Example 2.5 Suppose that the inlet stream in the CSTR system (Figure 1.7) comes from an upstream unit over which we have no control. Then cA,Fi, and T1 are disturbances. If the coolant flow rate is controlled by a control valve, then Ft is a manipulated variable, while Tc is a disturbance. Also, if the flow rate of the effluent stream is controlled by a valve, F is a manipulated variable; otherwise, it is an output variable. With respect to the output variables we have the following: T,F,Tcq, and V are measured outputs since their values can be known easily using thermocouples (T,Tso), a venturi meter (F), and a differential pressure cell (V). The concentration cA can be a measured variable if an analyzer (gas chromatograph, infrared spectrometer, etc.) is attached to the effluent stream. In many industrial plants such analyzers are not available because they are expensive and/or have low reliability (give poor measurements or break down casily). Consequently, in such cases cA is an unmeasured output variable