Tham khảo tài liệu 'automated continuous process control part 12', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 7 3 2003 8 02 PM Page 210 210 PROCESSES FOR DESIGN PRACTICE Figure B-2 Scrubber. enter . You may read Section 2-5 again to review the questions related to this process testing. Once the process characteristics are obtained and the controller tuned using the formulas of Chapter 3 the tuning can be tested by changing the set point or by introducing a disturbance by starting or stopping a fan. Designing the Feedforward Controller. It is known that for this scrubber unit the input flow can vary much and very often. That is it is common to have one fan working . fan 2 with a flow of 25cfm and suddenly the other two fans turn on to have a total input flow of 100cfm. When this occurs the outlet HCl will increase until the feedback controller can take hold by adding more NaOH to bring the HCl back to set point. We must be sure that the HCl ppms do not violate the EPA regulation. Thus under normal operation the set point to the controller must be set low enough to ensure staying within regulation under this upset condition. Although the low set point guarantees not violating the regulation during any upset condition it costs extra money during normal operation because it requires extra NaOH flow. If we could provide a tighter control less deviation from set point under upset conditions we could raise the set point and thus save a portion of the extra NaOH. Feedforward control can provide this tighter control. The idea is to measure the flow entering the scrubber and if this flow changes manipulate the NaOH 7 3 2003 8 02 PM Page 211 PROCESSES FOR DESIGN PRACTICE 211 valve. That is do not wait for a deviation in HCl before taking action. There is a flow sensor transmitter with a range of 0 to 100cfm measuring the input flow to the scrubber. Chapter 6 shows that a way to design a feedforward controller is to use a first-order-plus-dead time transfer function K t and t0 describing how the disturbance feed to the scrubber in this case affects the .
