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Compressed Air

Purchasing Tips

Your goal is to deliver the right amount of compressed air at the right pressure at the lowest cost. When you buy compressors, consider things such as standards and applications.

Unfortunately, as of February 2005 there is no standard comparison for air compressors. Institutes such as the Compressed Air and Gas Institute (CAGI) and Pneurop are developing simplified performance testing standards. Some of these standards have been incorporated as addenda in the International Standards Organization (ISO) Standard ISO 1217, Displacement Compressors Acceptance Tests. When you compare the energy efficiency of compressors, remember that that not all manufacturers are members of CAGI and may test compressors using different standards.

Compressed Air Energy Efficiency Reference Guide

A well-controlled compressor runs in one of two ways. Some compressors will run as “base load” units where they will be either at full load or off. Some will need to run in “trim” or “swing” duty where they vary their output to take up the remainder of the load. In one-compressor systems, the compressor runs in trim duty.

For base duty, the most important thing to consider is full-load efficiency rating of the compressor. You can check efficiency by consulting the Compressed Air and Gas Institute (CAGI) data sheets published by the manufacturer. Most manufacturers also publish these sheets on their Web sites. For more information on the data sheets, consult Appendix B, Packaged Compressor Efficiency Ratings in the Compressed Air Challenge (CAC) publication, Improving Compressed Air Performance – a Sourcebook for Industry (referred to in these web pages as the “CAC Sourcebook”).

For trim duty, the most important thing to consider is part-load energy consumption. Part-load ratings are not included in the CAGI data sheets, so ask your supplier for the part-load package kW ratings for the compressors that interest you. The average percentage load at which the trim unit operates will determine your savings. Ask for the “package” power ratings that show how much power the compressor-assembly uses rather than just “brake horsepower.” Brake horsepower is only the power delivered to the compressing element shaft. It does not take into account all of the other losses.

Note: Not all compressor manufacturers rate their compressors under the same conditions, making it difficult for buyers and suppliers to compare data. For an explanation, refer to Appendix B, Packaged Compressor Efficiency Ratings in the CAC Sourcebook.

When buying a compressor, note that the purchase price is one of the smallest components of the life cycle costs. Often the purchase price differs only a small percentage between units of different brands. But energy costs and maintenance are by far the largest life cycle cost components for air compressors. So when choosing compressors, examine these costs closely.

Things to Consider When Buying a Compressor System

(In this section, we mention various compressor operating modes. For a discussion of operating modes, see the Compressor Operating Modes in
How Much Will I save?)

  • First do a compressed air system audit. Audits often show that you needn't buy a new compressor: leak or load reductions may be enough to meet your demand and save money.

  • When it comes to operating costs, note that most compressors consume more energy than their nameplate rating states. For example, a typical air-cooled, 125 psi 100 hp screw compressor consumes about 90 kW, the equivalent to 120 hp. It is common practice to load compressor motors at higher than their nameplate ratings. (The service factor of a motor is the measure of the continuous overload capacity at which a motor can operate continuously without overload or damage. For example, a motor with a service factor of 120% could be run continuously at 120% of its nameplate load.) For more information on calculating costs, read Compressed Air System Economics and Selling Projects to Management in the CAC Sourcebook.

  • Be sure that the compressor you buy has an operating pressure rating matching your system. If you combine a lower-pressure-rated compressor with higher pressure compressors, the lower pressure one may operate in the less-efficient modulating mode if it can operate successfully at all.

  • Buy a compressor to meet your peak load, with an added margin of 20 to 30 percent for growth. Choosing grossly oversized compressors, can increase operating costs, especially when compressors must run in modulation or load/unload modes.

  • Consider how compressor size affects reliability. Compressors occasionally fail, and when a large stand-alone compressor fails, production losses can be high. A well-designed system has automatic-start backup capacity compensating for the temporary loss of the largest compressor. Two smaller compressors, rather than a single large one, increase reliability, with the added benefit that smaller compressors often cost less to operate.

  • When buying compressors to operate in load/unload, Variable Speed Drive (VSD), or start/stop mode, the compressors must be connected to appropriately-sized storage receiver tanks or the expected savings will not materialise. The old rule of thumb of two gallons of storage for each cubic feet per minute (cfm) of output formerly used by manufacturers has been proven inadequate for good compressor control. For more information, read Compressed Air Storage and Compressed Air System Controls in the CAC Sourcebook.

  • With electronic pressure controls, you can set up your system pressure easily and accurately. When buying such controls, make sure that the compressor can automatically unload and shut down when not needed, rather than run unloaded for a long time. To maximise efficiency, ensure that the controls and compressors you use are designed to work well with each other and not stand alone.

  • Choosing the right cooling medium to cool compressors can greatly lower operating costs. A once-through water cooling system can cost much in water and sewer charges. Air-cooled compressors use ambient air, but you must install them away from dust, and in locations where ambient conditions are cool enough to cool the compressor, yet not drop below freezing. To cut operating costs, consider recovering compression heat. (See the Heat Recovery in Operation and Maintenance Tips.)

  • Don't buy a high pressure compressor to supply low pressure applications. Avoid regulating 100 psi air down to 30 psi or lower. Low-pressure applications can often be supplied by separate low-pressure sources.

  • Consider buying an efficient air dryer. For information, read
    the Air Dryer section in Operation and Maintenance Tips

  • In some provinces, local power utilities offer financial incentives for energy-efficiency strategies, including air compressors.

Next: Operation and Maintenance Tips