Compressed air: 7 common pitfalls in system design

I’ve seen a lot of pictures of brand-new compressed air installations proudly displayed on compressor-related social media sites by equipment suppliers. In looking at these through the eyes of an experienced energy-efficiency auditor, I am often quite disappointed, knowing the systems will provide adequate compressed air volume and quality but at higher energy cost to the customer than necessary.  I often wonder whether the equipment suppliers have tried to offer the more-efficient choices to the end user.

Of course, the blame often rests with the end user, who asks for and accepts the lowest-cost bid, not knowing that the purchase of a better compressor, dryer, filters and piping will reduce the operating costs for years to come and more than pay for any additional costs in a year or so. Energy is important to the life-cycle cost of the compressor and by far the most costly input – energy costs will greatly exceed the initial purchase cost of the compressor.

Here are some common pitfalls in replacing compressors:

  1. Reciprocating to screw conversion – When converting from a reciprocating compressor to a screw compressor, the customer will gain many benefits, but one of them is not usually energy efficiency, especially if a fixed-speed compressor is purchased and runs at low-duty cycle. Many customers are surprised to learn their new screw is consuming twice the previous energy consumed because of lack of storage receiver capacity and improper pressure set points.
  2. Fixed-speed instead of variable – Fixed-speed compressors are usually more efficient than screw compressors at full load, but if running at part loads, then the opposite is true, especially if the system has only one compressor. Most systems run at part loads for a significant length of time. Variable-speed-drive-controlled compressors, when installed in appropriate locations, will quickly pay for any additional cost in locations with variable-shift-oriented loads.
  3. Lack of storage capacity – Adding appropriate storage increases the installation cost, but the added capacity makes the system compressors run better. A receiver size of between 5 and 10 gallons of storage capacity for each cfm trim compressor output helps make the compressor much more efficient. Yes, I know that’s big, but so is the compressor power bill, if you add it up. And don’t forget to add a wet receiver before the dryer.
  4. Poor-efficiency dryers – Modern, energy-efficient air dryers have energy controls that can help save power over standard noncycling and fixed-cycle desiccant dryers. Consider cycling or dew-point-controlled dryers.
  5. Piping too small – Many installations use the same size piping in the compressor room as for the discharge of the compressor or will downsize if aluminum pipe is used. This leads to higher velocities in the pipeline, especially at direction changes, and excessive pressure loss. This loss causes the compressor to be less efficient and reduces the effective storage receiver capacity.
  6. Too many filters – Avoid overfiltering your compressed air. If most of your load is general-duty, with the odd end use that needs higher air quality, consider installing the extra filtering at the end use only. Too many filters adds pressure differential and reduces system efficiency.
  7. Pressure too high – If your system is running at greater than 100 psi, then you are running at less-than-optimum efficiency. Question why your system pressure is so high; often it is set by the compressor supplier. If it's an internal requirement for some reason, study your loads and find the cause of excess pressure requirements. Often it will be one or two items, such as low-cost connectors or undersized regulators that can be optimized. Then the compressor discharge pressure can be reduced.

Ron Marshall is a compressed air energy efficiency expert and a compressed air trainer with website at www.compressedairaudit.com

Join Ron at the Compressed Air Efficiency LinkedIn discussion group at https://www.linkedin.com/groups/12065406