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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When controller output is at mid-range,
both valves are fully open.
Click image to enlarge For this case the operating lines depend on the sizes of the two valves and on the flow coefficient for the exchanger. Flow simulations are the only way to analyze processes with parallel flow paths and/or two or more valves. The configuration in Figure 3 contains both.

As before, there're two extremes, one being for constant ΔP and the other for constant total flow. The operating lines in Figure 4 reflect the same liquid flow when all liquid goes through the bypass (valve on the flow through the exchanger is closed) and all liquid goes through the exchanger (valve on the bypass is closed). All operating lines contain significant nonlinear characteristics that are likely to raise some controller tuning issues.

If equal-percentage valves are used, another issue arises. When output of the liquid outlet temperature controller is 50%, both valves are 50% open. But when an equal-percentage valve is 50% open, it only passes around 20% of its maximum flow. So, for constant ΔP, total flow is restricted; for constant total flow, a large ΔP is required across the exchanger.

A split-range configuration is preferable, especially when equal-percentage valves are used (as they usually are).

Figure 5 incorporates the split-range logic shown in Table 1. When controller output is at mid-range, both valves are fully open. As controller output increases above mid-range, the bypass valve remains fully open and the exchanger valve closes. As controller output decreases below mid-range, the exchanger valve remains fully open and the bypass valve closes.

Dead zone
Figure 6. Dead zone: Equal-percentage valve likely is preferable despite the flat
region just above mid-range. Click image to enlarge
Figure 6 presents operating lines for the two extremes (constant ΔP and constant total flow). Valve characteristics for temperature controller outputs less than 50% pertain to the bypass valve (the exchanger valve is fully open). Valve characteristics for temperature controller outputs greater than 50% pertain to the exchanger valve (the bypass valve is fully open).

Operating lines clearly suggest the exchanger valve should be equal-percentage; the operating line for the linear valve is very nonlinear. Perhaps there's a slight advantage to using linear characteristics in the bypass valve but most likely an equal-percentage valve would be installed.

For both cases in Figure 6 the operating line for the equal-percentage valve exhibits a flat region just above mid-range (temperature controller output of 50%). This flat region is more pronounced for the constant ΔP case. Within this region output of the liquid outlet temperature controller barely affects liquid outlet temperature, resulting essentially in a "dead zone" just above mid-range.

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