The environmental engineer measures the velocity head at various traverse points using the pitot tube assembly shown in Figure 5.11.2. The engineer collects the gas samples at a rate proportional to the stack gas velocity and analyzes them for carbon monoxide (CO), carbon dioxide (CO2), and oxygen (O2). Measuring the velocity head at a point in the flowing gas stream with both the type S pitot tube and a standard pitot tube with a known coefficient calibrates the pitot tube. Other data needed to calculate the volumetric flow are the stack temperature, stack and barometric pressures, and wet-bulb and dry-bulb temperatures of the gas sample at each traverse.
Table 5.11.2 gives the equations for converting pitot tube readings into velocity and mass flow and shows a typical data sheet for stack flow measurements (Morrow, Brief, and Bertrand 1972).
TABLE 5.11.1 LOCATION OF TRAVERSE POINTS IN CIRCULAR STACKS (Percent of stack diameter from inside wall to traverse point)
Number on a Diameter
Number of Traverse Points on a Diameter
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