Maximum Heat Transfer Rate
The operating line in Figure 2 suggests that the maximum steam flow is approximately 93 lb/min. This corresponds to a fully open steam valve, resulting in a liquid outlet temperature of approximately 290°F and a shell pressure essentially equal to the steam supply pressure.
Figure 4 -- Functional but sluggish:
Allowing the liquid outlet temperature to specify a steam flow set point in excess of 93 lb/min would result in windup in the liquid outlet temperature controller. You could impose a maximum limit on the steam flow set point, but there’re better methods for preventing windup:
External reset. The measured value of steam flow is the external reset input to the liquid outlet temperature controller.
Integral tracking. When the steam valve opens fully, the measured value of steam flow is used for the integral term within the liquid outlet temperature controller.
Inhibit increase/inhibit decrease. When the steam valve opens fully, the liquid outlet temperature controller isn’t allowed to increase the steam flow set point.
These methods have the additional advantage that you needn’t precisely know the maximum steam flow. Although some digital systems provide more than one of these options, any will suffice.
The flow controller in Figure 4 regulates the steam flow by manipulating the control valve on the condensate. Changing the condensate valve opening affects the condensate level within the exchanger, which affects the heat transfer area exposed to the condensing steam, which affects steam flow. This loop will function but its dynamics are far slower than those of most flow loops.
Figure 5 -- Valve on condensate:
I previously recommended some settings for a flow controller — these come with one condition: the flow measurement must directly sense the flow through the control valve. This is the case in Figure 1b with the control valve and flow measurement in the steam supply but not in the configuration in Figure 4, where steam flow is measured but condensate is flowing through the control valve. At equilibrium, the steam flow must equal the condensate flow, but the two can significantly differ during transient conditions. So, the recommended settings won’t work for the configuration in Figure 4.
Having established that we can control the steam flow using a valve on the condensate, let’s now consider the temperature-to-flow cascade configuration illustrated in Figure 5b. Because condensate flow (in lb/min) must equal steam flow (in lb/min) at equilibrium, the process operating line for the control valve on the condensate is the same as the process operating line for the control valve on the steam, but with one exception: the minimum steam flow is zero when the control valve is on the condensate whereas the minimum steam flow is 31 lb/min when the control valve is on the steam. The maximum steam flow is exactly the same for the two configurations. With the control valve on the condensate, the process operating line is essentially linear from a steam flow of zero up to the maximum.