Furnace Control System for a Rapidly Changing Demand

Let’s consider the figure below:

In the system above, the output of temperature controller TC represents the process demand, and this signal is applied as one of the inputs, each to a high (>) and a low (<) signal selector block (module). The second input to the high (>) selector is obtained from the measurement of the fuel flow FT_f, additional, the second input to low (<) selector is obtained from the airflow measurement FT_a. The output from the high selector is applied as the set point of the airflow controller FC_a, and the output from the low selector is applied as the set point of the fuel flow controller FC_f. Note the airflow measurement is applied to the ratio module X before it forms the measurement input to the airflow controller. The oxygen controller adjusts the ratio for the amount of combustion air needed for complete combustion.

As long as the demand is constant under operating conditions, the airflow and the fuel flow controllers FC_a and FC_f respectively, regulate their respective flows to meet the demand. That is, under steady-state conditions both control loops operate in parallel. Additionally, note that the air controller FC_a has an output that increases with increasing measurement, and the fuel flow controller FC_f has an output that decreases with increasing measurement. This situation of parallel control loop operation changes under a process upset.

Let’s consider the case of an increase in demand; that is, the output of temperature controller TC increases. This implies that the temperature measurement has fallen. In the case of the high signal selector (>), the demand signal is greater than the instantaneous fuel flow FT_f signal, making the demand signal the one to be selected. This higher signal will increase the set point of the airflow controller FC_a, which will increase the airflow. At the same time in the case of the low signal selector (<), because the demand signal is greater than the airflow signal FT_a the airflow signal will be selected, but the airflow signal is always larger than the fuel flow. Since we need more air than fuel, even at steady-state conditions, the airflow measurement is always greater than the fuel flow measurement; As we started all this from a steady state in the furnace, this will represent an increase in the set point of the fuel controller, which will increase the amount of fuel supplied to the burners. The net result will be an increase in the amount of heat from the furnace to meet the new demand.

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Let’s now consider a case under a demand decrease. This means that the temperature measurement has increased and the output of temperature controller TC has fallen. In the case of high signal selector (>) the demand signal is smaller than the fuel flow FT_f, and the fuel flow signal FT_f will be the one to be selected. This smaller signal will decrease the set point of the airflow controller FC_a, which will decrease the airflow. At the same time in the case of the low signal selector (<), because the demand signal is smaller than the airflow FT_a signal the demand signal will be selected. This smaller signal will decrease the set point of the fuel flow controller FC_f which will decrease the fuel flow. The net result will be a decrease in the amount of heat from the furnace to meet the new demand. From the foregone, it will be seen that under process upset conditions the control loops act in series.

Also read: How to Perform Mass flow Measurements with DP sensors