Your facility personnel typically know when not enough air compressors are operating. The demand side system air pressure drops below what is required for production triggering phone calls and complaints. But do they know when too many compressors are running increasing your plant s energy costs? The Compressed Air Challenge, sponsored by the Department of Energy, has impressed upon us that compressed air is a very expensive utility. Typically, only 15% of the input power to a plant air compressor goes to producing compressed air. In addition, compressors running in a partially loaded condition, or worse totally unloaded for hours on end, waste considerable amounts of energy and maintenance resources. Many compressors can use as much as 70% of full load power while delivering less than 30% of full load capacity. The controls on individual compressors perform a critical function in helping your compressors match compressed air supply with the facility s compressed air demand. If the supply exceeds the demand, system compressed air pressure will rise and a compressor will need to reduce its output or risk exceeding system pressure limits. Rotary screw, centrifugal, and reciprocating compressors with varying individual compressor controls exist and most can be efficient, when properly applied and operated. The paradox is that even compressors with energy efficient part-load controls can be very inefficient when operated together or in concert with other capacity control types or brands of air compressors. This is where modern multiple compressor controllers can alleviate the part-load energy waste and the effects of wide swings in pressure on your compressed air system and production. There are many multiple compressor controllers available today using different logic to solve the same problem. Some controllers merely start and stop compressors based on system pressure; some rely on time of day to determine which compressors to run, while others will only work with specific types of compressors or those from only one manufacturer. The most advanced controllers can address all of these issues. Let s examine the evolution and application of some of the more popular types. Cascading Pressure Type Cascading pressure type multiple compressor controllers have been in use for a long time. Multiple compressors are controlled based solely on system pressure. As system pressure falls below a set point, an additional air compressor is brought on line. The actual sequence of which compressor is brought on line is pre-determined. In many cases, the largest horsepower compressor is started first and subsequent compressors are selected on descending horsepower order. If the system air pressure continues to fall to a lower pressure set point, the next pre-selected compressor is brought on line. This type of controller requires the facility to operate with a cascading pressure band. For example, the lead compressor will load or turn on when system pressure falls to 100 psig. The next compressor in the sequence may turn on when system pressure falls to 95 psig and the one after that at 90 psig, and so on. As on-line capacity exceeds system demand, system
Pressure (psig) Pressure ( psig) pressure will rise allowing the last compressor brought on line to unload or turn off (in our example somewhere around 100 psig) The previous compressor brought on line will require system pressure to rise to 105 psig before unloading or turning off. As the number of compressors to be controlled increases, so does the pressure band and energy consumption. In a nominal 100 psig system a 2 psi increase in discharge pressure results in a 1% increase in input power to the compressor. In addition, higher system pressures will increase the consumption of all unregulated uses and leaks. Often, systems with a cascade type controller lack necessary storage capacity and require a wide pressure band. When the pressure differential between the compressors maximum allowable operating pressure and plant s minimum required pressure is tight, too many compressors end up on line for the demand required, and the full benefit of individual compressor energy saving controls is not realized. 110 105 100 95 90 85 80 Multiple Control #1 #2 Production minimum requirement #3 #4 Traditional cascading set point control scheme Courtesy: Compressed Air Challenge Networking Type The use of microprocessor controllers on individual air compressors improves response time to system demand changes and allows for network type sequencing of multiple similarly equipped air compressors. Networking multiple compressors is an advancement, which allows all networked 110 105 100 95 90 85 80 compressors to operate within a tighter control band. Typically, the control scheme is to have only one compressor operate at part load and all remaining compressors run close to or at 100% capacity at the target pressure. The controllers are connected via a cable allowing them to communicate with each other. A network type multiple compressor control system usually requires that all compressors are the same type from one manufacturer, are equipped with the same model microprocessor controller and will only control air compressors. Since communication among the network controllers requires cable, it is often impractical to connect compressors located in remote areas of the plant. Smart Controllers Network Controls Unload pressure Single set point control pressure Load pressure Production minimum requirement Basic single set point control scheme Courtesy: Compressed Air Challenge With today s advancement in programmable logic controllers, modern sequencers have evolved with the ability to determine the most efficient combination of available compressors to meet the ever changing plant demand efficiently and effectively.
