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Air Compressor Energy Savings: Finding the Low-Hanging Fruit

Jonathan B. Maxwell, PE, Energy Resource Solutions, Inc.

A typical 300-hp air compressor costs about $100,000 per year to operate1. At this level of expense it makes sense to invest some time and thought into making sure this “silent” energy hog runs as efficiently as possible.

This white paper reviews some of the many ways to measure compressed air system performance and identify savings opportunities. It also notes costs and some funding sources that readers may find helpful to offset some of them.

Monitor LeaksLeaks commonly constitute 25% of total compressed air use. If you’re running a three-compressor plant and think you’re on the verge of needing to buy another, hunt down your leaks first. You might be able to avoid a $50,000 capital expense, not to mention the associated project headaches. Walking the lines and listening for leaks is the only way to find individual cracks.

Leaks are among the lowest of the low-hanging fruit on the energy savings tree. Payback time typically is measured in weeks or months instead of years, and repairing them doesn’t require capital funding. That’s easy picking. To flog this metaphor to death, I will add that one must perform this harvest every season for maximum system health.

Ultrasonic leak detectors are the essential tool for the job. They concentrate your listening ability by focusing on the sound frequencies that compressed air emits. Plants are just too loud. Leak detectors range in cost from $500 to $15,000. We use a mid-level unit that cost about $2,000. Some utility companies and state energy programs lend leak detectors out for free to their customers, which is a great way to try one out and perhaps get some free training. Wisconsin’s Focus on Energy and California’s PG&E Energy Center, for example, both have tool lending libraries that include ultrasonic leak detectors. NSTAR Electric in Massachusetts doesn’t lease the detectors, but runs an award-winning program that co-funds the leak detection study and pays directly for leakage reduction—$8 per cfm eliminated.

If you lack the confidence to do it yourself, call your compressed air system contractor and request that they do

a leak audit with you. Then, follow them around. You’ll find it’s not rocket science. After that you can do it yourself. That way you can pay for your training out of your maintenance budget and not have to leave your site.

If you don’t have the time, then request that leak detection be part of your compressed air service contract. Many progressive compressor contractors have expanded their service options to include energy studies and routine compressed air leak detection.

One procedural tip: Finding leaks is a different exercise than repairing them, so plan on doing a leak detection sweep and tagging them plainly in the first round so they’re easy to re-find. Then do a second sweep to repair.

You’ll also want to make a simple table of where the leaks are while you tag them, and maybe estimate the leak loss rate for each, since that helps justify the effort. There are many different ways to convert sound to cubic feet per minute (cfm). At Energy Resource Solutions (ERS), we have an algorithm, but basically it amounts to louder means more air loss. Table 1 shows a table of leaks we tabulated at a dairy products facility.

Table 1: Air Leak List

At a Raytheon plant in Andover, Massachusetts, ERS worked with staff there to identify over 400 leaks that wasted 750 cfm in our estimation. The leak study was conducted right before the company replaced its old compressor plant.

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1 Energy costs calculations in this paper use the February 2008 national average industrial retail price of $0.064 /kWh.

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By eliminating the leaks, they were able to buy a pair of compressors 100 hp smaller than previously in place. National Grid funded part of the leak detection, repair, and central plant replacement efforts.

Savings on a continuously pressurized system will be $25 to $125 per year for every cfm eliminated, depending on your lead compressor’s part load efficiency. A 1/32-inch diameter hole leaks more than 1 cfm.

Monitor Compressed Air Flow Now, let’s move backwards in the compressed air system from the sprawling distribution system to the main headers and consider monitoring compressed air flow. Admittedly, this is a luxury we don’t see in most systems. It can, however, be invaluable in diagnosing compressor sequencing and loading problems.

Technology is available to measure air flow and the cost is decreasing. Penetrating thermal mass flow meters cost from $400 to $2,000 depending on pipe diameter and can be installed pretty quickly. Such meters require drilling pipe and are intended to be left in place. As our firm ventures out to perform studies at customer facilities we are finding more plants with these devices attached to the main headers, distribution lines, or integrated into flow control systems. Also, temporary portable non-invasive ultrasonic devices that strap on existing pipes are available. They are relatively expensive for occasional use at $8,000 to $10,000, and are sensitive to the particulars of the installation, but they have a role in compressed air studies. GE Sensing will rent them direct for about $1,200 per week.

