| OCR Text |
Show grammed to deliver the combustion air requirement index for the sample gas. (The GC analyses are automatically started and run every two hours.). UTILIZATION CRITERIA FOR USE - In the field of combustion, regulation always plays a double role ensuring a given heat input while maintaining a constant combustion quality. Any change in the characteristics of the fuel obviously affects these two functions and they will have to be corrected by the control system. We have already indicated the numerous examples where typical closed loop control cannot be implemented. The COMBURIMETRE, a purpose-built sensor of variations in gas characteristics, is most appropriate for these special cases. It also deserves to be emphasized that in respect of natural gases, the calorific value is related to the stoichiometric air requirement by a practically constant factor, such that both can be corrected by a same, single action. IMPLEMENTATION ALTERNATIVES - A comburimetre can be utilized in a control system in several ways, the specific mode of utilization being Temperature controller generally determined by the type of installation. Three examples of Utilization will now be described. A) Control by adjustement of the AirlGas Ratio - The diagram of Figure 7 illustrates an open loop control configuration involving actuation of the cam controlling the gas input according to the air input. According to the changes in the characteristics of the gas, the COMBURIMETRE acts via the cam to change the gas flowrate, to ensure both a constant combustion quality and no modification to the heat input, the latter always being operated by the temperature controller. B) Control by Adjustment of the Gas Pressure - The diagram of Figure 8 illustrates an open loop control system operating by adjusting the gas pressure. The law of variation of pressure P as a function of Valid must be such that the product IF x Valid is constant. For a given opening of the gas valve, the same air factor will thus be obtained with gases having different Valid indexes, particularly since the gas flowrate is directly proportional to IF/Id in incompressible subsonic flow, as is usually the case with control valves. Temperature I~.~ .e _ _______________________ ~~e Air valve ComburimeHre Burner I ~I Set Point GAS Gas valve Fig. 7 Valva RESPONSIVE AIR FACTOR CONTROL SYSTEM 210 |
