| OCR Text |
Show ,. Gas input ,. Air input ,. Production rate ,. Heating time ( DISTURBANCES) GOALS) ,. Product quality If- Efficiency ( MEASUREMENTS) ,. Temperature ,. Flue gas composition (02 ,C02 ,CO> Fig. 1 : DISTURBANCES AFFECTING FURNACE OPERATION CLOSED LOOP AND OPEN LOOP CONTROL - As a rule, control of the furnace temperature and of the percentage of oxygen in the combustion products in accordance with the classical closed loop scheme (Figure 2), should correct the settings effectively enough to cancel the effects of these disturbances. 6 control % °2control * Air to gas flow rate ratio operator Fig. 2 ! CLOSED LOOP CONTROL In fact however, inasmuch as there is a broad variety of industrial furnaces, there are numerous exceptions to this rule. The response times of large furnaces, of glass melting furnaces for example, to control inputs can be up to several hours. The difficulty of closed loop temperature control in such cases is obvious. In addition, the large quantity of heating zones involved, as well as ingresses of unwanted air, can make it impossible to actually measure the oxygen content of the combustion products. Indeed, in some applications, combustion must take place with an air deficiency to obtain an oxygen-free furnace atmosphere. It accordingly becomes necessary to rely on open loop control and, if changes in one of the characteristics of the gas are likely to disturb furnace operation, a dedicated control loop must be implemented for the specific disturbance (Figure 3). MEASU - CHARACTE-FURNACE Fig. 3 OPEN LOOP CONTROL (Example of AIR/GAS ratio correction) MEASURING GAS CHARACTERISTICS - It is consequently indispensible to have an accurate and reliable sensor to indicate the value of the incriminated gas characteristic at all times. In practice, in the field of industrial utilization of gas, there are three basic gas characteristics of importance and subject to change : relative density, stoichiometric air requirement and calorific value. Relati ve density variations, at a constant supply pressure to the burners, bring about variations in the gas flowrate. Variations in the stoichiometric air requirement, both isolated and combined with relative density variations, bring about change in the air factors of the air-gas mixtures. And finally, variations in calorific value at constant flow bring about variations in heat output. 206 CHOOSING A SENSOR -- USEFULNESS OF A "COMBURIMETRE" - The choice of sensors enabling automatic measurement of the above-mentioned characteristics is limited. Only the following are available : densimeter, calorimeter, Wobbe meter and chromatograph. The densimeter and the calorimeter both provide only part of the information generally required for combustion control. Measuring relative density alone is of little value. Measuring calorific value alone can be sufficient in certain cases, provided gas flowrate data is also available from volumetric metering. Present calorimeters nevertheless present two major weaknesses hindering their utilization in industrial process control : firstly, they have a slow response time due to their very principle, which relies on the heat exchange between fluids, and secondly, they are so complex as to afford only a moderate reliability with the precision models reserved for laboratory use. There are of course more simple calorimeter designs with an adequate reliability, but these are too easily affected by industrial environmental conditions and have poor accuracy. The Wobbemeter, related in concept to the calorimeter, has the same drawbacks : slow res- |
