Compressed Air System Assessments


Energy costs going up? We all know they are! What are you going to do? You already optimized your lighting and HVAC, but where do you go next to lower energy costs? How about compressed air? You have heard that“big hits” in energy reduction with quick pay backs are available, but where do you go to get a compressed air energy savings program or audit, and what exactly does the audit do? What is the scope of supply to ask for?

The perception that great savings opportunities exist in every compressed air system with rapid payback is real. In a recent tabulation of our suggested compressed air energy reduction programs that were implemented at 80 various plants, there was an average 30% reduction in electrical costs. These projects had a simple payback of less than one year. As energy costs rise, the cost of compressed air usually exceeds 30% to 50% of the total electrical energy cost. This equals a range of 10% to 15% of the total electrical energy cost. This is usually accomplished with less than a one-year payback. That is exciting! It is often much more dramatic than that in smaller plants, where the largest electric load may be air compressors.

Compressed air systems represent a unique opportunity for the development of energy savings programs. Compressed air is one of the largest energy users in industrial plants and represents a major savings opportunity. In addition, an energy program targeted on compressed air can also provide a great vehicle to help change the “energy culture” in industrial plants. This can set the stage for additional efficiency projects.

No other end use has the same profile for such significant savings yet being sufficiently complex that plant managers often miss capturing the full potential of energy savings without technical support throughout the design and implementation process.

Where Do You Go to Get a Compressed Air Audit?

There are many sources for compressed air audits, some more effective than others for specific applications. Compressed air is not a high technology issue, but since there is not often a formal degree in “Compressed Air Systems,” it is a technology not fully or widely understood by many. This is particularly true where the total system is involved. Following are some of the more common sources.

1. Compressed Air Equipment Manufacturers

Many compressed air equipment manufacturers, particularly air compressor manufacturers, have “in-house” — developed audit teams or groups. Most of these personnel are well trained in basic compressed air theory as applied on the supply side, specifically with their specific type and brand of equipment.

Compressed air systems are divided into two completely different and separate areas, which are interdependent and must operate together. Supply side includes all compressors, dryers, receivers, and filters inside the compressor room. Demand side is the rest of the production and compressed air-consuming end uses in the site.


    • 60% to 75% of the economic savings are projects in the demand side—improvements that not only save money but also often improve quality and productivity. Capital equipment knowledge and experiences does not always produce results on the air-using side.
    • The major equipment selection and available options will usually come only from the manufacturer’s lineup of equipment.

2. Compressed Air Dealers and Distributors

Various compressed air equipment sales and service organizations offer partial or full audit services. Their personnel may be very well trained in their brands of equipment and others. Familiarity of the demand-side issues would depend on their breadth of services and experiences. Like many other issues, the abilities of these groups should be evaluated on a case-by-case basis.


    • The major equipment selection and available options will usually only come from their available lineup of suppliers.
    • The commitment to full measured audits with trained personnel and instrumentation, etc. often is limited. Their sales and service personnel often have other priorities. When experience and proper measurement equipment is called for, some distributors are aligned with professional independent compressed air consultant organizations to work with them and their clients.

Compressed air dealers, distributors, and/or service organizations may only have limited experience with equipment lines other than the ones they are selling. In addition, those that sell and service major equipment such as compressors and dryers are not often deeply involved with the overall air system and familiar with using process equipment such as regulators, cylinders, positioners, dust collectors, transporters, vacuum systems, cabinet cooling, aeration, mixing, etc.

3. General Energy Management Companies

These firms offer energy services in a large range of areas, such as HVAC, power quality, motors, fans, lighting, controls, etc. They often have limited specialized expertise and experience in compressed air. Customers often find that many of the control schemes that have been adapted to their air system by such general energy management companies do not work well with rotary screw or centrifugal compressors. Often these very efficient “generalists”will also align themselves with independent compressed air consultant organizations.

4. Energy Audits Supplied By DOE-Sponsored University Students

University students led by their instructors as part of their training perform these services. Often the reports are accurate about the general specifications and some magnitude of potential. Due to limited specific industry contacts and knowledge, they may fall short, particularly on the demand side. The funding is usually not there for an in-depth analysis and follow-up.

