In this Ask the Experts feature, which runs regularly in Plant Services, expert instructors from the Compressed Air Challenge (CAC) tackle questions on compressed air systems and associated technology.

The Compressed Air Challenge, an educational foundation administered by the Compressed Air & Gas Institute,  is a voluntary collaboration of industrial end-users; manufacturers, distributors and their associations; trade organizations; consultants; state research and development agencies; energy efficiency organizations; and utilities. The CAC has one purpose in mind—helping facilities enjoy the benefits of improved compressed air system performance.

Question: We repaired our compressed air leaks. Why didn’t our compressor electricity use go down?

Improve compressor turndown to capture energy savings

Ron Marshall, Winnipeg MB:

One common problem is overestimating the leak flow during a leak audit. Tools like leak detector guns and acoustic imagers often have convenient lookup tables or automatic estimating calculators to help size the flow of each leak, but these commonly show higher than actual flows. Let’s face it, when we do a leak audit, we want some positive results, so we tend to believe the higher number, but if it is not accurate then the savings will be lower.

Fixing compressed air leaks reduces the amount of air your system consumes, but it doesn't always reduce electricity consumption. The reason is often poor compressed air system turndown. Turndown is the ability of your compressors to reduce power consumption as air demand decreases. Some compressors continue to use significant power even when producing less air. If your compressors aren't properly controlled, repairing leaks may simply cause them to spend more time running unloaded rather than saving electricity.

The solution is to make sure your compressor system can turn down efficiently as demand decreases. This may require adjusting compressor controls, improving sequencing, shutting down an unnecessary compressor, or even purchasing a compressor that can more effectively turn down. The Compressed Air Challenge (CAC) Fundamentals and Advanced training courses explain how compressor controls work and how to turn reduced air demand into real energy savings. Remember, fixing leaks reduces air demand, but effective system turndown delivers the electricity savings.

Compare compressor run times, airflow and production levels

Frank Moskowitz, Phoenix, AZ:

Many facilities repair compressed air leaks and expect to see an immediate drop in compressor electricity consumption, but that does not always happen. In some cases, the compressor was already operating at its minimum power level for much of the day, so reducing air demand does not significantly change electrical usage. It may even cause the compressor to go into blow off if it is a centrifugal design. Other factors, such as system pressure settings, compressor control strategies, artificial demand caused by excessive pressure, or additional equipment brought online after the repairs, can offset the savings from leak reduction.

Another common reason is that compressed air demand can fluctuate significantly with production activities. While leak repairs reduce wasted air, the compressor may continue running for the same number of hours due to increased production, poor sequencing between multiple compressors, or storage receiver and distribution system inefficiencies. To verify the benefits of a leak repair program, it is important to compare not only electricity consumption but also compressor run times, loaded versus unloaded operation, airflow measurements, and production levels before and after the repairs. A detailed performance analysis often reveals that the energy savings are present but are being masked by other operational changes within the facility.

Optimize compressor controls to match demand

Paul Maguire, Natick, MA:

A common reason, highlighted by the Compressed Air Challenge, is that fixing leaks does not always cause a compressor to use less electricity if the compressor is still running in the same operating mode. Many industrial compressors are controlled to maintain a pressure band and may continue to idle, unload or modulate even after air demand is reduced. In those cases, the plant is wasting less compressed air, but the compressor is still drawing nearly the same amount of power. The savings are real, they just may not show up as a large drop in the electric bill until compressor controls, sequencing or operating pressures are optimized.

For example, imagine a manufacturing plant with a 100 HP compressor that had several leaks repaired during a shutdown. Before the repairs, the compressor ran loaded much of the day. After the leaks were fixed, air demand dropped, but the compressor continued operating in unload mode between production cycles. The system became more efficient and the leaks were eliminated, yet the electricity meter showed little change because the compressor was still running and consuming significant power while unloaded. This is why the Compressed Air Challenge recommends looking beyond leak repair alone and evaluating compressor controls, system pressure and sequencing to capture the full energy savings from a leak reduction project.

Adjust compressor sequencing and pressure bands to improve efficiency

Ross Orr, Colchester, CT:

I have an example where that exact thing happened. This was a multi compressor system, all units had variable frequency drives (VFDs) and were all connected to a factory sequencer. We identified and repaired a number of leaks and saw a reduced demand on the flow meters; however, the system efficiency dropped (kW input/measured flow).

Although one compressor could have been shut down with the reduced demand, the controller not only kept the same number of compressors operating but now they were operating at a less efficient operating point. This was confirmed by looking at the CAGI data sheets we had.

Our solution was to remove two units from the sequencer and base load them (these particular models were more efficient with a heavier load), then make adjustments to the sequencer and the pressure bands that now increased the efficiency enough to make a real difference.   

Find hidden energy costs caused by small cumulative leaks

Paul Shaw, Middletown, CT:

Here is my experience.  A large manufacturer with numerous assembly lines, hoses, vibrators and pneumatic tools initiated a comprehensive leak detection and repair project across its compressed air system. The facility operates more than 200 small machines, along with many additional tools and workstations.

During the assessment, it was discovered that nearly every production line had small air leaks caused by mismatched metric tubing and fittings connected to SAE fittings at the main header & sub-headers. While each individual leak was minor, the cumulative impact across hundreds of machines, combined with leaking hoses and worn pneumatic vibrators, resulted in air losses approaching 50% of the system’s total flow.

The projected savings from eliminating these leaks exceeded 300 cfm, equivalent to more than 63 kW of power and approximately $45,000 annually at an electricity rate of $0.12/kWh. Based on these estimates, the project was expected to achieve a six-month payback.

After the repairs were completed, the system was data logged again. Surprisingly, the measured savings were less than half of what had been anticipated, prompting a deeper investigation.

Applying the Compressed Air Challenge’s systems approach, the assessor re-evaluated both the system and the operating data. The analysis revealed that insufficient storage capacity was causing a second compressor to start unnecessarily during periods of sudden demand, increasing operating costs.

In addition, because pressure drop across system components varies with the square of the flow rate, the reduction in system flow increased the average pressure in the main air header. As a result, many point-of-use regulators that had previously been adjusted upward to compensate for pressure drop were now operating at pressures significantly higher than required, causing the connected loads to consume more flow.

Do you want help in finding out what to do to reduce your system’s operating costs? Compressed Air Challenge’s Fundamentals and Advanced training can help you formulate a plan. More information at the Compressed Air Challenge website.

The contact information of the featured experts in this article can be found on the Instructor Website.