Automated controls improve compressor efficiency

To keep your plant running, you must provide electricity, chilled water, air handling, compressed air, heating, cooling, and perhaps other services to your production line. Without them, you can't make product, much less profit. In a basic economics course, these are referred to as "fixed costs." But nothing could be further from the truth.

By Bob Dannenfelser, Systems Engineer, Ingersoll-Rand September 15, 2002
Key Concepts
  • Compressed air systems are a major factor in a plant’s total electricity usage.

  • Adjustable frequency drives enable air compressors to operate more efficiently.

  • Automated controls further improve the efficiency of air compressors and are good for the plant infrastructure.

    Controlling multiple compressors
    Adjustable frequency drives
    Retrofitting compressors with automation

    To keep your plant running, you must provide electricity, chilled water, air handling, compressed air, heating, cooling, and perhaps other services to your production line. Without them, you can’t make product, much less profit. In a basic economics course, these are referred to as “fixed costs.” But nothing could be further from the truth. These can vary month-to-month, line-to-line, and product-to-product.

    Resources “manufactured” in your plant allow you some measure of control in their costs. Installing a more efficient furnace reduces your heating costs. Wrapping insulation around chilled water lines can save both heating and cooling costs. Frequently called the “fourth utility,” compressed air is a common resource required for a broad range of manufacturing applications (Fig. 1). Depending on the plant, air compressors can be responsible for as much as 25% of a plant’s electric bill.

    In larger plants, compressed air frequently comes from several machines. If these machines are not working as a team with one controller having system responsibility, they are not working as efficiently as they could be. While several air compressors are running, some may not be producing any usable air.

    The controls on each compressor may not be as energy efficient as they could be, either. When plant demand has been satisfied, a typical compressor will run unloaded for several minutes until the stored supply of compressed air drops. Then it loads again to meet demand. When running unloaded, the compressor motor does use less electricity than while under load. Still, it is running, producing no usable air, perhaps for as long as 10 minutes before returning to its loaded state.

    Controlling multiple compressors

    Individual compressors don’t know what the plant’s needs are, or if one compressor is more efficient than another. A system controller, using a setpoint for the plant, or even for different zones within the plant, makes decisions on which compressors to run, and in what order, to meet the plant’s air pressure demands. Bringing compressors online to meet demand, and then turning them off as demand is reduced maximizes energy efficiency and better regulates air output. The system controller can rotate lead responsibility among the units it controls based on hours run, day of week, or at the operator’s discretion.

    Another benefit is that key information on all compressors is available to the plant engineer at one screen. This key information includes knowing which units are on, which units are lead, individual compressor status, and plant system pressure. Trips to and from the equipment to gather information, report data, troubleshoot, and make adjustments are no longer necessary. If a fault occurs, a maintenance technician can go to the correct compressor the first time, already knowing what the fault is, so he or she is armed with the proper tools and parts to get the unit back online quickly.

    Adjustable frequency drives

    Getting just the right pressure by loading and unloading compressors is, at best, an approximation. But technology makes it easier to achieve accurate downstream pressure. The latest adjustable frequency drives (AFDs) make it possible to control a compressor’s output to within 2 psi of the desired setpoint. Thus, in a compressor system, one unit with an AFD can trim the total output of the system far more accurately than traditional methods.

    The benefit of integral AFD control is the increased energy efficiency of the system. This efficiency comes from three aspects.

    • Fully loaded compressors run as efficiently as possible.

    • Compressors with AFD control run as efficiently as fully loaded units, even when running as little as 50% of full speed.

    • Large compressors running unloaded can adversely affect the power factor within the plant, producing a ripple effect of inefficiency.

      • If a compressor with an AFD is fairly large (at least 200 hp) while other motors in the plant are comparatively small (10-20 hp), the capacitor bank within the AFD improves the power factor of the entire plant, so that all motors run more efficiently.

        Depending on compressor usage, number of shifts, daily operation, the cost of electricity, the amount of air leakage, and many other factors, the payback period for an adjustable frequency drive could be 10—36 months. For typical operations involving three shifts, with varying loads among shifts, and an electricity cost between 5 and 6 cents per kW, payback for an AFD is usually 18—24 months. The dollars saved can prove to be significant for 8—12 years after payback, before major replacement costs are considered.

        Retrofitting compressors with automation

        Typically, older equipment has start/stop push-buttons, manual adjustments for load and unload, and very little else. If a failure occurs, often many manhours are required to identify the problem before the repair can begin. The compressor cannot support production if it is down. Retrofitting with automated controls can breathe new life into an older compressor.

        On typical compressor automation systems, English language messages provide quick and easily understood information on compressor status, faults, elapsed time, and more. The automation systems make this information available to the central controller and any graphical supervisory systems. Critical information and key indicators are available on one computer screen. Details on problems make diagnostics easy and repairs quick. Typically, key operating details of a plant air system can be graphically displayed in a central office with mouse-click commands of graphic elements. Color-coded images provide critical information on all units. Equipment screens, tables, trends, and histories can be recalled, making data available in reports. Data can also be networked to other key personnel in the plant.

        Connected to a phone line, this information becomes accessible to anyone with the proper password without needing to be physically present. This can be quite handy at 2 a.m. from the supervisor’s home. Decisions can be made and activity directed by phone much quicker than driving to a plant to assess the situation before applying corrective measures.

        When alarms occur, the supervisory control and data acquisition (SCADA) computer can automatically page maintenance technicians, supervisors, and even vendor’s service personnel to respond quickly to an equipment failure. Most SCADA systems also include data from other equipment in the plant. Different screens control different equipment. Supervisory screens can show the entire plant at a glance (Fig. 2).

        Sophisticated levels of automation have been available on the “demand” side of the factory for more than three decades. The potential benefits of applying the same to the “supply” side are only beginning to be appreciated. Knowing where, when, and how the air, electricity, heat, and cold are consumed makes the efficient allocation of these increasingly expensive resources possible and necessary. While enhanced utility automation does not directly make plant air, distribute energy, or produce cold or heat, it provides the tools for those plant personnel responsible for controlling costs to do their jobs more accurately, quickly, and economically. With automated controls, plants can provide a direct contribution to improving the bottom line.

        Edited by Jack Smith, Senior Editor, 630-288-8783,

        More Info The author is available to answer questions regarding air compressor automation. Mr. Dannenfelser can be reached at .