should be on the outside to preserve a constant internal diameter. For the probe lining, either borosilicate or quartz glass probe liners are used for stack temperatures to approximately 900°F (482°C); quartz liners are used for temperatures between 900 and 1650°F (482 and 899°C). Although borosilicate or quartz glass probe linings are generally recommended, 316 stainless steel, Incoloy, or other corrosion-resistant metal can also be used.
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