Most compressed air systems at process plants offer significant opportunities for improvement. A rigorous evaluation of an existing or new system can establish sustainable best practices for maximizing reliability and performance and minimizing lifecycle costs. In addition, it can forestall product contamination and scrap as well as the possibility of environmental damage.
The same optimization strategies and techniques apply to new systems and improvement of existing ones. They reflect a key common denominator: energy costs alone generally exceed the purchase price of a compressor in its first year of operation. Often, addressing the inappropriate application of compressed air and the proper control of the compressed air system provide the greatest savings.
Efforts must begin with analysis of demand. Every new plant will have specific initial requirements while an existing plant has established consumers. However, both of these likely will change over time, so always consider anticipated growth.
HOW MUCH AIR IS NEEDED?
Start by creating a "demand profile" form and listing each steady demand, based on manufacturers' specifications, including the required pressure and operating flow rates at minimum, average and peak conditions. Identify and separately list all intermittent demands with compressed air "on/off" cycle times in seconds or minutes. Include the number of shifts and resulting variations and potential future additions. Indicate whether the air is for process or general plant use or serves as breathing air.
The type and size of compressors needed and the air quality necessary for the applications are very important factors. Consider the air quality requirement for the applications in selecting the compressors and the air treatment. While a specific process or plant standard may call for an oil-free air compressor, some sites have successfully installed lubricated compressors with proper filtration and excellent maintenance to provide oil-free air. In other cases, where a plant requires some oil-free air but has an almost equal demand in other applications that can tolerate a given lubricant level, two separate systems may make sense. International Organization for Standardization (ISO) standards classify the level of particulates, degree of dryness and amount of lubricant in the air.
Operating at the lowest possible system pressure allows for the most efficient operation. For example, in many compressors, reducing the compressor discharge pressure 10 psi can cut specific power by 5%. Many plants supply compressed air to the main distribution system at a pressure that is at least 5 psi above the required minimum, which is determined either by the most significant uses or one most sensitive to lower pressure.
Figure 1 shows components of demand and their typical levels. "Artificial Demand" relates to increased air consumption required by any unregulated uses due to system pressure in excess of what actually is necessary. "Inappropriate Uses" refers to applications where replacing compressed air with other options potentially can improve effectiveness or efficiency. "Leaks" covers unintended losses.
Leaks typically represent 10–20% of total demand. In the past, recommendations were to maintain the leakage rate below 10%. However, compressed air now is recognized as a very expensive utility and standards at many plants call for holding leaks to less than 5% of peak flow.
A corollary is to provide automated equipment to shut off air to applications when not in use.
A few end uses requiring higher pressure can lead to maintaining the entire system pressure at an "artificially" elevated level, resulting in increased energy consumption. In some plants, it's possible to modify end uses to operate at a lower pressure. In other cases, a motor-driven booster can handle continuous demands for higher pressures and an air amplifier can take care of intermittent ones.
Sometimes a low-pressure blower can replace compressed air. There are many examples of such switches for sparging processes for aerating or agitating liquid. Depending on the height of the column and specific gravity of the liquid, the design blower discharge pressure usually runs 4–15 psig. For a 15-psig design pressure, the energy saving is approximately 60%. Before making such a move, check for any adverse effects on the process and necessary changes to the distribution piping.
Electric motors often can replace air motors, reducing energy consumption by 80%. Similarly, where applicable, swap out double-diaphragm pumps with electric-motor-driven pumps.
Another often very worthwhile move is to create vacuum for continuous applications with a vacuum pump rather than compressed air venturi methods.
COMPRESSED AIR QUALITY
For most industrial applications, ISO 8573.1:2009, the international standard for compressed air quality, defines the level of contamination permissible (Table 1). The standard identifies three primary forms of contamination: solid particles, water (vapor) and oil. It classifies contaminants and assigns an air quality level ranging from Class 1, the highest purity level, to Class 9. A user or supplier can specify an even-more-stringent requirement, Class 0.
There also are other possible gaseous contaminants. Their acceptable level depends on the application; the purification methods will depend on the specific requirements. Compressed air users must understand what are considered potential contaminants in their applications, the effects of these gases, and the methods to achieve successful and sustainable contamination control.
Process and laboratory air. Process air sometimes is defined as air that comes in contact with the product or has incidental product contact. Compressed air must meet the requirements of the process equipment it serves. A minimum pressure dew point of 35–39°F, supplied by a refrigerated air dryer, may not always suffice. A regenerative desiccant dryer to provide a substantially lower dew point may be necessary — but will consume more energy. The requirements of the air consumer also many demand greater filtration. You can locate these additional filters centrally at the compressors or at points of use.
Packaging and instrument air. Most plants define packaging air as air used in packaging lines that does not come in contact with the product in any way.
Instrument air is defined as air used for instrumentation and controls. Most instrumentation engineers specify oil-free air compressors. Where specifications just require oil-free air, proper filtration often may suffice. The pressure dew point of compressed air used inside a building should be at least 18°F below the building's minimum temperature. For instruments used outside, the pressure dew point should be at least 18°F below the lowest ambient temperature. Monitor these temperatures and install an alarm to alert operators of a possible problem.
Where a single system must provide process, packaging and instrument air, use the highest quality air required. Many process plants instead rely on two systems — one for "plant" air, which may not be dried, and the other, generally at a much higher quality, for instrument air or some processes. Many plants will use dry nitrogen as a backup for the instrument air system. This is especially important when short-duration power interruptions occur and minimum pressures must be maintained for all instruments and valve actuators.
