Energy Savings in Compressed Air Systems
Evaluating Compressor Efficiency
Waste Heat Recovery and the Importance of Maintenance
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Achieve Significant Savings Through Improved Energy Management
Cost-justifying More Efficient Compressors
Justify a more energy-efficient compressed air system.
Look beyond the purchase price.
     As indicated previously, over a 10 year period, the electrical power costs of a compressed air system impact the bottom-line far beyond initial price and maintenance. How can we evaluate the true impact each element of the compressed air system has on the electrical power costs? 
The air compressor package. 
         The air compressor package has a very significant impact on overall operating power costs. There are other pieces of equipment in a compressed air system that support the air compressor. These also impact on power costs, both directly and indirectly. But the core of it all is the air compressor package. Energy wasted at the air compressor site can never be recovered.
What are the components of an energy efficient compressor?
  • Compressor element (airend) - performance can vary up to 20% depending on what airend size and style are used.
  • Drive motor efficiency - there are high efficiency motors available which can save up to 5% in power consumption. Referring to the previous example, a 100 hp compressor would save $3,750 annually.
  • Compressor controls - are an important part of the air compressor package, matching compressor supply to demand. As outlined previously, the right control type (dual control) is essential for efficient operation. Maximum savings of 45% are possible. Any reduction in air usage in a system accomplished by good demand side management (adequate storage and flow controller) can be translated into real power cost savings by the control system, and none does this better than dual control.
State-of-the-art control systems generate large savings.
Sequencing compressors/controls.
     A microprocessor-integrated sequencer allows the system to maintain a stable system pressure and ensures that only needed compressor units are operating at their most efficient level.
     User-friendly, PLC-based sequencers can mix and match compressor supply to demand, including automatically shutting off units not needed, and bringing on backup units as required.
     Sophisticated sequencers not only sequence and select units as required, but ensure that no more than one unit in a multiple-unit installation will be operating at inefficient part load. All other units will be operating efficiently at full load.
     Electrical power savings result from operating fewer compressors at a lower pressure than with conventionally controlled compressors.
Efficiency Rules of Thumb
? Most air compressors deliver 4 to 5 cfm per hp at 100 psig discharge pressure. 

? Every 2 psig of pressure increases or decreases the power draw of an air compressor 1%. 

? Every 10 degrees Fahrenheit change in inlet air temperature affects the efficiency about 1%. Colder temperature increases and warmer temperature decreases efficiency. 

? Power cost for 1 hp for 3 shifts, 7 days a week (8,760 hours) at 10 cents/kWh = about $750/year. 

? A 50 hp compressor rejects approximately 126,000 Btu per hour. Approximately 119,000 Btu/hr of this is recoverable. 

? Size control air receiver located after compressor for about 1 gallon capacity per cfm of compressor capacity. 

? Size storage air receiver for about 2-4 gallon capacity per cfm of compressor capacity. This results in an effective demand side control management system. 

? Total pressure drop across all compressed air system components, including piping, should not exceed 15 psi.

Reduce pressure drop in system components.
     When buying or replacing equipment, make sure it maintains low pressure drop over its entire service life. Also, ensure that filters and dryer are sized and maintained properly. The total pressure drop across all compressed air system components, including piping, should not exceed 15 psi.
Evaluating Compressor Efficiency / Waste Heat Recovery and the Importance of Maintenance 
© Copyright 1998 Maintenance Resources, Inc.
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