2257 Sequencing

While not actually a separate topic deserves special attention since it has great potential for energy savings. A significant blight in many industry processes is the overlapping of adjacent and opposing processes. A common example is HVAC reheat, where air that has just been cooled with an energy source is now being heated. This is analogous to driving around with the accelerator fixed and controlling the vehicle speed with the brakes; even if the detriment to the brake system is neglected, the effect upon vehicle fuel economy is obvious. There are many examples of heating/cooling overlap, some deliberate but most inadvertent—but all make overall energy consumption higher than necessary. One very effective tool in combating or correcting this is effective equipment sequencing. Ignoring some of the imperfections of real-world control systems, some control systems that appear sequenced will actually overlap occasionally, or over time. Some things that contribute to inadvertent overlap are:

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