Compressed Air System

The top 10 targets of a compressed air audit

The top 10 targets of a compressed air audit.

By Rich Merritt, senior technical editor

The easiest but potentially the most expensive way to improve your compressed air operations is to hire someone to do a compressed air audit. A team of experienced, professional compressed air wizards will visit your plant, spend days measuring pressures and examining your system, and give you a list of all the things that are wrong. In many cases, along with repairing leaks, poor piping and other relatively minor problems, they’ll advise you spend tens of thousands of dollars on new compressors and controls.

Following the recommendations of an audit usually pays for itself in a short time by saving those tens of thousands of dollars in operating expenses.

And they’ll be right. Following the recommendations of an audit usually pays for itself in a short time by saving those tens of thousands of dollars in operating expenses.

You are likely to benefit from such an audit, but it may make sense to know what the audit team is likely to find so you can identify the typical problems yourself. To that end, we asked some top compressor manufacturers and service companies to tell us what they usually find. Listed below are their 10 most typical, highest-payback audit items.

One caveat, though: Fixing these before doing a full system audit can make it more difficult to justify the higher-cost improvements. “Often, many parts of a system upgrade that improve the quality, reliability and repeatability of the system are financed in conjunction with the reductions in waste,” says Mark Krisa, air audit manager at Plant Air Technology (www.plantair.com). “Energy reductions associated with your efforts cannot be incorporated into future return on investment projects.”

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In other words, if you fix all the low-hanging fruit, when you do an audit the payback on the investment won’t be as high. It’s a numbers game, but it might be important in determining who pays what, and whose budget it comes out of. The numbers game may decide which you do first: the professional audit or your own list of low-cost repairs.

1. Plug those leaks

“One of the most common problems is leaks,” says Wayne Perry, technical director, Kaeser Compressors (www.kaeser.com). “Studies indicate that as much as 35% of the compressed air produced in the market today is wasted to leaks, and everyone has leaks.” Identifying and correcting them may save not only the purchase price of a compressor, but reduce the amount of energy needed to run the compressor.

“It has been our experience that plants which have no formal, monitored, disciplined, compressed air leak-management program will have a cumulative leak level equal to 30% to 50% of the total air demand,” adds Henry van Ormer, engineer and owner of Air Power USA (www.airpowerusainc.com). Every 8 cfm to 12 cfm leak can cost you $800 to $1,200 per year.

Van Ormer suggests setting up a short-term leak inspection program so that every sector of the plant is inspected once each quarter to identify and repair leaks. “Inspections should be conducted with a high-quality ultrasonic leak locator during production and nonproduction,” he recommends. “A record should be kept of all findings, corrective measures and overall results.”

Afterward, he suggests setting up programs to monitor the air flow to each department and making each department responsible for identifying its air usage as a measurable part of the expense for that area.

If you get rid of leaks, you might cause other problems. “Elimination of waste, such as leakage and artificial demand, may result in reduced loading on compressors that are not equipped to turn down efficiently,” says Mike Bakalyar, manager, enhanced services, Gardner Denver (www.gardnerdenver.com). Dynamic efficiency may actually degrade, resulting in very little positive effect on energy usage (Table 1). Waste has been reduced, but the cost recovery shifts to compressor controls.

Table 1. Demand can affect efficiency
  Leaking system Tight system Tight plus controls
Process demand* 1,500 cfm 1,200 cfm  
Demand reduction   300 cfm (20%)  
Power 259 kW 243.5 kW 206.9 kW
Dynamic efficiency 5.8 cfm/kW 4.9 cfm/kW 5.8 cfm/kW
Annual energy cost* $108,780 $102,270 $86,898
Net savings   $6,510 (6%) $21,882 (20%)
*Example system at 90 psi and $0.05/kWh


“The campaign to reduce leaks must be complemented with configuration and control improvements that will allow the air generation to turn down with the reduced demand,” he says.

2. Down with overpressurization

Excessive pressure increases leaks and wastes money. “Some end users will actually increase pressure in an attempt to compensate for capacity issues,” Perry says. “In fact, increasing pressure will have the opposite effect on air flow and often exacerbate the problem. There is also a proportional relationship between pressure and power consumption – for every 10 psi in excess pressure there is a 5% increase in power cost.”

Norm Fischer, president, Centrifugal Equipment Service (www.cescontrols.com), says too-high pressure will amplify problems, not solve them. “The easy answer to many system problems is to jack up the pressure. Unfortunately, the leaks will leak more, and unregulated users will waste more air and more energy.”

