A steam-heated evaporator used to concentrate a feed stream by evaporating water is shown in Fig. E8.14. The mass fraction of solute in the exit stream x is measured and controlled by adjusting the steam flow rate, S. The control valve is fail-open. Should the composition controller be

Question

A steam-heated evaporator used to concentrate a feed stream by evaporating water is shown in Fig. E8.14. The mass fraction of solute in the exit stream [latex]x[/latex] is measured and controlled by adjusting the steam flow rate, [latex]S[/latex]. The control valve is fail-open. Should the composition controller be direct-acting? Justify your answer.

Accepted Answer

First consider qualitatively how solute mass fraction [latex]x[/latex] responds to a change in steam flow rate, [latex]S[/latex]. From physical considerations, it is clear that if [latex]S[/latex] increases, [latex]x[/latex] will also increase. Thus, if [latex]x[/latex] is increasing, we want [latex]S[/latex] to decrease, and vice versa. For a failopen (air-to-close) control valve, the controller output [latex]p[/latex] should increase in order to have [latex]S[/latex] decrease. In summary, if [latex]x[/latex] increases we want [latex]S[/latex] to decrease, which requires an increase in controller output [latex]p[/latex]; thus a direct-acting controller is required.

Question 8.14: A steam-heated evaporator used to concentrate a feed stream ...

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