Compressed Air Equipment
In This Guide
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Introduction
Compressed air is used widely throughout industry and is often considered the "fourth utility" at many facilities. In a typical industrial plant, compressors use 8 to 25% of the total electricity consumed. In many cases, air compressors use more electricity than any other type of equipment. Inefficiencies in compressed air systems can therefore be significant. Energy savings from systems improvements can range from 20 to 50% or more of electricity consumption.
Technology Options
The U.S. Department of Energy (DOE) estimates current U.S. compressed air systems use 90 billion kilowatt-hours (kWh) of electricity annually, with energy costs of about $1.5 billion. Plant air compressor systems can vary in size from a small unit of 5 horsepower (hp) to large systems with over 50,000 hp. Energy savings can be realized in three major areas of compressed air systems:
- Equipment package improvements
- System design improvements
- Operations & Maintenance improvements
A compressed air system consists of a supply side, which includes compressors and air treatment, and a demand side, which includes distribution and storage systems and end-use equipment. Improving and maintaining peak compressed air system performance requires addressing both the supply and demand sides of the system and how the two interact. A properly managed supply side will result in clean, dry, stable air being delivered at the appropriate pressure in a dependable, cost-effective manner. A properly managed demand side minimizes wasted air and uses compressed air for appropriate applications.
Here are some of the most common areas to target for improvement:
Leaks: Leaks have been estimated to waste as much as 30% of a compressor systems output and energy usage. This can be reduced to less than 10% with focused maintenance and leak detection systems. Each 1/8" leak in a compressed air system is estimated to represent nearly 42,000 kWh of wasted energy per year.
Distribution Piping Pressure Drop: Most piping systems were not designed to account for future expansion and increased air usage. Consequently, many systems are undersized. This can result in significant line losses and a drop in pressure. Pressure drop reduces efficiency and creates the need for increased pressure, which adds to overall costs. Strategically located pressure gauges alert you to the pressure drop, and quantifies the drop.
Point-of-use Conservation: Monitoring and controlling compressed air use at each user is extremely important. Pressure measurement and regulation at each usage point manages usage and creates awareness. Many plants have specific operations that require higher pressure than the remainder of the facility. Investing in point-of-use compressors or booster systems could provide the additional pressure required without increasing the overall system requirements.
Eliminate Inappropriate Uses of Compressed Air: There are many compressed air applications that can be done more effectively or more efficiently by a method other than compressed air. DOE’s Office of Industrial Technologies (OIT) provides information (www.oit.doe.gov/bestpractices/pdfs/compressed_air2.pdf) thats list many of these inappropriate uses and suggested alternatives and actions.
Compressor Over-pressuring: After point-of-use management tools are in place, leaks are repaired and inappropriate compressed air uses are minimized, the system pressure requirements will most likely be lower. Reassess compressor pressure with the understanding that every pressure reduction of two psig reduces the total system horsepower 1%.
Compressor Control: Any operation that uses independently controlled multiple compressors is wasting energy. A multiple compressor control system can reduce energy usage by up to 10%.
Coalescing Filter: If the filter has not been changed in several months, then the filter could be plugged. This adds a pressure loss up to 10 psig. Incorporating routine filter maintenance into basic facility protocol could recover the cost of replacement in several months.
Moisture Removal Drain Traps: Even if you enter a compressed air system room after leaks have been fixed you may still hear the hiss of lost air. Electric solenoid style traps can each blow off up to 37 cubic feet per cycle.
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Additional measures include:
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There are many resources available to facility and energy managers responsible for compressed air system energy efficiency, including:
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Efficiency Benefits
Compressed air is one of the most expensive forms of energy at an industrial facility. Annual electricity costs for operating a typical 100-hp compressor system can reach or exceed $50,000 per year, often eclipsing the acquisition costs of the system. A properly managed compressed air system can save energy, reduce maintenance, decrease downtime, increase production throughput and improve product quality. Given that energy expenses represent the majority of the life-cycle costs of a compressor system, DOE studies estimate that 20 to 40% of this energy demand could be eliminated with energy efficiency improvements.
Manufacturers
Manufacturers of compressed air systems include:
- Ariel Corp.
- Crawford Inland Compressor
- Elliott Turbomachinery Co., Inc.
- JUN-AIR USA, Inc.
- Kaiser Compressors, Inc.
- Lans Co.
- Paxton Products
- Thomas Industries, Inc.
- Vortron Industrial, a division of Vortech Engineering, Inc.
Air Compressors EZ Search by Thomas Register features a database of air compressor manufacturers.
Purchasing Tips
Compressors are not by their nature very energy efficient. In fact, it can take 7 to 8 hp of electric power to produce 1 hp of compressed air power. Energy costs for operating a compressor can exceed the initial cost of the compressor in the first year alone. Buying an energy-efficient compressor saves you tens of thousands of dollars in the long run.
Look for a compressor that has the best energy saving features. The most efficient compressor on the market today is the double acting reciprocating compressor. However, it has been replaced in many plants by oil-flooded rotary screw compressors. Once the standard for plant air, the double acting reciprocating compressor is now used mostly for higher pressure or specialty applications (such as when oil-free compressed air is required). The demise of these compressors is due partly to high initial costs, complex installations and the need for experienced maintenance crews.
This leaves the majority of plant air being provided by rotary screw compressors. The best rotary machines have energy saving features built into their design. The most efficient rotary compressor is one that is fully loaded. When an application has long periods of little or no demand for compressed air you should have a compressor that can shut itself off. Or, at the very least, you can blow the oil reservoir down and operate at reduced horsepower. The construction of a rotary screw air compressor is important when it comes to keeping energy costs lower. For maximum efficiency, the rotors should have an asymmetrical rotor shape, which provides a tighter seal between the grooves of the rotors to reduce slippage, eliminate vibration and increase efficiency from 3 to 5% over standard rotor designs.
When comparing compressors, be careful to compare products of similar components. Look beyond the sticker price to the rated efficiencies, ease of installation, ease of maintenance and service support. The initial price is important, but the long-term value is accounted for in terms of energy efficiency, reliability and maintenance costs.
Emerging Technology
A promising and emerging technology hybrid permanent magnet (HPM) synchronous motors represents a cost-effective means for companies to upgrade the overall efficiency of their compressed air systems. HPM motor technology can drive rotary air compressors and deliver significant benefits, including lower life cycle costs, increased reliability and minimized environmental impact. Compressors driven by HPM motors are controlled by variable speed inverters that match compressor output to demand, enabling the air compressor to maximize its air production and minimize its energy consumption.