2. Flow through the bypass at times is 70% or more of the total, making the configuration in Figure 8(b) preferable. But, at other times, flow is less than 30% of the total, making the configuration in Figure 8(a) preferable.
Figure 9. Minimizing interactions:
Where degree of interaction is degrading loop performance, flexibility of digital controls permits implementing configurations that account for the nature of the process. In Figure 9 a summer determines the position of each control valve. Each summer has two inputs, one from the temperature controller and one from the flow controller. The configuration reflects the following observations:
1. To increase flow without affecting liquid outlet temperature, both control valves must be opened. In Figure 9 the flow controller output is connected to a positive input to both summers.
2. To increase liquid outlet temperature without affecting flow, the exchanger valve must be opened and the bypass valve closed. In Figure 9 the temperature controller output is connected to a positive input for the summer associated with the exchanger valve but to a negative input for the summer associated with the bypass valve.
Coefficients on the inputs to the summers in Figure 9 aren't necessarily 1.0, permitting some tuning. But even so, it's unlikely that the configuration can completely eliminate interaction between the two loops. However, it only needs to reduce the degree of interaction to the point where both loops can be tuned to deliver good performance.
Cecil L. Smith is president of Cecil L. Smith, Inc., Baton Rouge, La. E-mail him at email@example.com. He is the author of "Practical Process Control," just published by John Wiley.