Compressed Air Systems
Quick tip: Air compressors commonly use more electricity than any other system in a manufacturing plant.
General Description:[1]
Compressed air is used widely throughout industry and is often considered the “fourth utility” at many facilities. Almost every Arkansas industrial plant, from a small machine shop to an immense pulp and paper mill, has some type of compressed air system. In many cases, the compressed air system is so vital that the facility cannot operate without it. Compressed air is a safe and flexible source. Plant air compressor systems can vary in size from a small unit of 5 horsepower (hp) to huge systems with more than 50,000 hp.
In many industrial facilities, air compressors use more electricity than any other type of equipment. Inefficiencies in compressed air systems can therefore be significant. Energy savings from system improvements can range from 20 to 50 percent or more of electricity consumption. For many facilities this is equivalent to thousands, or even hundreds of thousands of dollars of potential annual savings, depending on use. A properly managed compressed air system can save energy, reduce maintenance, decrease downtime, increase production throughput, and improve product quality.
A modern industrial compressed air system is composed of the compressor, prime mover, controls, treatment equipment and accessories, and the distribution system. Many modern industrial air compressors are sold “packaged” with the compressor, drive motor, and many of the accessories mounted on a frame for ease of installation.
Compressed air is used for many things in an industrial facility including powering pneumatic tools, packaging and automation equipment, and conveyors. Pneumatic tools tend to be smaller, lighter, and more maneuverable than electric motor-driven tools. Although they have many advantages, pneumatic tools are generally much less energy-efficient than electric tools. Many manufacturing industries also use compressed air and gas for combustion and process operations such as oxidation, fractionation, cryogenics, refrigeration, filtration, dehydration, and aeration.
Potential Energy and Cost Savings Opportunities:
The below energy conservation opportunities or energy efficiency actions (EE ACTIONS) are provided as a partial list of potential savings opportunities in your plant. They are grouped as no-to-low cost, moderate cost, and long-term cost investments. Consider each for your plant and feel free to contact us for clarification or any assistance you may need in assessing specific projects.
No-to-low cost investment
EE Action: Eliminate inappropriate uses of compressed air [2]
Compressed air is one of the most expensive sources of energy in a plant. The overall efficiency of a typical compressed air system can be as low as 10%-15%. For example, to operate a 1-horsepower (hp) air motor at 100 pounds per square inch gauge (psig), approximately 7-8 hp of electrical power is supplied to the air compressor.
Users should always consider other cost-effective forms of power to accomplish the required tasks and eliminate unproductive demands. Inappropriate uses of compressed air include any application that can be done more effectively or more efficiently by a method other than compressed air. The table on the right provides some uses of compressed air that may be inappropriate and suggests alternative ways to perform these tasks.
| Inappropriate Use | Suggested Alternatives/Actions |
| Clean-up, Drying, Process cooling | Low-pressure blowers, electric fans, brooms, nozzles |
| Sparging | Low-pressure blowers and mixers |
| Aspirating, Atomizing | Low-pressure blowers |
| Padding | Low to medium-pressure blowers |
| Vacuum generator | Dedicated vacuum pump or central vacuum system |
| Personnel cooling | Electric fans |
| Open-tube, compressed air-operated vortex coolers without thermostats | Air-to-air heat exchanger or air conditioner, add thermostats to vortex cooler |
| Air motor-driven mixer | Electric motor-driven mixer |
| Air-operated diaphragm pumps | Proper regulator and speed control, electric pump |
| Idle equipment* | Put an air-stop valve at the compressed air inlet |
| Abandoned equipment** | Disconnect air supply to equipment |
*Equipment that is temporarily not in use during the production cycle
**Equipment that is no longer in use either due to a process change or malfunction
EE Action: Minimize compressed air leaks [2]
Leaks are a significant source of wasted energy in a compressed air system, often wasting as much as 20%-30% of the compressor’s output. The best way to detect leaks is to use an ultrasonic acoustic detector, which can recognize high frequency hissing sounds associated with air leaks. These portable units are very easy to use. Costs and sensitivities vary, so test before you buy. Depending on your compressor type, there are other methods to determine the total plant leakage and associated costs.