Breathing air. Air supplied to respirators, hoods and helmets and to special breathing air systems must satisfy U.S. Occupational Safety and Health Administration (OSHA) standard 1910:13d. It requires drying, filtration and treatment to meet specific levels, including for carbon monoxide, with an alarm system. Compressed breathing air must comply with the requirements for Type 1 – Grade D breathing air as described in American National Standards Institute (ANSI) Compressed Gas Association (CGA) Commodity Specification for air ANSI/CGA-G7.1. The OSHA standard references this specification and is essentially consistent with ANSI. Breathing air also must be tested for contaminants such as methane, nitrogen oxide, nitrous oxide, halogenated hydrocarbons and other hydrocarbons. Grossly contaminated or oxygen deficient air may not be able to be purified to levels acceptable for breathing.
The CGA Standard Commodity Specification G7.1, Grade D, commonly is specified for plant breathing air systems. At a minimum, this air must conform to OSHA standard 1910.134 (revision effective April 1998) or, in Canada, Canadian Standards Association standard CAN3-Z180.1-M85. Check to ensure compliance with all local standards.
Many companies use lifecycle costing derived from compressed air data and bid response forms. These forms help in judging among possible options, including air-cooled, water-cooled and various types of compressors and ancillary equipment. The data forms could include: package power, pressure, flow, inlet conditions, dew point for dryers, cooling water or ventilating requirements, service life and necessary maintenance. Purchase decisions often consider actual energy and maintenance costs for a 10-year period (Figure 2) as well as equipment reliability.
The inlet air to the compressor should be as cool, dry and clean as possible. Clean is defined as a minimum amount of dust and foreign matter. The air cannot contain contaminating gases such as ammonia, chlorine, sulfur, carbon monoxide, etc., that can affect the compressor, piping, process or pneumatic system components. Compressed air must be free of corrosive contaminants and hazardous gases.
For lubricant-free positive-displacement-type compressors (screw and reciprocating), lowering the inlet air temperature 5°F will improve compressor performance approximately 1%. Where practical, consider outside air for many applications. If the inlet piping takes air from a remote location, such as outdoors or from conditioned plant air, increase the pipe one size for every ten feet of length. Do not place the intake point near cooling tower or exhaust fan discharge, as this could contaminate the inlet air to the compressor. If not supplied with the compressor, install an inlet vacuum gauge to monitor the condition of the intake filter.
Adequate compressor room ventilation is essential for heat rejection. Usually, however, it is not cost effective to air condition the area. If the compressor ventilation air is ducted out of the room, the total static pressure of the combined inlet and outlet ductwork generally should not exceed 0.12 in. of H2O. You many need to install a ventilation fan in the ducting for heat recovery or to address large ducting losses.
It may be possible to recover almost all the heat generated by the air compressors to significantly reduce your plant's total energy consumption. For more on heat recovery, see: "Air Compressor Heat Recovery Is a Hot Topic."
OPTIMIZATION AND CONTROL
A control strategy should strive to match system demand with compressors operated at or near their maximum efficiency levels. This should result in compressors running at their lowest possible input power and total energy consumption for all demand conditions. Excessive part-load or no-load operation is wasteful — avoid it where practical. Some examples of multiple compressor sequencing are cascading systems and rate-of-change systems.
A cascading system overlaps the pressure setting in the compressors installed so that an increase or decrease in pressure starts or stops the appropriate compressor, loaded or unloaded. This type of system generally requires a large pressure band and considerable storage volume.
Today, modern multiple-compressor systems can benefit from sophisticated controls, like Smart Sequencers (Figure 3), to efficiently match compressor operation and air delivery to the system requirements at the lowest energy consumption. This type of sequencer can be used with any combination of compressor types and manufacturers. For additional information, see: "Taming Multiple Compressors."
Reducing compressor discharge pressure 2 psi will cut input power 1% for many types of compressors. Proper sequencing controls should consider fluctuations in demand, available storage and the characteristics of the equipment supplying and treating the compressed air.
Effective control strategies require documented data to monitor:
• flows (use mass flow meters that compensate for pressure and temperature);
• power and energy consumption;
• pressure and pressure drop (∆P) before and after major components such as dryers and filters;
• temperatures (sensors often come with the equipment); and
• pressure dew point of the system.
Another optimization strategy relies on a pressure/flow controller. This is a device that serves to separate the supply side of a compressor system from the demand side of a compressed air distribution system. The controller maintains a constant demand-side pressure with varying demand loads.
For this controller to work properly, the supply-side pressure generally must exceed the demand-side requirement by a minimum of 10 psi. The compressors operate at an elevated pressure and increased horsepower, but pressure on the demand side can be maintained at a lower stable level to minimize actual compressed air consumption. Storage, sized to meet anticipated fluctuations in demand, is an essential part of the control strategy.
Using a pressure/flow controller may not be necessary in all cases. Each compressed air system differs in supply, distribution and demand aspects. So, it's essential to properly evaluate the benefits of such a controller for the particular system. Additional primary and secondary air receivers often may serve as an alternative to, or in conjunction with, a pressure/flow controller.
Get More Details
The subjects covered in this article are discussed in much greater detail in "Best Practices for Compressed Air Systems," which is available via www.compressedairchallenge.org. The 325-page manual addresses topics such as distribution piping systems, specific end uses, measuring and estimating the cost of leaks, monitoring systems for optimum performance, and self-auditing opportunities.
WILLIAM SCALES, P.E., is CEO of Scales Industrial Technologies Inc., Carle Place, N.Y. Email him at firstname.lastname@example.org.