How expensive is compressed air? It’s probably much more expensive than you think. It takes about 8 hp of electrical energy to produce 1 hp of work with compressed air. Do you think your electric power is expensive? Your air costs eight times as much! This is often overlooked in energy studies because many people don’t fully understand the interaction of all the elements within a total compressed air system. If you’re willing to apply some common sense, your plant holds a gold mine of opportunities to save money.
Pneumatics is not complex; simple algebra and physics are about as complicated as it gets. Don’t make it hard, just apply common sense, understand the terminology, and observe what’s going on. Every process in your plant that needs compressed air has minimum flow (cfm) and pressure (psig) requirements for optimum performance. Whenever you supply air at a higher pressure than required, you increase energy cost, but gain no increase in productivity or quality.
Do you know the lowest effective pressure and flow requirement for each end use? Do you measure and monitor these parameters to stay on target? If so, you need not read any further. If you don’t measure and monitor how much compressed air you use, how do you know how much it costs you each month? You may be clueless, but somebody knows how much you pay for electricity every month. You can’t manage the the cost of your compressed air if you don’t monitor and measure it.
In order to actually reduce energy cost, you must reduce the pressure and flow from the compressor. Any action that does not carry back to this is no real savings.
Typical energy cost of air
Half of the air produced in industrial plants is not used for production. Air compressors driven by electric motors will use a surprisingly large amount of energy every year they’re in operation. It’s not unusual for the annual cost of power to operate a compressor to equal its initial purchase price.
For example, the initial price of a 100-hp compressor may range from $30,000 to $50,000, depending on the type and options. Furthermore, that same 100-hp compressor, operating 6000 hr/yr — at a power rate of $0.07/kWhr and with a motor efficiency of 90% — will cost $34,800 to run for one year. This duty cycle translates to 3 shifts, 5 days a week. At 0.06 kWh, 8000 hr/yr with an air supply that produces 4.0 cfm/hp, 1 cfm costs $100/yr in energy, and 1 psi costs $398/yr for every 100 hp. Keep these numbers in mind as we identify basic opportunities on the demand side of a compressed air system.
You can determine the appropriate annual electric power cost of your compressors with the following formula. First, multiply the horsepower of the compressor by 0.746, then by hours of operation, your power rate, then divide that number by the motor efficiency. Everyone in the plant should know the total power cost for operating your compressors. This is especially important for anyone working with air-operated equipment.
Becoming aware of the real costs associated with compressed air use is only your first step. Unless you like to fight these battles alone, it’s essential that you get everyone involved. Once you’ve done a little homework, it should be no problem getting management behind you in this endeavor — especially one they learn how much money can be saved simply by following some common-sense practices. As this article explains, the savings aren’t tied to just the cost of compressed air. Huge gains in worker and machine productivity are just waiting to be discovered.
Poor piping design
Piping networks are the most overlooked characteristic in air systems. Even though friction pressure loss may be calculated as low for the pipe, convoluted piping, crossing tee connections and dead heads cause significant turbulence- driven backpressure. This not only wastes power, but also can cause unloading controls to become ineffective. Poorly selected filters, dryers, etc., without regard to pressure loss, merely compound the problem.
In a well laid out system, the interconnecting piping from the compressed air supply to the point of use (and the header distribution piping) should create no pressure loss. Following are some of the more common piping errors.
Tee connections — When a feed line of compressed air breaks into a flowing stream or air, the turbulence caused by perpendicular entry often creates a 3- to 5-psid pressure loss. The actual pressure loss is a factor of relative pipe sizes, flow rate, and other factors.
Based on the values introduced in the example on the previous page, a pressure drop such as this would cost almost $800 to $1200 every year, with absolutely no increase in production. More importantly, the backpressure can send a false unload signal to controls, causing premature unloading or auxiliary compressors to come on line needlessly.
Using a 30° or 45° directional angle entry instead of a 90° tee will eliminate this pressure loss. The extra cost for the directional entry is relatively small, but it is a onetime investment that pays for itself again and again.
Dead head connections — The figure at left shows how air flowing into opposite ends of a tee connection can create a pressure loss of 10 psig. Replacing the dead head piping with a long elbow fitting and a 30° directional entry, below, reduces the pressure loss to 0 psig. This represents 300 hp worth of air — about $1200/psi in our example, or $12,000 annually. Moreover, the backpressure created by the dead-head piping can cause the same control problems as the tee described above.
90° elbows —A standard 90° elbow causes turbulence equal to about 25% more pipe length than a long or swept elbow. Again, the cost of the component is negligible, but even taking the cost of labor into account, is a long-lasting investment well spent.
Undersized piping — Size pipe by the length and flow required, not by the port size of the components. Pressure-drop charts show loss based on entry pressure, pipe ID, and flow. Use these charts to select a pipe size that will register no pressure loss. When in doubt, compare the material cost of the next size pipe up or down. You may find very little difference in the installed cost. Most of the material cost in piping installation is for labor, valves, and fittings. You don’t have to run the same valve size as the pipe; you can often install a reducer to accommodate the smaller port size. This is an especially cost-effective way to minimize pressure drop for new installations.
