Selecting compressed air system controls

Compressed air systems are critical for most manufacturing facilities. They are typically the third most expensive operating system after HVAC and lighting, and in some industries, the most costly utility.

01/01/1999


Compressed air systems are critical for most manufacturing facilities. They are typically the third most expensive operating system after HVAC and lighting, and in some industries, the most costly utility.

Industrial air compressor systems range from ten to several thousand horsepower. Their installed and annual operating cost runs from several thousand to over a million dollars. The most expensive item in operating a compressed air system is production loss due to an improperly maintained system and inadequate controls. Comprehensive air compressor controls should provide efficient volume and pressure regulation of compressed air delivery.

The advent of low-cost, reliable electronics has revolutionized controls available for air compressors. Electronic control packages provide energy efficient operation, improved condition monitoring, equipment protection, and information necessary for preventive and predictive maintenance.

There are three main types of industrial air compressors: reciprocating, rotary screw, and centrifugal. Each type requires unique controls to operate efficiently and prevent failure.

The choice of compressor that best fits the needs of a plant should include evaluating available controls. A comprehensive compressor control system can have a significant effect on operating and maintenance costs. Energy savings of 15%-20% and long term maintenance savings of 7%-10% are common for advanced control systems vs conventional electropneumatic controls.

Energy savings

Use and misuse of energy by conventional compressor controls occur in two areas: pressure control and multicompressor operation. Overpressurization of the air system is the most common and generally most significant energy misuse caused by inadequate compressor controls.

Most conventional packages with control logic use proportional changes in air system pressure to make alterations in capacity control. This method requires a 10-20 psi pressure change in the air system to fully load and unload a compressor. The method is crude but relatively inexpensive and straightforward to implement with simple electropneumatic controls.

The result is compressors typically operating at average pressures 25-35 psi greater than the minimum required by equipment. This approach wastes a significant amount of energy.

The second common energy inefficiency associated with conventional compressor controls is the lack of multicompressor coordination. Compressors are designed to operate most efficiently at full load capacity. Usually multiple compressors operate at partial load or no-load to regulate the supply of air to meet demand.

Most plant air demands vary from full production, to reduced production, to nonproduction periods such as shift changes, breaks, and weekends. Conventional controls on individual compressors are difficult to set up and maintain for adequate response to changing plant air demands. The result is more compressors operating than are needed.

Often, several compressors operate in an inefficient part-load condition, fighting each other with one loading up in response to another unloading. This lack of multicompressor, system-wide control typically wastes an additional 10%-15% of energy by having more compressor capacity online than needed and operating the equipment in an inefficient part-load condition (Fig. 1).

Efficient air capacity control

Controls should have the ability to modulate compressor capacity based on a single, target pressure. An analog pressure transducer is required.

Based on the type of compressor and modulation method, a tunable, pressure control dead band is used to regulate load/unload cycle times. Advanced systems use proportional integral derivative (PID) loop control for capacity modulation control. PID loop control continuously adjusts reactive time of the modulation control providing good pressure control and optimization of load/ unload cycling of compressors. This method of control is applicable to all three types of compressors.

Centrifugal compressors have a limited throttling range dictated by the potentially damaging phenomenon called surge. To prevent surge, while maintaining constant plant pressure, centrifugal compressors are equipped with a relief valve, called a blow-off or recycle valve, which exhausts excess air produced by the compressor and not needed by the plant air system.

To further complicate centrifugal compressor control, the surge characteristics of centrifugal compressors vary with changes in discharge air pressure, intake air temperature, and inlet pressure. Advanced control systems for new and retrofit applications compensate for these changes, ensuring maximum throttle range and minimum wasteful exhaust.

Rotary screw compressors present a different type of pressure control problem. Capacity control of rotary screw compressors is accomplished by load/unload or modulation control.

Load/unload control operates the compressor at full load or unloaded. Unfortunately, this method is not very effective at maintaining a narrow range of air pressure. Plant air demand is inherently different than what compressors can deliver. There is constant cycling of the load/unload control, continuously delivering either too much or too little air.

To minimize the cycling effect, pressure control setpoints for load and unload are typically 10-15 psi apart. The result is plant air pressure swings of 15-25 psi. The solution is having compressors available which can modulate their capacities to maintain a tighter pressure control and avoid energy-wasting overpressurization.

Modulating rotary screw compressors presents another control problem. The energy-efficient throttling range of a rotary screw compressor is limited. As the rotary screw modulates its output, compressor efficiency (cfm/kw) degrades significantly.

Most conventional modulating controls throttle the capacity 30%-50% before fully unloading the compressor. This method of modulation control is commonly called auto-dual control.

While modulation is capable of good pressure regulation, many compressor controls still incorporate a large differential change in discharge pressure, typically 10 psi, to effect a change in capacity. Overpressurization is still a common energy wasting symptom of modulation control. Part-load inefficiencies of modulation control introduce further energy waste.

Advanced control systems modulate capacity over the rotary screw compressor's optimum throttling range, and when applied to multiple compressors, maintain a narrow minimum pressure range while allowing only one of the compressors to operate in the inefficient part-load condition to maintain pressure (Fig. 2).