Figure 1: Simultaneous Flow and Power Measurement Can Reveal Control Problems

The combination of compressor power monitoring with simultaneous flow monitoring can reveal sequencing and part loading instruction problems not otherwise apparent from power monitoring alone. Figure 1 above was taken

from such a site. Coincident measurement revealed that Compressor 1 (of 4) idled for 26 hours per week when there was no demand for air.

This is low-hanging fruit. At most, the cost to correct problems such as this is a new compressed air sequencing system, and at least a matter of adjusting setpoints.

Monitor Compressor PowerShort term electrical monitoring of compressor is the heart of most compressed air energy monitoring systems. The easiest and most common technique is to use battery-powered data loggers. Combining logger electric data with known compressor part load performance curves can provide good estimates of air use and often is the fastest way to determine if system upgrades are economical.

Over the last ten years, the cost of data logging hardware has gone down and their associated software packages have improved immensely, reducing the metering cost per point. This allows engineers to lower costs and meter more equipment and parameters at each site. ERS now routinely monitors all compressors instead of just the one(s) that reportedly modulate. Also, the migration from dial-up remote data access to the latest web-based mediums has made it easier to conduct longer-term metering.

The two biggest decisions to make regarding compressor electrical measurement are what to measure (current or power), and how frequently to measure it (every 1 second, 1 minute, 15 minutes, or 1 hour).

Power vs. Current Both true rms real power logging and current logging with power estimation have their place. Real power monitoring is ideal, but is not always practical. There are cost constraints and sometimes physical limitations where a panel simply will not hold three CTs and a logger, or when there is limited suitcase space.

When monitoring relatively constant loads, a spot real power or power factor measurement combined with current logging can be a very cost-effective approach. Battery-powered loggers such as the Onset HOBO® U12 data logger can cost less than $200 including a split core current transformer (CT). The accompanying HOBOware® software is so user-friendly we sometimes import data from other meters and use the software to study it.

Even when load varies, current logging can work in many applications. The key is properly accounting for voltage and power factor. Voltage is straightforward and relatively

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constant for a piece of equipment, so spot measurements will suffice.

Power factor, on the other hand, varies with motor size and loading. The good news is that the power factor versus current relationship is pretty predictable. ERS has developed algorithms that estimate real power based on logged current, observation of motor nameplate data and spot voltage measurement, and that accounts for motor part load power factor effects. Figure 2 illustrates the relationship. If using the below chart, be sure to adjust the nameplate full load amps for the actual compared to nameplate voltage.

Figure 2: Estimating kW When Measuring Current

Real power devices are bulkier and more expensive because they require three CTs, three voltage clips, and more memory, but are the approach of choice when uncertainty must be minimized or when there is concern that power factor is unpredictable. Real power loggers such as the HOBO Energy Logger Pro™ start at about $1,000. As the cost of real power meters decreases and the equipment becomes smaller and more portable, direct real power is increasing.

Measurement IntervalThe type of compressor controls has a major impact on the proper measurement interval.

For three types of screw compressor controls—inlet throttle, variable frequency drive (VFD) modulation, and oil-free load-unload type compressors—longer measurement intervals alone are typically fine. The part load performance curves for three types of controls are linear. This means that simply averaging air and energy use over a longer period of time will give the same result as if air and energy use was estimated over many shorter intervals with the same total period. For such compressors, setting the interval at one or two minutes, or even five

minutes, and logging data for a month is fine.

That approach is inappropriate for other types of controls. Many screw compressors are oil-flooded and cycle between loaded and unloaded state to meet part load. For these compressors, not only is the average power important, but also the duration and characteristic pattern of the operating cycle. The cycle time can be as short as 15 seconds or as long as 5 minutes. The wasteful unloading and reloading portions of the cycle can be 5% or 50% of the total operating time. Those differences have a critical effect on assessment of the system efficiency and air load and on the optimal cut-in and cut-out pressures.