Lowering the pressure may solve problems. “Lower system pressures mean less mass required, therefore fewer compressors running,” Fischer adds. “Compressors are usually more efficient when run at lower pressures.”

But you have to convince production, Fischer says. “Often, the greatest struggle is gaining the confidence of the production people that the system is reliable enough to respond when required, so they will lower the pressure requirement closer to the actual design requirement.”

3. Double-check air requirements

Often, production overestimates the amount of air it needs. “If production is allowed to define their own compressed air requirements based on as much as they want whenever they want it at any pressure, the system will never operate efficiently,” Krisa says.

Dave Booth, systems specialist at Sullair (www.sullair.com), agrees. “The entire paradigm under which the compressed air system operates must change,” he advises. “We must shift from the principal goal of maintaining a minimum pressure and that higher pressure is OK to the goal of maintaining a consistent and stable pressure. Plants must change their focus from ‘maintaining air supply’ to ‘supplying air to meet demand.’ More air and more pressure is simply more cost.”

Van Ormer says, “More often than not, it is one process that needs a certain minimum pressure. These claims should always be reviewed. In one audit, the rest of the plant could run on 80 psi but the compressed air system had to run at 98 psi because the grinding area – with only 20% of the demand -- required it. Testing revealed that the actual inlet pressure to the tool was 63 psig at load. In other words, we had a 35 psig pressure loss from the header to the tool. Further tests indicated that the optimum inlet pressure for these particular tools was 75 psig.”  The plant installed a larger feed line and a regulator to deliver full flow to the grinders at feed pressure. The header pressure was lowered to 85 psi. Results after 18 months showed that tool repair went down for the grinders, production increased by 30% and total air demand fell from 1,600 to 1,400 cfm. Total cost for the regulator, piping changes and adding quick disconnects on nine grinders was $1,362. Annual electrical savings are about $18,000 per year.

In those cases where you have a small area that actually needs high pressure, van Ormer suggests setting up a secondary, smaller, high-pressure unit or an appropriate booster, rather than drive the entire plant system at the higher pressure. “Expecting the supply system to support a black hole is not a realistic design criterion,” Krisa adds.

4. Angle connections: all teed off

One of the simplest fixes in a compressed air system is to replace tee connections with directional angle entry connections. In a piping system where a feed line of compressed air is trying to feed into another air line, the turbulence caused by a 90 degree entry often causes a 3 psi to 5 psi pressure loss. Such a loss can cost you $1,200 per year at every one of those tees.

“More important, the back pressure sends a false unload signal to the controls, causing premature unloading or extra compressors to be on line,” van Ormer says. “Using a 30 degree to 45 degree directional angle entry instead of a tee will eliminate this pressure loss. The extra cost of the directional entry is usually negligible.”

Even worse is a dead-head tee connection (Figure 2), where compressed air enters at opposite ends of the tee, causing extreme turbulence. “In one instance, the pressure loss was almost 10 psi,” van Ormer says. “This is 300 hp worth of air, or about $12,000 annual power cost.” To avoid such a situation, he suggests using two directional angle connections spaced so the incoming air does not cause such turbulence.

5. Bad piping

Convoluted piping, piping restrictions, old pipes and incorrect pipe sizes often lead to pressure loss. In a well-laid-out system, the interconnecting piping from the compressed air supply to the process and header distribution piping should create no pressure loss. In many cases, it is easy to simply replace a section of pipe to gain efficiency. “With 150 cfm of air at an entry pressure of 100 psi, a 1¼-in. pipe has almost three times the friction pressure loss of 1½-inch pipe,” van Ormer says.
 
Booth looks at it more simply. “If you cannot walk up to your compressed air piping system and in a brief glance obviously figure out how the air gets from the compressors through the contaminant removal system and to the plant and then on to the points of use, you probably have a problem,” he says. Look at your piping. Is it logical? Does it make sense? Would you install it that way?

“Piping is a major consideration, especially in older facilities or shops that have grown and expanded,” Perry says. “Cast-iron piping will rust over time, releasing rust and scale into the compressed air and creating buildups at various points in the system. This not only degrades air quality, but reduces the effective internal diameter of the pipe and obstructs air flow creating unwanted pressure drops and velocity problems.”

