Читать книгу Continuous Emission Monitoring - James A. Jahnke - Страница 106

Since the flow through the critical orifice is dependent on gas density, molecular weight of the sample gas also affects the dilution ratio. Fluctuation of flue gas moisture and CO₂ concentrations can cause errors in the dilution measurements since their concentrations are respectively low (18) and high (44) relative to the usual combustion gas molecular weight of ~30. Also, with regard to earlier Scenario 4, the use of multi‐blend calibration gases containing CO₂ initially caused considerable confusion in conducting calibration error (drift) and linearity tests. The higher molecular weight of CO₂ increases the density and will cause lower readings if the dilution system was previously calibrated with a cylinder gas containing nitrogen (MW 28) only, as a background gas. Errors due to fluctuations in molecular weight have been calculated to approach up to 7%, depending upon the initial calibration conditions and measurement conditions (McGowan 1994).

This issue was first resolved by Miller (1994) who calculated relative sonic velocities for a variety of multicomponent gas mixtures. A molecular weight correction expression was developed by Appel (1994) for changes in flue gas composition valid under the condition that the dilution system is calibrated initially with single‐blend, nitrogen background cylinder gases:

(3‐6)