EE ACTION: Locate compressor air intake in the coolest possible location [2]
The effect of intake air on compressor performance should not be underestimated. Intake air that is contaminated or hot can impair compressor performance and result in excess energy and maintenance costs. When inlet air is cooler, it is also denser. As a result, mass flow and pressure capability increase with decreasing intake air temperatures, particularly in centrifugal compressors. Often, only small amounts of ductwork are required to implement this opportunity.
EE ACTION: Reduce system air pressure to the minimum required
Compressed air system energy savings can be about 1% for each 2 psig drop in system air pressure. Distribution pressure drop should not be more than 10%, so the system pressure (pressure in the receiver tank) should be set to slightly more than 10% above the pressure required by your end-use application. For instance, if an assembly tool requires 65 psig to operate properly, you may set your system pressure to 75 psig. It is typical to see plant systems set to 120-140 psig when only 60-90 psig is required, wasting significant energy. Alternatively, incremental reductions in pressure can be used to determine the minimum acceptable end-use pressure.
EE ACTION: Remove condensate with minimal air loss
Removing condensate is important for maintaining the appropriate air quality level required by end uses. However, significant compressed air (and energy) losses can occur if condensate removal is done improperly. Solenoid-operated drain valves and zero-loss traps are available, saving significant energy compared to manual condensate valves.
EE ACTION: Ensure compressors are shut off during nights and weekends
It is not uncommon for the plant’s compressed air system to be left on overnight or throughout the weekend. The reason may vary as the plant may not have a standard operating procedure, or the person responsible may have left early for the day, or nobody has been assigned the responsibility. Nevertheless, the use of an electronic controller can eliminate unnecessary electricity usage when the plant isn’t operating.
Moderate investment
EE ACTION: Preventative maintenance program
Excellent maintenance is the key to good reliability of a compressed air system; reduced energy costs are an important and measurable by-product. Due to the high cost to produce compressed air, the benefits of good maintenance far outweigh the costs and efforts involved. Good maintenance can save time, reduce operating costs, and improve plant manufacturing efficiency and product quality.
All components in a compressed air system should be maintained in accordance with the manufacturers’ specifications. Manufacturers provide inspection, maintenance, and service schedules that should be strictly followed. Because the manufacturer-specified intervals are intended primarily to protect the equipment rather than optimize system efficiency, in many cases, it is advisable to perform maintenance on compressed air equipment more frequently. Look for significant pressure drops across any system component as an indicator of needed maintenance.
EE ACTION: Redesign compressed air distribution system
As plants expand their operations, it is not uncommon to add a separate air compressor to serve that new area. Or, the compressed air is supplied to the new area by a single connection point, without a piping redesign. Either of these two approaches is not optimal. So, a distribution system redesign can lead to reduced system pressure drop and, often, a reduced number of compressors needed to deliver process air.
EE ACTION: Install smaller, separate compressors for small low pressure needs or small low load times
Sometimes a single system requires significantly lower pressure air as compared to all others. The high pressure air must be throttled down to the lower pressure and this is an energy loss. In certain circumstances, it is economically feasible to use a separate small compressor to provide the lower pressure air. Similarly, a small compressor can be used during nights and weekends when the compressed air load is very small (often required by one or two specialty systems/equipment)
Long-term investment
EE ACTION: Utilize natural gas or diesel engine-drive compressor to reduce electrical peak demand
When the economics are right (high demand charges, low fuel costs, and distinct peak demands), the use of natural gas or diesel engine-drive compressor to reduce electrical peak demand can have a reasonable return on investment. As the plant’s peak demand reaches a predetermined value, the electric motor-driven compressors are shut down as the engine-drive compressors are activated.
EE ACTION: Install a variable speed drive compressor
The newest-technology variable speed compressors have very high efficiencies, even at part-load conditions. The use of a single variable speed compressor can be used as a ‘topping’ unit as all other compressors operate at a higher efficiency, close to full-load, state.
References
[1] exerpts from Improving Compressed Air System Performance: A Sourcebook for Industry, Industrial Technologies Program (U.S.), 2003, United States Department of Energy, Washington, D. C., pp. 128 (cited 9/13/10)
[2] Energy Tips – Compressed Air, Industrial Technologies Program (U.S.), United States Department of Energy, Washington, D. C., (cited 9/13/10). To find more tip sheets go to EERE Publication and Product Library.