5. Independent Compressed Air Equipment and System Specialists such as Air Power USA

These companies are not aligned with any particular manufacturer or product line. They should have no financial interest in any equipment selection that is recommended or purchased. They are free of any pressure from any manufacturer to sell equipment to help“keep the factory operating.”

To be successful, these companies have a significant comprehensive library of engineering data sheets on older and current compressors, dryers, filters, vacuum systems, venturi systems, cabinet cooling, material conveying, etc. They also enjoy strong relationships with compressed air equipment manufacturers and distributors of all brands, many of which have lasted for decades.

In addition to full compressed air audits, reviews, etc., their services also usually include:

    • Providing project design and management
    • Developing “Best Practices” guidelines
    • Developing and running training workshops
    • Verifying savings for utility and government incentive programs
    • Managing and capturing incentives to help fund compressed air projects
    • Developing corporate air programs for multi-plant clients
    • Implementing training workshops for plant staff in proper air system maintenance and operation and in the proper use of their equipment and systems
    • Offering “Persistence Programs” to help sustain project savings and foster a fundamental change in “energy culture” in the plant, a culture that frequently serves as the major barrier in implementing energy projects

A recent trend has also been to identify and quantify the impacts these air programs have on air quality and plant productivity.

These organizations offer completely independent evaluations and recommendations with no financial interest in any recommended equipment. They have a wide variety of specific compressed air system knowledge and experience. The substantial investment in state-of-the-art instrumentation and continued systems measurement implemented by experienced, trained personnel has helped develop many new high-efficiency system design parameters. Most have significant Fortune 500 company references.

2. What Should a Compressed Air Review Cover?

    • Evaluate the existing air compressors as to installation, general apparent performance and condition, suitability of unloading control system, cooling water, air supply, etc.
    • Relate the opportunities that exist for energy conservation with these unloading controls. Evaluate an appropriate networking air management system, if applicable. Provide a general opinion of alternative types of equipment and controls that may be more preferable and/or more efficient, if any.
    • Evaluate the compressed air treatment equipment (air coolers, dryers, filters, drains, etc.) regarding installation, general apparent condition and performance, suitability for application, general effect on efficiency, and energy costs.
    • Evaluate specifically identified filters, separators, traps, etc. as to potential effectiveness for water, oil, and particulate control. Provide a general opinion of alternative types of equipment that may be more effective and efficient, if any.
    • Evaluate the general piping distribution system and air receiver sizing and placement.
    • Evaluate the potential effectiveness of demand-side control systems, if any, or with subsystems as required, such as feeds to surge demand, off-line storage to cover permissive starts, overall correction of short-term, high-peak demand to a low average usage.
    • Look into specific areas of low pressure and air distribution problems. Evaluate compressed air quality in those areas and the effects of production and quality.
    • Identify the effective electric energy cost of compressed air at the facility — i.e., the cost per cfm, cost per pound of pressure — and translate this to the cost of leaks and otherwise wasted air. Locate, identify, quantify, and assign a cost of loss or recovery to specific examples of compressed air savings opportunities on the demand side:

a. Locate unregulated flows
b. Locate open drains, timer drains, condensate handling
c. Locate blow-offs, venturi generators, air movers for spot cooling
d. Evaluate dust collector operation and effectiveness
e. Locate cabinet cooling, air vibrators, air motors, air pumps
f. Locate misapplied high-pressure air, agitation, aeration, material transport, etc.
g. Evaluate control and use of air-operated diaphragm pumps
h. Evaluate opportunities with air cylinder selection and operation
i. Recommend specific actions and/or equipment to eliminate or minimize “demand-side waste”
j. Identify other areas of energy recovery, heat recovery, motor efficiency improvements, etc.
k. Identify demand side realignment opportunities to minimize the costly effect of “demand changes” and varying rates by time of day
l. Estimate the “cost of implementation” and the “energy cost recovery”
m. Evaluate critical support systems, ventilation, cooling water, electric drive motors, etc.