Advanced smart controllers will not only monitor system air pressure but also monitor system demand (flow). These modern controllers can efficiently sequence various types of compressors along with different types of capacity control systems. Smart controllers can also integrate dryers, pressure flow controllers and other ancillary equipment. Smart controllers are designed to be pre-programmed with information unique to your compressed air system. Data such as individual compressor size and type, response rate, system storage capacity, full and part load performance characteristics, capacity control type and system operating set points will be stored. A smart controller will continuously monitor the percent load of the individual compressors along with the system s compressed air pressure and flow demand in cubic feet per minute (cfm). By combining the monitored data with the data stored in memory, a smart controller will consider the type of compressors (rotary screw, centrifugal, reciprocating) and capacity control (variable displacement, variable speed, throttled inlet, on-line / off-line) available to ensure only the minimum kw is on line to satisfy the demand. s will be brought on line when system pressure falls below the target pressure. However, compressors are taken off line based on system pressure and the system demand or flow required by the plant. The plant s compressed air flow demand (cfm) data is collected by a flow meter (or flow meters) and then sent to the controller. The controller then determines which compressors to turn off and which to leave on line. Smart controllers will also take full advantage of different capacity controls on individual compressors to maintain maximum system efficiency. As a plant s production expands so does demand for compressed air. Typically, the increase in compressed air requirements is met by adding air compressors of different sizes and manufacturer. For example: In a system, which includes multiple rotary screw air compressors of different sizes and vintages, all with pneumatic modulation inlet valve type control, the compressors meet the system demands effectively but not efficiently. It is not uncommon to have all available compressors operating at part load since total supply side capacity typically is greater than system demand. As the compressors modulate to reduce output and meet the demand the pressure bands of each overlap. Since each compressor is modulating and reducing output, system pressure will not rise and the plant is left with all air compressors running at part load
wasting considerable energy and increasing maintenance costs. By integrating a smart controller into the system, the plant s compressed air demand is monitored along with system pressure. When plant compressed air demand decreases to a level which can be met by the total capacity of one less air compressor, one compressor will be turned off and left in a stand-by mode. The reduced number of on-line compressors will all be operating closer to full load saving considerable energy. As demand falls further another appropriately sized compressor will be taken off line; or since the smart controller is programmed with the capacity of each compressor a larger capacity compressor may be taken off line and the smaller compressor previously taken off line is brought back on. Throughout this process the operating pressure band is relatively narrow (2 3 psi) reducing energy and maintenance costs associated with higher operating pressures. In another example, consider a system with centrifugal compressors equipped with inlet guide vane controls and a rotary screw compressor equipped with modulating inlet valve control. The smart controller will direct the rotary screw to run fully loaded (at its most energy efficient point) and trim with one of the centrifugal compressors via its inlet guide vanes, maintaining excellent energy efficiency. As system demand decreases, the controller will continue to turn down the centrifugal, reducing output until maximum turn down is reached. If output capacity is still greater than demand, the controller will begin to turn down the next centrifugal. By monitoring the demand flow, the controller will know whether to slightly modulate the rotary screw compressor or completely unload it. The load can then be supplied by one or both of the centrifugals or completely unload one of the centrifugals allowing the rotary screw compressor to operate fully loaded. Once again this is typically achieved within a relatively narrow pressure band of 2 3 psi. Smart controllers have many additional practical capabilities including the ability to display the plant s compressed air consumption in cubic feet per minute; power required to produce the compressed air in kilowatts, system efficiency data in cubic feet per kilowatt, peak, average and minimum demands throughout the day, equipment status and alarms, etc. A single smart type controller can control compressors in different or remote rooms or even combine multiple systems. Multiple plants can be controlled by a single smart controller through radio
communication. In addition to compressed air equipment many modern smart controllers can control, as well as monitor and trend, other equipment and utilities in a plant such as chillers, pumps boilers, etc. Generally compressed air systems have become more modern and plant requirements more complex. Advanced smart controllers will operate a system reliably, provide operators with the information needed to properly manage it, and above all turn off air compressors that are not required.. For more information contact Scales Industrial Technologies, Paul Shaw (203) 630 5555 or Niff Ambrosino (973) 890 1010. Messrs. Shaw and Ambrosino are both qualified instructors of Advanced Management of Compressed Air Systems for the Compressed Air Challenge workshops.