Major-Components-Permanent-Gas

Unfortunately, irrespective of its shape, in large flue ducts, neither the flow velocity, the temperature nor the sample composition is likely to be constant over the total cross-section of the duct. Consequently, samples must be taken across the duct and, simultaneously, the flow velocity, temperature and pressure must be taken at each sample points. Consider a flue duct depicted in figure 1. By using a Pitot tube and a manometer the flow velocity and the absolute pressure in the duct can be measured at any point in the duct. From the total length (L) of the Pitot tube, the length (l) exposed outside the flue duct and the angle (a), the position of the sampling point can be calculated. In addition, with a simple valving system, and the use of an evacuated sample bulb as the sample container, a gas sample can be taken simultaneously from the Pitot tube. The tube sampling arrangement is shown in figure 2.

Figure 2 The Pitot Tube

The gas velocity (V) is given by the expression,

V = (1)

The gas flow his measured from the pressure difference between the central aperture of the Pitot tube and the peripheral apertures and then the valve is rotated. The central tube is, thus, connected directly to the evacuated gas sample container and a sample withdrawn down the center Pitot tube for analysis.

Figure 1 indicates four symmetrically located sample points, but if the flow varies widely across the duct, or the duct is not circular in cross section, significantly more than four sampling points will be necessary. From the flow velocity, pressure and the gas composition (obtained from the analysis) the total flux of each component through the tube can be calculated. From such data, and other pertinent measurements, the heat balance of a combustion unit can be calculated.