Maintenance is the customer of controls and energy engineering is the customer of monitoring. And I discussed potential problems that can occur when combining monitoring and control in the same system. In this article, I will get more specific about building practical systems that address both controls and monitoring.
A large chemical plant in Celje, Slovenia, planned to retrofit a kiln used to produce titanium dioxide. To make space for the new equipment related to the retrofit, the plant needed to relocate its high-pressure compressed air system feeding an adjacent pressing process used to dry the material before firing it in the kiln. However, a comprehensive compressed air audit using cloud-based software showed the plant did not need to relocate the system.
Many OEMs of air compressors, dryers, sensors and master controls are integrating monitoring features and capabilities into their components. It would seem a no-brainer to keep it simple and use those sensors and systems for both control and monitoring. What could be simpler?
Baseline measurements include flow, power, pressure, production output, and other relevant variables impacting compressed air use. These data evaluate trending averages to develop Key Performance Indicator (KPI) and Energy Performance Indicator (EnPI) parameters and establish base‑year performance. The focus of this article is the application, evaluation, and analysis of baseline measurements to provide information necessary to improve Compressed Air Supply Efficiency.
A chemical packaging facility had done everything right when they last upgraded their compressed air system a few years ago. They installed a Variable Speed Drive (VSD) air compressor and implemented other energy efficiency measures, but plant expansions caused increased system demand, which exceeded the capacity of the system. The packaging lines were now seeing low pressure, causing shut downs in production. And projections showed plant demand would increase even further.
Often, multiple centrifugal air compressors are set up to simply react to air demand, which requires the system to not only meet the new demand, but also make up for air depleted in the main header. This typically results in too much supply, which results in bypassing the air to atmosphere. The result is wasted energy use.
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This article is going to identify two air compressor control situations that will preclude translating air use reduction in the production area into lower input energy into the air compressor.
The plant produces both molded and blow molded plastic parts on a 5 day per week, three shift schedule. Production and maintenance sometimes occurs on weekends, occasionally requiring the air compressors to run on a 24 x 7 basis so the practice was to leave the compressed air system always pressurized. The system consisted of three modulating lubricated screw compressors one sized at 150 hp and the others 125 hp (3 units), each controlled with their local compressor controllers.
Load-sharing is an important part of a multiple centrifugal-compressor master control system. It minimizes blow-off based on the available turn-down. In addition, remote start-stop saves more energy if load floats between different ranges. Finally, adding a screw compressor and implementing a hybrid control system might save the most energy and provide the best back-up. In any case, a well-instrumented system allows engineers and operators to assess, optimize and tune the system.
When a system has the right combination of VFD and base-load air compressors, how do you coordinate their control? What tells the air compressors to run and load, to have just enough (or no) base-load air compressors and a VFD running, all the time air is needed? Appropriate master controls are needed. These controls are often called “sequencers” or “master control systems”.