Controlled air storage systems

These systems create a reserve supply of compressed air and use it to meet plant demand peaks, rather than have individual compressor controls try to react to demand change. This method of control is used for pressure regulation and to operate the plant at a reliable minimum pressure. Energy losses due to overpressurization associated with leaks and artificial demand are reduced in certain applications.

With adequate storage, a system can prevent a low-pressure event from slowing production when a compressor goes offline. Storage can be used to provide lost capacity temporarily, while a backup compressor is started and brought on line. The disadvantage of this system is that to create a reserve supply the compressors must produce air at an artificially high pressure and then store the excess in a large tank. This approach produces an expensive standby system, justified by reducing potential production losses.

Extra energy is expended but countered by the energy savings of lower demand associated with overpressurization and by reducing compressor load to meet occasional demand peaks. Overall energy savings are considerably less than efficient compressor controls. Controlled storage can be a good solution in applications where existing controls do not have modulation capability. However, controlled storage systems do not contribute to longevity of compressors.

Machine protection and preventive maintenance

While energy efficiency is the most measurable advantage of advanced compressor controls, machine health monitoring and protection lower maintenance costs, extend the useful life of a compressor, and reduce plant production problems associated with an unreliable system. Condition monitoring provides vital information for preventive maintenance such as dirty inlet air filters, fouled interstage coolers, and excessive wear of components.

By monitoring and trending key parameters such as inlet and interstage pressures and compressed air temperatures, routine -- rather than emergency maintenance procedures -- can be implemented. Conventional control systems use inexpensive gauges and pressure and temperature switches to monitor and protect compressors. All too often, they can't protect the compressor from damage.

Advanced control systems use reliable pressure, temperature, and vibration transducers to provide critical information for machine monitoring. Controllers can scan instruments many times per second, alerting operating personnel of impending problems and automatically shutting down a compressor to avoid catastrophic damage. Some controllers have logging capabilities to trend information for preventive maintenance and to assist maintenance personnel in troubleshooting problems.

System control and information management

One of the most common problems with air systems is the lack of universal compressor controls. Control systems available on different compressors are not easily combined into an efficient working system.

The majority of plants acquire compressors from different manufacturers over the years. This purchasing approach presents costly problems in efficient operation, maintenance, and reliability. Operating and maintenance personnel have difficulty sustaining the knowledge necessary to efficiently support numerous types of controls.

To operate a multicompressor air installation effectively and efficiently, it must be controlled as a system. Advanced controls are available for multicompressor plant air systems. Compressor controls are also available for retrofit and new applications to provide a common interface for operating and maintenance personnel, regardless of the compressor manufacturer.

Multicompressor pressure control

Sequencer or central controller has the advantage of low cost per compressor and is generally available for systems with up to 10 compressors. A modern sequencer should have a single pressure transducer located in the air header. Logic should maintain a target pressure within

The sequencer should automatically start and stop compressors, as well as load and unload them. Controls should automatically rotate the order of loading and unloading to optimize compressor combinations for varying demand conditions. A disadvantage of sequencers is they are not well suited for compressors with modulation control.

Distributed control is an alternative to sequencer. Each compressor has an intelligent controller that communicates with others. This type of multicompressor control is easy to install, has fast system response, and offers the capability of easily sharing information with a remote location.

Advanced systems offer multimaster capabilities, where any local controller can operate as the master control for the system. This system provides redundant system integrity for reliable energy savings. Other useful features available are automatic scheduling of combinations of compressors and operating pressure based on time of day or day of the week, communication with other plant-wide monitoring and control systems, and a data highway for information exchange with preventive and predictive maintenance programs. -- Edited by Joseph L. Foszcz, Senior Editor, 847-390-2699, j.foszcz@cahners.com

Key concepts

Electronic controls save energy, monitor compressor condition, and improve maintenance.

Energy is wasted by poor pressure control and inefficient multicompressor operation.

Monitoring compressor condition trends helps spot the beginning of problems, avoids catastrophic shutdowns, and indicates preventive maintenance.

Calculating wasted energy

A rule of thumb for 100-psi air systems is that for every 2 psi of overpressurization, there is an increase in required energy of 1% to deliver the same scfm of air. For example, three 250-hp compressors operating year-round at 10 psi more than needed costs an additional $12,250/yr in energy costs, based on a rate of $0.05/kwh or $16/hp.

Advantages of advanced controls

- Energy savings (typically

10%-25%)

- Lower maintenance costs

- Improved reliability and air quality

What to look for in compressor controls

- Electronic pressure control

- PID control for modulating capacity

- Multicompressor system pressure control

- Analog compressor condition monitoring and protection

- Communication interface for system monitoring and control

- Vendor experience with similar installations

More info

The author is available to answer questions about compressor controls. He can be reached at 800-837-2292.

Two previous articles discussed compressor controls: "Efficient Compressed Air Management: A Systems Approach" (PE, February 1997, p 111, File 4010); and "Benefits of Remotely Monitoring Rotary Screw Air Compressors" (PE, April 1998, p 77, File 4030/ 5540). Texts of Plant Engineering articles are available online at www.plantengineering.com. Copies of articles may be purchased by calling 847-390-2692.





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