These ranges are just examples and can be changed as needed and should be verified with testing. This method also assumes that the process is relatively linear between each region.
David G. Rolfe, P.E., process controls engineer
Ashland Chemical, Columbus, Ohio
Switch between two controllers
You can use two controllers in parallel, but only use one at a time. Add a switch to sense when the flow exceeds what the controller with the low-flow PID settings can handle. Use the switch to change to the other controller that is tuned for the high-flow conditions. You can also put a limit switch on the temperature control valve and set it at a point just above the low-flow cycle range to perform the same function as the flow switch.
Terry V. Molloy, P.E., president
CMES Inc., Novato, Calif.
Opt for feed-forward control
Use feed-forward control. Install flowmeters on each water stream and use them to calculate the percentage of the total flow. Use the data as a feed-forward input to the temperature controller. We did just that and reduced variability in a three-user system from Â± 30 degrees F to about Â± 5 degrees F.
Curt Caveney, principal process development engineer
Eastman Chemical Co., Kingsport, Tenn.
Base feed-forward on water rate
Add feed-forward control based on water flow rate. Reset the feedback when the flow rate is stable. Many hotel hot water heaters successfully use such a strategy.
Andy Schwartz, chairman
Keeshan & Bost Chemical Co., Manvel, Texas
Try controller with two modes
The process dynamics are very different when operating at minimum water flow compared with normal load conditions. Effective values for both the process gain and the process time constant tend to increase inversely with the water flow rate. The temperature offset in rising (or falling) loads is inversely proportional to the controller gain. This means that if the temperature controller is tuned for the minimum flow condition, the controller gain and reset rate will be too small when the load increases, resulting in excessive offset during load ramping. On the other hand, if the temperature controller is tuned for the normal flow condition, the control action will be too aggressive at minimum flow, resulting in oscillation.
You should try a gain schedule controller or a controller with two control modes. We do not believe that feed-forward control by itself will handle the problem because it will not compensate for the change in loop dynamics as the load changes from zero to normal.
Irving Lefkowitz, vice president of engineering
ControlSoft Inc., Cleveland
An electric cable bundle was supported by cable hangers. The hooks at the ends of the cable hangers were hooked over the top of a metal strip (see top figure). The electric cables had to be lowered to the ground to provide access to whatever lay behind them. When the cables were returned to their supports, they were put back as shown in the lower part of the figure. This increased the load on the upper hooks. One hook eventually failed, increasing the load on the adjacent hooks. They then failed. A 200-ft. (60 m) length of cables fell, injuring one man. How can we avoid this situation in the future?
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