Fundamental Steps for Reducing Energy, Operating Cost, and Improving Performance in a Compressed Air System

by: Scot Foss, Plant Air Technology

The following are the required steps of action that should be taken to identify and quantify the constituents of demand including waste and inappropriate usage, current energy, and the annual operating cost per condition. In taking these steps, you will also significantly reduce the operating cost of the system by 25-35% of the current condition. The steps are prioritized in the correct order of sequence. Please keep in mind that this is a simple, straightforward approach. It is the author’s intent that these actions can be taken with a minimum cost and optimum result. Large systems with multiple compressor rooms, multiple entry points, multiple pressures, or complex compressors and controls will require special attention or the assistance of a professional, experienced auditor.

1.   Learn how to get the compressors into a base load & trim condition at the desired supply pressure. Understand cascading different types and control arrangements to trim down more than one compressor. It is highly unlikely that you will be successful getting the system to operate this way with any of the compressors in modulation. In the event of centrifugals, you will want to fully load the base compressors on the natural curve and modulate the last compressor. Stagger the set points on the last two centrifugals assuming the turn down will be equal to or less than the throttling capability of the two units. If you have multiple compressor rooms, base load one room and then follow the above instructions to trim in the other room. Signals may be a complication. You will likely shut off a compressor in the exercise at least a good deal of the time.

2.    Determine the demand based on the compressor loading in the no load, low load, and normal operating loads. You will want to get some diversity of top to bottom in each condition. Measure the energy and determine the cost at the various conditions including dryers, pumps, fans, etc. Apply the hours of service for each condition against the electrical cost. Distribute maintenance, operators, inspection, depreciation, inventory, rental, and water costs proportionate to time per condition

3.   While doing the low load test, walk the production floor and check to see what is on that may also be on during the low and normal production loads. Estimate this volume and subtract it from the total low load. The balance is the leak load. Make sure that the valves and solenoids are open on the production machinery with the equipment off and reevaluate the low load. The difference should be added to the low leak load during production hours. You now have the leak load for the various conditions. You can also determine how much of the leaks is in the piping and how much is on the production machinery.

4.   While you are in the plant, check the critical pressure applications and other regulated samples and determine if the regulators are adjusted to the supply pressure (not necessarily seated). Between leaks, no regulator applications, and tracking or max set regulator applications; estimate the percentage of total demand, which is not regulated. Ratio the absolute pressures or weights between the current average supply pressure and the lowest that you think that you can operate at after retrofitting the point of use applications to reduce the differential pressure and high rate of change applications that cause the pressure to drop in the system. These are cause and effect applications. The difference in the ratio between where you are at current and where you can operate at with the demand improvements is expressed as a percentage. Multiply the percentage times the total demand, which is not regulated. This will produce the artificial demand in volume.  You have now quantified the well applied production, leaks, and artificial demand in low load. Do the same for the other conditions. This is the beginning of quantifying the constituents of demand. Since you know the cost of air for each condition on an hourly basis, you can apply cost to these individual issues by condition.

5.   During low and normal production identify inappropriate applications for air such as open blowing, aspiration, cooling, etc. Quantify them and break them out from the production volume in the constituents of demand. We have now separated the things we can change from the unchangeables in production.

6.   Install a master/local signal header and learn how to use it. Adjust the compressors to the desired demand pressure. The differential pressure across the clean up equipment and aftercoolers will be absorbed in the drive energy at .5% per 1 psig compared to having the system drop a proportional amount of the delta pressure. Make sure you adjust the compressors for the demand pressure required, not the previous supply pressure.

7.   Learn the five concepts of storage and how to measure negative rate of change events in the system. Install the desired storage required to handle the demand events at an acceptable pressure drop. Adjust the trim portion of the supply to create the required control storage so that you can operate at the lowest pressure possible and only drop to the minimum demand pressure when handling events with control storage. Minimize the differential pressure and optimize the capacity of control storage to keep the base power down. Make sure that the storage is sufficient to handle a compressor failure and the time and allowable pressure decay to get the back up compressor up to full load without dropping below the minimum acceptable demand pressure. This will assure that the back up compressor does not turn on inadvertently. Note the change in power. 

8.   Install a demand controller and make sure that the pressure drop is minimal (less than 2-3 psig at full flow). Adjust it to the minimum demand pressure plus the control variance (should not be more than 1-2 psig at worst). You may have to make a supply pressure control adjustment to correct so that you have sufficient control storage without tracking the controller on supply during your largest event.

9.   Use an ultrasonic leak detector for leak benchmarking and identification. First pass, tag enough leaks to reduce the leak load by 50%. Tag them by color code base on large and medium leaks. Fix the leaks and make sure that an appropriate amount of supply energy is reduced. You may have to review the supply profile and readjust the set points. Review the types of leaks you found and identify what changes in supply items such hose, hose fittings, assembly techniques, push lock fittings, tube, disconnects, filters, regulators, and thread compounds need to be made to minimize more similar leaks in the future. Establish the first pass benchmark and revisit the value every couple of weeks and bring the low load value back to benchmark. When you get comfortable with the level, lower its smaller percentage than the last time as it will be more difficult to maintain.

10.  Apply low pressure positive displacement blower at all open blowing, sparging, and cooling applications where possible. Replace the balance of open blowing with low volume, high thrust nozzles 

11.  Apply all checked and metered, dedicated storage to all points of use which have higher volume intermittent usage. Adjust metering to flatten the load as seen by the supply 

12. Check the demand controller for accuracy and sensitivity. Lower the unit if possible. Check the supply system again and adjust set points and the system’s profile as needed.

13. Benchmark the demand flow for the various use conditions making sure that there is no significant increase. In the event that an increase occurs over previous benchmarks. Review the demand analysis procedures from the past and take corrective actions where needed.

Even a fumbling attempt at this approach will net a substantial reduction in total cost and energy. There are many areas that have not been explored in improving the system because of their complexity in diagnosis and solution including coincidental demand, load shaping, a variety of multiple compressor controls, information management, and unit interface.

Reprinted with permission from R. Scot Foss, president of Plant Air Technology, Charlotte, N.C., a company specializing in system auditing and design.  This article is based on his book, "Compressed Air System Solution."  A portion of the proceeds from sales of the book is donated to children’s charities.  To order a copy of the book, please contact Southern Corporation.