A hot liquid is cooled by cold water in a counter-current heat exchanger: shown in Fig. E8.15:

Temperature $T_{h 2}$ is to be controlled by adjusting flow rate, $w_{c}$ . The temperature sensor/transmitter (TT) is direct-acting. Based on safety considerations, should the control valve be air-to-open or air-to-close? Should the feedback controller be direct-acting or reverse-acting?

Question

A hot liquid is cooled by cold water in a counter-current heat exchanger: shown in Fig. E8.15:

Temperature [latex]T_{h 2}[/latex] is to be controlled by adjusting flow rate, [latex]w_{c}[/latex]. The temperature sensor/transmitter (TT) is direct-acting. Based on safety considerations, should the control valve be air-to-open or air-to-close? Should the feedback controller be direct-acting or reverse-acting?

Accepted Answer

First consider qualitatively how exit temperature [latex]T_{h 2}[/latex] responds to a change in cooling water flow rate, [latex]w_{c}[/latex]. From physical considerations, it is clear that if [latex]w_{c}[/latex] decreases, [latex]T_{h 2}[/latex] will increase. Thus, if [latex]T_{h 2}[/latex] is decreasing, we want [latex]w_{c}[/latex] to decrease, and vice versa. But in order to specify the controller action, we need to know if the control valve is fail open or fail close. Based on safety considerations, the control valve should be fail open (air-to-close). Otherwise, the very hot liquid stream could become even hotter and cause problems (e.g., burst the pipe or generate a two phase flow).

For an air-to-close control valve, the temperature controller output [latex]p[/latex] should increase in order to have [latex]w_{c}[/latex] decrease. In summary, if [latex]T_{h 2}[/latex] decreases we want [latex]w_{c}[/latex] to decrease, which requires controller output [latex]p[/latex] to increase; thus a reverse-acting controller is required.

Question 8.15: A hot liquid is cooled by cold water in a counter-current he...

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