OCR Text |
Show air-requirement gas likely to be tested and the other hand, one should avoid too large a margin of excess air since the sensitivity and the accuracy of the oxygen analyzer are both heightened as the percentage of oxygen measured is decreased. Thus, to calibrate the device for another type of gas, one would select the most suitable standard gas and oxygen quantity respectively, according to these same criteria. GAS COMBUSTION AIR REQUIREMENT MEASUREMENT CAPABILITY The Comburimetre is designed to measure the combustion air requirement index (Valid) of a gas. But a different version of the meter can also serve to measure the stoichiometric combustion air requirement Va, using fixed volume ratio proportioning pumps instead of the pressure and flow controllers. This alternative embodiment has been named "Volumetric Comburimetre" (Comburimetre volumetrique). The theoretical relation between stoichiometric air requirement and the percentage of oxygen in the products of combustion then is O2 Va = R - (1 + R) x -- 0.21 a relation in which R represents the ratio of the air flowrate to the gas flowrate (a fixed ratio independant of the gas relative density). This equation follows from Equation (1), if it is assumed, as it can be assumed for natural gases, that the humid flue gas volume (V'fo) is approximately equal to the sum (1 + Va). COMBURIMETRE TESTS AND PERFORMANCE - The laboratory experiment was supplemented by lon&term tests in an industrial environment [2J. These demonstrated the reliability of the Comburimetre when operated for several months with no maintenance other than routine calibrations. The meter's inherent qualities make it perfectly suitable for control systems. It is highly insensi ti ve by design to di turbance from industrial factors. Its response time is less than 30 seconds at 90% of full scale. The zirconium dioxide cell's response being instantaneous, the 30-second figure actually corresponds to the time required for transfer of the sample through the conditioning system and the analyzer. The instrument's accuracy is of the order of ±l% of the measured value of the Valid ratio as computed from the voltage accross the terminals of the analyzer's cell. Figure 6 shows the results of a comparative test between the comburimetre and a chromatograph the integrating computer whereof has been pro- Va va 12,1~ ________ -. __________ ,-________ ~ __________ ~ ________ ~ __ 12,2~---------+----------~---------+----------4----------4 __ • • 12,3+-________ -4 __________ ~----------+_---=~~_4--~~----~ 12,44-~~~~~--------4_--------4_--------~--------~ • Chromatograph Comburimetre 12,5~---------+----------4_--------_4----------4_--------_4- hours 4 6 8 10 12 14 Fig. 6 COMPARATIVE TEST OF CHROMATOGRAPH AND COMBURIMETRE 209 |