May I ask about the working principle and process of this problem?

Temperature is the primary parameter, while flow rate is the secondary parameter. The temperature at the outlet of the pyrolysis gas is adjusted by controlling the flow rate of the refrigerant.

Selective control system for pyrolysis gas outlet temperature and refrigerant flow rate. Under normal operating conditions, a stable refrigerant flow is maintained to cool the pyrolysis gas. After cooling, the outlet temperature of the pyrolysis gas must not be too low; if the temperature is too low, the moisture contained in the pyrolysis gas will form hydrates that can block the pipes. To this end, the refrigerant control valve should be a air-operated (fail-closed) valve, closing in the event of a fault in order to prevent excessive cooling of the pyrolysis gas ； The temperature controller (emergency controller) operates in direct action mode, with pure proportional control and a large proportionality coefficient; its output drops rapidly when the temperature becomes too low ； The flow controller (normal controller) is used in reverse, with proportional-integral action. Under normal conditions, the pyrolysis gas temperature is higher than the set temperature (minimum temperature); the controller receives a positive deviation, and it outputs a high signal. The selector (low selection LS) selects the output signal of the flow controller and sends it to the control valve in order to regulate the refrigerant flow. When the temperature at the pyrolysis gas outlet is below the set temperature (minimum temperature), the temperature controller outputs a signal whose level decreases rapidly; the selector then selects this signal from the temperature controller to send it to the control valve, thereby quickly reducing the flow rate of the refrigerant and preventing the pyrolysis gas temperature from dropping too low.

Thanks to your explanation, I get it now; this is a selection loop! The low-selection method is being used here; as for the criteria for judgment, it’s as you mentioned. Thank you!

I think we need to analyze whether to use FIC or TIC under normal operating conditions. 1. The valve of FO is controlled by LS; the purpose is to allow the override (or selector controller) to open the valve. In other words, it is only when the main controller tries to close the valve that the override controller intervenes to prevent it from closing. 2. FIC is a constant-value controller and, in theory, does not close the valve; only TIC closes the valve, as it has a process channel for cooling the process medium – when the amount of pyrolysis gas is low, TIC closes the valve. 3. At this point, FIC takes over the control of the valve to prevent it from closing, with the aim of maintaining a certain cooling flow rate, similar to the function of a minimum limit setting. So my understanding is: TIC is the master controller, and FIC is the override controller.

Without understanding these process requirements, it is impossible to determine which controller should be used under normal operating conditions for this selective control system (override regulation).