Measuring pressure loss in piping sections will identify the worst culprits. If you find a severe pressure drop through some convoluted sections, or determine that the pipe is too small, the cost of changing the pipe often pays back quickly. “Upgrading to copper or aluminum piping provides excellent value for money and ideal delivery characteristics,” Perry says. “When upgrading, ensure that the physical piping diameter is sized to deliver the required air flow with minimum pressure drop.”

Interconnecting piping between two or more compressors often needs attention. “This is the piping area where we find the most opportunities for improvement,” van Ormer says, “particularly in systems installed after the late 1970s. Older systems were put in more carefully.”

He cites an example of a system that had six compressors, each producing 750 cfm at 100 psi. Three units were installed in 1968, with a 6-inch discharge to a 10-inch header. Their calculated pressure loss was 1 psi. This system was installed “by the book,” and runs fine. The other system was installed in 1978, using 2-inch lines to a 3-inch header. Its calculated pressure loss is 8 psi, but the actual measured pressure loss was 18 psi, with 10 psi caused by turbulence from high velocities and tees.” The second system costs $60,000 more per year to run.

The rule of thumb in piping, van Ormer says, is “run it as large as you can and when you have to go to smaller lines or hoses, make it as short as possible.”

6. Get rid of obsolete restrictions

Clogged filter elements, forgotten manual drain traps and neglected separator cartridges can cause significant drops in pressure and negatively impact capacity and reliability, not to mention creating air-quality issues. 

One often overlooked item in the air piping system that causes pressure loss is equipment that is left installed but is no longer in use. Such things as old, unused orifice plate flowmeters, filters, separators, etc., are often left in the air system even though they are no longer in use. Since they are not used or maintained, they often fill with sludge, rust, scale, etc., causing ever-increasing blockage and pressure drop as the air flows past. This requires a corresponding increase in header pressure to maintain the required process pressure. You may want to go on a hunt for equipment that’s no longer being used and disconnect it from the compressed air supply.

In one example, a pet food plant was running a 150-hp rotary screw that produced 750 cfm. “The discharge pressure was 120 psi, and actual pressure at packaging was 90 psi,” van Ormer says. “Investigation of the main header from the compressor room to process found an old, forgotten inline filter full of rust and scale. The filter was removed, the discharge pressure was reduced to 100 psig, and this produced an annual electrical energy savings of $6,570.”

7. Insufficient storage

Perry says insufficient storage is a common problem. “Across the board in manufacturing and processing, the value of an appropriately sized air receiver and appropriate compressed air piping is underestimated,” he says. “These tanks provide a first stage of moisture separation to help maintain compressed air quality. However, their primary function is storing and delivering compressed air to help meet periods of peak demand and to prevent excessive compressor cycling.”

All air systems will do better with storage between the user and the process. The amount of effective storage for any system is where the operating control band is equalized by the back pressure in the system. In one example, a 280-hp, two-step controlled, lubricant-cooled rotary screw compressor was running 24 hours per day, seven days a week at a relatively level load of 70% flow. The unit had very low storage capacity and would unload, idle for 15 to 25 seconds, then reload.

The bleed-down time for this unit was one minute to reach full unloaded power. “The unit did not stay off long enough to reach the low power point and spend time there,” van Ormer explains. “Correcting the effective storage to almost 10 gals per cfm created a two-minute idle allowing full blowdown to the low idle input power and a full one-minute run at this low power before reloading. This resulted in an annual electricity cost reduction of more than $14,000.”

8. Inappropriate use

Unregulated use of compressed air, and using compressed air for inappropriate purposes, wastes a lot of energy. Considering that it costs eight times as much to use air as it does to use electricity, you may want to reevaluate unregulated air-powered cabinet coolers, blow-offs, vacuum generators, mechanical pumps, air motors and hoists, vibrators, aeration, spraying and a host of other equipment.

Compressed air is readily available in a plant, and the cost of using it is not always understood. “Therefore, when a need was identified, air was usually the easy answer,” Fischer says. “Sometimes it’s even used for cooling people at workstations, blowing dust, or to power vortex-type coolers and air to keep food clean.”

“Open blow, refrigeration and vortex cooling may all be replaceable with heat tube cabinet coolers with a potential savings of 3.5 kW to 4 kW each on a 30- by 24- by 12-inch average cabinet,” van Ormer says. “The initial cost is usually in the $700 to $750 range with a potential resultant power savings of $1,000 to $2,000 per year each.”

