In a previous article, Temperature Control of Air Heaters, we provided a general overview of control for process heat systems and explained the difference between closed- and open-loop controls. Briefly, a closed-loop system is output driven; the output of the system is continually measured and fed back to control components which adjust the operation of the tool to bring its output into alignment with a pre-set target. An open-loop system has no feedback loop and, as a result, the output of the tool does not impact its continued operation. For example, in a closed-loop process heat system if the inlet air increases in temperature the tool output temperature will briefly increase before the control system brings it back to setpoint, however in an open-loop system the tool output temperature will increase and no corrective action will occur.
In this article series we will expand on this topic, exploring when it is advantageous to use a closed-loop system and when it is acceptable to use an open-loop system. The first installment looked at the benefits of automating your system, the second installment looked at developing a better understanding of your process. This third installment will look at when an open-loop system is a good choice for your application.
Flexible Process Requirements
Many applications do not require pinpoint temperature control of the process heat system. Often, these applications require some amount of heat and fluctuations in temperature won’t have a profound impact on the final product. Processes with a fairly wide band of acceptable temperature and a stable environment surrounding the system are a great fit for an open-loop system. The reduced control will not detract from the system’s ability to perform the work necessary.
Cost Constrained Systems
Sometimes the budget is tight and a low-cost solution must be found. In this scenario, it is possible that a properly sized open-loop system will give you the best value for your dollar. Open-loop systems are less complex than closed-loop systems, requiring fewer control and measurement components. As the systems are simpler, this naturally leads to a lower capital cost.
While open-loop systems offer a lower degree of control compared to closed-loop systems, there are times when they are the best fit. If you are having difficulty determine which control system will work best for your process, contact an expert at STANMECH, we are happy to help!