For example, consider three different compressors in a hypothetical example. One has throttle control, and two have load-unload control. One of the unloading systems has a small receiver; the other large. All run at the same average power. The graph on the left in Figure 3 below shows average power over five-minute intervals for the three systems. It is unenlightening. High speed data logging in the graph on the right in Figure 3 reveals very different patterns. The throttle is unchanged and averages 19% output capacity. By calculating the percentage of time that the compressor runs at over 90% power, we find that the larger receiver system produces about 58% of capacity and the smaller receiver system produces 50% capacity. That’s significantly more cfm/kW and illustrates why large storage capacity is such a good idea.

Figure 3: Short and Long Metering Intervals

Without high speed logging, the opportunity to upgrade the smaller receiver system to large might not have been revealed.

In a recent real project, ERS used two Onset HOBO U12 data loggers to log the current on a screw air compressor. One of the loggers had a fast sample rate—two seconds. The logger filled up in a day. At the same time, we installed a second logger and collected data in five minute intervals over two weeks. This showed us the overall load over time.

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Together, the data loggers gave us a better understanding of the compressor’s performance and we could accurately estimate existing energy use and forecast savings potential. In this particular case, the high speed logging helped us identify a sequencing problem, illustrated in Figure 4 below. Savings will result from changing the sequencing controls to stop two compressors from alternating air production. It is unlikely that we would have discovered the problem solely with 15-minute interval logging.

Figure 4:Compressor Sequencing Pattern Problem

Financial Assistance While it is impossible to cover all the different programs that fund compressed air upgrades around the country, it is worth pointing out a few of the more prominent ones in addition to those mentioned previously. On the west coast, Tacoma, Washington has a program designed specifically for compressed air studies and upgrades. PG&E customers can participate in the “AIM” compressed air program. Oregon manufacturers can take advantage of both state Business Energy Tax Credits and the Energy Trust’s general Production Efficiency program. Seattle City Light also has a general industrial program that can fund compressed air projects. In California, Lockheed Martin runs industrial efficiency programs for PG&E and SCE that fund compressed air system upgrades.

On the east coast, virtually any manufacturer in New England or the mid-Atlantic is eligible for funding for compressed air upgrades. Check with either your utility company or state efficiency program. ERS manages Efficiency Maine and approves dozens of compressed air system upgrade projects each year. Efficiency Vermont is similar and NYSERDA also has a flexible industrial program that is growing in New York. Progress Energy and Duke Power in North Carolina are rolling out similar general programs. NSTAR offers compressed air-targeted assistance.

Centrally, Xcel Energy in Colorado, the Dakota Electric Association, and Reliant Energy in Houston all offer compressed air-focused programs, the last as a subset of a retrocommissioning program. The large Texas IOUs, Austin Energy, Wisconsin Focus on Energy, and Commonwealth Edison fund customizable energy savings programs that can include compressed air. AmerenUE in Missouri is expected to roll out a custom program in 2009.

SummaryWhen ERS staff first started performing energy audits, the average cost-effective cost savings we were able to identify averaged between 10 to 20 percent of total use. After almost 20 years of customers performing upgrades, market transformation, code changes, and the like, when we do energy audits today, we still find between 10 and 20 percent savings potential. It’s not because nothing has changed in these plants. It has.

Management has been investing in efficiency. But new technologies, lower equipment costs, and increasing energy rates means there is plenty of low-hanging fruit. That’s exciting and bodes well for the future of American industry.

Jon Maxwell is Director of Engineering at ERS, an energy consulting firm based in Haverhill, MA. Mr. Maxwell works out of Texas. He is a mechanical engineer and has been helping large commercial and industrial customers save energy for 18 years. To learn more about ERS, please visit www.ers-inc.com.

About Onset Onset Computer Corporation has been producing small, inexpensive, battery-powered data loggers and embedded controllers since 1981, and has sold over one million loggers that are used around the world by over 50,000 customers. The company manufactures a broad range of data logger and weather station products that are used to measure temperature, humidity, light intensity, voltage, and a broad range of other parameters. Onset products are used widely in research, commercial, industrial, and educational applications.

Onset Computer Corporationhttp://www.onsetcomp.com(800) 564-4377 / (508) 759-9500Fax: (508) 759-9100sales@onsetcomp.com

Copyright © 2008 Onset Computer Corporation. All rights reserved. Onset and HOBO are registered trademarks of Onset Computer Corporation.

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