He also suggests using venturi air amplifier nozzles or air inducers whenever possible, which will reduce blow-off compressed air by 50% or more.

9. Watch those pumps

Air-operated diaphragm pumps tolerate aggressive conditions relatively well and can run dry, which makes them a favorite with plant personnel. But is an air-operated pump the best solution? Electric motor-driven diaphragm pumps are readily available, and may work just as well. A 2-inch, air-operated diaphragm pump, pumping water at 130 gpm, will use 25 hp worth of compressed air at a cost of $9,947 per year. A 3-hp electric pump may well do the same, at an energy cost of $1,989 per year.

If air-operated pumps must be used, consider adding controls to shut the pumps off when they are not needed. Pumps waste the most air when they are pumping nothing. Also, check to see if the pump is running at the lowest possible pressure. Simple controls can increase pressure when needed.

10. Maintain the system

Poor air quality adversely affects overall plant operations. What you want is air that is clean and dry, and that requires maintaining the filters, separators and driers. Neglecting recommended maintenance can let oil get into the plant air and cause production problems from dripping tools to fisheyes in paint systems.

Poor maintenance also affects efficiency. Van Ormer says they did an audit and found three 150-hp compressors with 9.5 psi inlet pressure instead of the normal 14.2 psi. This reduced the effective output from 725 cfm to 501 cfm, or a 31% loss. The plant had to run all three compressors at full load to supply the 1,400 cfm demand. Investigation discovered dirty and restrictive inlet conditions. Correcting the problem resulted in almost $45,000 per year in energy savings.

Change air/oil separators, filters and other components at the optimum time, not when they clog up and cause a pressure loss.

What about that audit?

“Most of the lasting benefits and big opportunities identified in air audits are really common-sense solutions,” Booth says. “Most involve simple maintenance issues, misapplications and general problems caused by neglect and not fully understanding the consequences of mismanaging a compressed air system.”

But many plants can benefit from more sophisticated analysis by professional auditors who might recommend, for example, changes to the control system. “The most common problem identified in complete air system audits is the improper application — or at worst, the complete lack — of compressor controls,” Perry says.

The pros might recommend a serious system redesign. “Poorly designed systems cause the end user to decide that the solution is to run more compressors and run them at a higher pressure,” Fischer says. He cites symptoms such as compressors fighting each other, too many compressors running, compressors running “just in case” they might be needed, and fluctuating plant pressures.

Those problems are more difficult to find and fix than the leaks, inappropriate equipment and rusty pipe problems described here. Unless you are a compressed air wizard yourself, you may need an audit to tell you what’s wrong with your system controls and overall design.

Fortunately, as we’ve seen in this article, an audit frequently finds enough problems to pay for itself in the short term. So, clean up your compressed air system using the tricks explained here, and call in the pros to give it a final check.

Lack of ownership

Several of the compressor wizards we interviewed prefaced their list of audit items with a note bemoaning the sad state of affairs in most plants. Mark Krisa, air audit manager at Plant Air Technology (www.plantair.com), said it best: “The common issue that exists in almost every facility is the attitude toward acceptance of responsibility for the problems. If the intention is to correct the problems, the organization as a whole has to take responsibility for the problems.”

Krisa says that in many plants the production department determines compressor requirements. “This is classically based on keeping production happy,” he says. “If the operating goal for the compressed air system is to keep production from complaining, then production has no involvement in resolving problems, only in creating them.” Production may overestimate its needs for compressed air, misuse it and misapply equipment.

“Without formal changes to how the system is approached and an assignment of responsibility, the system will ultimately return to the initial state of operation, regardless of what efforts are made to purchase equipment to make the system better,” Krisa advises.

Dave Booth, systems specialist at Sullair (www.sullair.com), says it’s a lack of understanding. “Most plants really do not understand what it truly costs them to operate their system and what effects it can and does have on their overall production process and quality,” he laments. “If you don’t know what it costs or how it operates, how can you even begin to consider evaluating savings or other potential improvements and changes?”

Other companies agree that most plants don’t understand the economics of compressed air. They regard it as just another utility they don’t have to pay for. Henry van Ormer, engineer and owner of Air Power USA (www.airpowerusainc.com), says the typical cost of air (at $0.06 per kWh) is about $100 per year per cfm. Also, he says it can take up to 8 hp of electrical energy to produce 1 hp of work with compressed air. That means air power can be eight times as expensive as electric power, and just about anything you can do to improve compressed air operations will save a lot of money.