    • Each of the compressed air audit conservation projects should be listed in the executive overview giving the estimated implementation costs and predicted project savings.
    • A short-term action plan should be created prioritizing the projects, as required.
    • Long-term follow-up continuing operation plans should be outlined.
    • Flow meters, dewpoint monitors, data recorders, kW meters, etc. can be installed, as required. There is usually no additional charge for any measurement and trending equipment. This may require additional time. Service technicians should be available to check out all the machinery as to internal condition, if required. This is usually at an additional option and cost.

Case Study #1

Auditors concerned with their specific product line offerings may miss significant savings opportunities with alternate equipment. For example, an air compressor manufacturer’s audit team reviewed an air system for an aircraft manufacturer. A single large demand of about 4000-scfm for fuselage testing occurred randomly for 20 to 45 minutes during normal production. This was not a controlled action and when the demand hit, the complete system sagged, interrupting the entire plant production. There was a 5000-scfm class back-up 100-psig centrifugal compressor online, set up with “auto hot start” controls. The “permissive start time to full load” requirement for this unit was a little over one minute. The machine could not automatically come online in time to avoid the system collapse, if the testing demand came on with no warning, which was most of the time.

The decision was made to run the backup trim unit (1000-hp class) at idle continuously to avoid the unscheduled production interruptions. This procedure was followed for several years. The measured idle power input was 40% of full load power. This is higher than the normal quoted percentage and could be caused by improper adjustments.

Estimated annual electrical energy operating cost at idle (not including loaded time) at $0.05 per kWh [(1000 hp x .746 x 0.05 x 8760 x .40) / .95 me] equals $137,578 per year. The compressor manufacturer performed the system audit and concluded, “…the compressor power required during the testing is legitimate and needs to be supplied when testing is underway.”

The proposed solution, which will work, was to install a 250-psig storage vessel fed by a smaller (100-hp) compressor set up on auto start/stop control. This will keep the vessel full and regulate the airflow out to just below system pressure. This tank is sized to hold a 4000-scfm demand for four minutes or more, while the auto hot start unit comes on (Fig. 1). This off-line storage is a very effective way to handle a surge demand and allow time to bring a unit online. However,this is running a 1000-hp compressor.

During fuselage testing, where the pressure generally does not exceed 5 to 10 psig,no consideration was given to a “blower”instead of a compressor. The blower will supply the same volume at the lower pressure for less than 300 hp.

The limited number of testing hours per year plus installation costs may make this option not viable. However, it should have been reviewed in detail. When the client asked the manufacturer’s representative why they hadn’t looked at blowers, the answer was “…we don’t have a line of blowers.”

Case Study #2:

After the catastrophic failure of two older 1500-hp primary air compressors, a large steel rolling mill had a typical varying demand from 15000 to 19000 scfm with the average about 17500 scfm. The current air supply was one late-model 7500- scfm three-stage centrifugal with 30% turndown with inlet guide vanes. This unithad relatively good specific power efficiency. The remainder of the air supply was from older, smaller, and a great deal less-power efficient compressors and several rental back-up units.

Two different air compressor manufacturers representatives performed full audits of the compressed air system and came up with the same recommendations: Install two new 7500-scfm class three-stage centrifugals (1500-hp class each) with appropriate turndown and inlet guide vanes to cover the “demand profile” efficiently. This is a conventional answer to serve this type of volume.

The consulting/audit team arm of a local compressed air equipment distributor was called in. After a one-week review, the report offered the following recommendations:

    • Scrap the existing water-cooled unit and replace it with two air-cooled, two stage, lubricant-cooled rotary screws. This was recommended to save water cost! An additional 3-4 more of the same units were recommended to serve the demand.
    • No comparison to a closed cooling system.
    • No mention of the 8-9% reduction in specific power compared to newer centrifugals. This company, of course, does not sell centrifugals—only rotary screws.
    • No specific air reduction projects. Leaks were estimated at 30% per the CAC chart.
    • They assumed a 3000-cfm reduction in leak repair and some blow-offs being changed.
    • Install a 40,000-gallon air receiver and pressure flow controller to hold a steady 80-psig system pressure.

The projected savings was over $1,400,000 per year with no math or list of specific projects to back it up. Some things too good to be true ARE!

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