All I Really Need to Know I Learned from a Compressed Air Audit Chart

By: Neil Mehltretter

We’ve done numerous blog entries about the benefits of compressed air audits and leak detection.  We’ve talked about how they can baseline your system, build a demand profile, and be an invaluable tool for optimization projects. But, we haven’t discussed how to read an actual chart that you might get back from a reputable compressed air auditor. At first glance, the charts can be a bit intimidating, but if you know what to look for, you can learn a lot about your system. Being able to understand the charts will also put you in a better position to explain and justify any capital expenditures that might be needed when presenting the plan to upper management. Here are the basics of reading a compressed air audit chart.

We’re going to break-down an actual compressed air audit chart that we compiled for one of our customers using data collected over a 9-day period. But, before we look at that, it’s important to understand the total capacity of the system. This is the total amount of air (cfm) that the system is capable of producing. You would get this by adding the capacities of each compressor together. For the system we are going to talk about, the total capacity is 740 cfm. We know this piece of information, because prior to doing the compressed air audit, we conducted a thorough walk-through of the plant and noted all of the equipment, piping, controls, and other items of interest. To understand the chart fully, you’ll want to know your system’s capacity, so make sure you know how much air you’ve really got. Add up the capacities of all the compressors—even that old one in the dark corner, tucked away that everyone likes to forget about.


So, take that number you got for your total system capacity, mark it on the chart, and draw a line all the way across. Now look at the demand lines for each day. There’s probably a bit of overlap, but there should be a fairly clear peak. Mark the very highest point. This is the maximum plant demand.

Take a look at the distance between the total capacity and the maximum plant demand. This will help you see if your system is properly sized. In this case, there’s an additional 360 cfm available in the system (1), making it very oversized. System oversizing can cause compressor cycling, pressure drop, unnecessary wear and tear, increased moisture, and higher maintenance costs. When a system is as oversized as this one, there’s significant opportunity for savings by switching to smaller capacity compressors. While each case is unique, we’ve seen it time and time again where the simple payback was around a year for downsizing—or rather—“right-sizing” the compressed air system.

Next, mark where the lines all tend to “flat line”.  The difference between this point and the maximum plant demand is the actual production demand (2)—the amount of air needed to supply the production processes.

Mark off when production is online. For this chart, they had shifts running from about 5:00 am to 4:00 pm. Now, how far away is the minimum production demand value from 0 cfm on the chart? This can show non-productive loads, such as leaks (3). For this customer, they were wasting an additional 170 cfm to leaks—which is the equivalent of running a 40 hp compressor. This was costing them $20,000 annually in electricity, not to mention the maintenance on the compressor.

To sum it up for this particular chart:

1. They were using only half of the air they had available.
2. Peak production was under 30% of the available system capacity.
3. Leaks, artificial demand, and other non-productive loads made up the most of the air consumption.

If you approach your compressed air audit charts looking for these main items, you will catch key areas for improvement and/or places where your system is doing well. Finer points, such as control schemes, may not be as apparent at first glance, but those should show up in a thorough compressed air audit report. That was the case with this customer. The single, 150 hp compressor was running in modulation control. So in addition to being significantly oversized which already provided some hefty savings, there was substantially more savings due to this oversized compressor running in this inefficient control type.

So, take the time to take a look. With a little bit of practice, you’ll be reading between the graph lines and finding areas for optimization before you know it.


4 thoughts on “All I Really Need to Know I Learned from a Compressed Air Audit Chart

  1. This should help a lot of people understand the Air Demand Analysis report…good job! One area that you might want to expand upon is the total capacity of the system. If there is a back-up compressor in the system, then it may cause the total system capacity to appear significantly oversized relative to demand. If you deduct the back-up unit’s capacity from the total and compare the resulting capacity to demand, you get a better idea of whether the system is too oversized or not. Of course, if that back-up compressor is required to run to meet occasional peak demand, then it is not really a “back-up” compressor. A back-up compressor should only run when one of the other compressors in the system is down for service or repair.

    1. You make a great point about “back-up” compressors. We find that when a back-up compressor is purported to “never run” they seem to always run during an audit. We’ve visited sites where back-up compressors were locked out and tagged out during the installation and sometimes we’ve even seen major components from the compressor removed, only to return after the test and find that this compressor was up and running during the test. This skews the data and provides an incorrect picture of system operation. It’s like visiting the doctor’s office and “forgetting” to mention that nagging problem. If you need more air constantly, make sure that you include those back-up compressors or even those dedicated compressors for specific points of use in your audit. It could make all the difference.

      One last point about back-up compressors, after you’ve completed your audit, you should give serious consideration to purchasing a dedicated back-up compressor. Often times budget constraints reduce project size and back-up compressors are relegated to future purchases. Consider that for most plants, the cost of downtime is significant – maybe even thousands of dollars per hour. This lost opportunity for production can easily be offset by adding that back-up compressor. Plus, with the advent of a system master controller and a properly designed system (such as from an air audit), any back-up compressor gets the necessary run time to ensure that it will be ready and able to support the demand when it’s called on.

  2. It would be good to take into account the “efficiency” of the installed compressors while calculating the TOTAL CAPACITY. More often than not, compressors lose efficiency over time. While this is significant in case of pistons, it is not unusual to find oil injected screws to lose some efficiency over time.

    1. If there is any doubt about the condition of the compressor in regards to its air flow capacity, a flowmeter/ flow measurement is recommended. In our experience, this is certainly a concern with piston compressors and some oil-free rotaries with Teflon coating, but not very likely at all on oil-flooded rotaries, as the lubrication prevents metal-to-metal contact.

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