Look at Liquid Bypass

Such process-side control of tubular exchangers offers advantages.

By Cecil L. Smith, Cecil L. Smith, Inc.

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At many plants the performance of steam-heated shell-and-tube exchangers can significantly impact operations. The three previous articles in this series examined regulation of liquid outlet temperature of tubular exchangers via a control valve on the utility side, either on the steam supply or the condensate return. This concluding article looks at the use of one or two control valves on the process side, specifically, to bypass some liquid around the exchanger.

single control valve
Figure 1. Low cost option: A single control valve
can provide liquid bypass control.
Click image to enlarge.
Using a liquid bypass addresses two key issues:

Condensate return. Shell pressure is the steam supply pressure, so condensate return isn't a problem.
Fast response. Changing the fraction of liquid that flows through the bypass very rapidly affects liquid outlet temperature.

Single Valve
Figure 1 shows the simplest arrangement. To reduce costs only one control valve is installed in the liquid bypass line. There're no control valves on either the steam supply or condensate. Condensate flows out of the exchanger through a steam trap. Full steam supply pressure is available for condensate return.

The maximum heat transfer rate occurs with the bypass valve completely closed. All liquid flows through the exchanger, providing maximum ΔT for heat transfer and, consequently, maximum heat transfer. The maximum liquid outlet temperature (and associated steam flow) matches that of configurations with the control valve on either steam supply or condensate. However, because the bypass valve is completely closed, there's no way the control system can attempt to exceed maximum heat-transfer capability. There's also no possibility for windup to occur, assuming windup prevention mechanisms are configured in the customary manner.

With a control valve only in the bypass it's impossible to reduce the heat transfer rate to zero. The lower limit depends on the size of the bypass piping and the control valve installed in the bypass — a flow simulation can determine the values of these flows, allowing computation of the minimum heat-transfer rate and corresponding liquid outlet temperature.

Equal-percentage valve
Figure 2. Operating lines: Equal-percentage valve shows only modest departure from linearity in
both cases. Click image to enlarge
For this configuration the process operating line is a plot of liquid outlet temperature as a function of bypass valve position. This configuration provides an action opposite to that of ones we've previously discussed, namely:

Process. Opening the control valve decreases liquid outlet temperature. The process is reverse acting.
Controller. If liquid outlet temperature is rising, the controller should increase its output to the bypass valve. The controller must be direct acting.

Process operating lines for bypass configurations will have a negative slope; in contrast, those for the control valve on the utility side have a positive slope.

Blocking exchanger flow
Figure 3. Blocking exchanger flow: This
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