Extension of the Basic Control Scheme

Of the four controlled variables, the furnace outlet temperature was the only one that gave a noticeable offset from setpoint, before stabilizing and returning to its setpoint. This response can be improved if a ratio controller is used that adjusts the fiiel flow in ratio with the fresh feed flow. The temperature controller for furnace outlet temperature (7 ) can then be used as additional feedback. This is simulated in FCCUIB. The step response for a fresh feed change is shown in Fig. 35.7. 

There is now only a maximum temperature offset of 0.8 °F instead of 5 °F, hence the ratio controller has improved the dynamic response considerably. The ratio setting is 0.34, which means that for every pound increase in fresh feed, the fuel flow is increased by 0.34 scf 

The furnace outlet temperature controller has now slightly different controller settings Kc 0.02, 7, = 12.5 and 7 =0.0. 

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