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Automated Continuous Process Control Part 5

Tham khảo tài liệu 'automated continuous process control part 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | c04.qxd 7 3 2003 8 22 PM Page 70 70 CASCADE CONTROL a Figure 4-3.2 Cascade control schemes in heat exchanger temperature control. c04.qxd 7 3 2003 8 22 PM Page 71 OTHER PROCESS EXAMPLES 71 resets the flow controller set point. Any flow changes are now compensated by the flow loop. The cascade scheme shown in Fig. 4-3.2b accomplishes the same control but now the secondary variable is the steam pressure in the exchanger shell side. Any change in steam flow quite rapidly affects the shell-side pressure. Any pressure change is then compensated by the pressure loop. This pressure loop also compensates for disturbances in the heat content superheat and latent heat of the steam since the pressure in the shell side is related to the condensing temperature and thus to the heat transfer rate in the exchanger. This last scheme is usually less expensive in implementation since it does not require an orifice with its associated flanges which can be expensive. Both cascade schemes are common in the process industries. Can the reader say which of the two schemes gives a better initial response to disturbances in inlet process temperature Ti t The cascade control systems shown in Fig. 4-3.2 and b are very common in industrial practice. A typical application is in distillation columns where temperature is controlled to maintain the desired split. The temperature controller is often cascaded to the steam flow to the reboiler or the coolant flow to the condenser. Finally another very simple example of a cascade control system is that of a positioner on a control valve. The positioner acts as the inner controller of the cascade scheme. Figure 4-3.2 Continued. c04.qxd 7 3 2003 8 22 PM Page 72 72 CASCADE CONTROL 4-4 CLOSING COMMENTS So far no comments have been made regarding the action of the controllers in a cascade strategy. This is important because as learned in Chapter 3 if the actions are not chosen correctly the controllers will not control. The procedure to choose the action is