Measure or calculate the flow through the pipe to make your best size selection when there is a choice. With 150 cfm of air at an entry pressure of 100 psig, every 100 ft of 1¼-in. pipe has almost three times the friction pressure loss of 1½-in. pipe. Turbulence-driven pressure loss is a function of compressed air velocity in the pipe. To be safe, size interconnecting and distribution piping to velocities of 20 fps or less whenever possible.
With good piping design and controls operating correctly, leaks are the next critical target. Facilities that have no formal, disciplined, compressed air leak management program usually suffer from cumulative leakage equal to 30% to 50% of the total air demand. All plants can benefit from a structured, ongoing leak management program. The most effective programs are those that involve production supervisors and operators working in concert with maintenance personnel. Accordingly, all programs should consist of the following:
• Short term — Set up a continuing leak inspection schedule by maintenance personnel so that each primary sector of the plant is inspected once each quarter (or at least once every six months) to identify and repair leaks. A record should be kept of all findings, corrective measures, and overall results. Inspections should be conducted with a high-quality ultrasonic leak locator during production and non-production.
• Long term — Consider setting up programs to motivate operators and supervisors to identify and repair leaks. One method that has worked well with many operations is to monitor the air flow to each department and make each department responsible for identifying its air usage as a measurable part of the operating expense for that area. This usually works best when combined with an effective inhouse training, awareness, and incentive program.
More often than not, certain processes require a minimum pressure. These “requirements” should always be traced to their origin. Are they actual OEM specifications or simply the perception of an operator? Higher-than-necessary pressure can also be caused by excessive system pressure fluctuation, which is either a piping, regulator, or sometimes compressor control problem.
“My grinders need 98 psi to run properly. Therefore, the air system should run at 98 psig or higher.” When you hear these words, what is the operator really telling you? Probably, when the system header pressure falls below 98 psig, the grinders don’t work well. Production personnel probably don’t know the actual pressure at the tool or how much air the tool uses.
We found this scenario during a recent air system energy audit. The plant could’ve run at 80 psig, but it ran at 98 because the grinding area supposedly required it. Furthermore, grinding accounted for only 20% of the demand, so 80% of the plant was supplied with air at a much higher pressure than needed. We could calculate how much the higher pressure was costing, but rather than belabor the point, let it suffice that the energy used to maintain the higher pressure throughout the plant amounts to thousands of dollars a year.
Testing revealed that the actual inlet pressure to the tool was 63 psig at load, but the header pressure stayed at 98 psig. In other words, a 35-psi pressure loss occurred through the plumbing between the header pipe and each grinder. Further investigation revealed that the grinders needed only 75 psig.
In this case, operators found the recommended ½-in.- hose to be too heavy, so ⅜-in. hose was used instead. The smaller hose restricted air flow, which created a substantial pressure drop. Furthermore, the ⅜-in. hose used ⅜-in. disconnects, which contributed even more to the high pressure drop.
We changed the standard ⅜-in. quick disconnects (which accounted for a combined pressure loss of 23 psig per station) to industrial quick disconnects at $2.50 extra per set — a whopping $5.00 per station. Doing so reduced the combined pressure loss to only 5 psi per station.
The ⅜-in. hose was replaced with a 1-in. pipe running to the base of the station at a cost of $30 per station. A regulator was selected to deliver full flow to the grinders at 75 psig with 80 psig feed pressure. we were then able to reduce the header pressure to a controlled 85 psig. Results after 18 months:
• Tool repair costs went down for the grinders.
• Production was increased throughout the plant by 30% — even after installing more grinders and other new equipment.
• The cost of materials to implement the changes amounted to $1362.00 for all nine grinders.
Even with the increased production and additional equipment, the average total air demand fell from 1600 to 1400 cfm. The key to this success was measuring the end use workstation inlet pressure when equipment was idle and when working while simultaneously measuring the header pressure. If the header pressure stays steady, and the process inlet pressure falls, then the restriction is in the feed line from the header to the process.
Running at high pressure to serve a small demand — A compressed air audit should always check whether or not the highpressure air is actually required. If so, can the end use be modified to lower the pressure requirement? For example, installing a larger bore air cylinder may reduce the pressure requirement. If this cannot be done, local high-pressure could be supplied by a small, dedicated high-pressure compressor or a pressure booster rather than running the whole system at high pressure. Measure the air flow and pressure requirements and the cycle time. Knowing this data allows calculating the most effective and efficient solution.
Other causes — A final but overlooked item in the air piping system that causes pressure loss is equipment left installed but no longer in use. Such components as flow meters, filters, and separators often are left in an air system even though they no longer are used. Because they are not used or maintained, they often fill with sludge, rust, and scale, causing ever increasing blockages and pressure drop as the air flows past. This requires a corresponding increase in header pressure to maintain the required process pressure.
Regulators wide open
Often a regulator at the point of use is opened to header pressure because a droop was not allowed for. But sometimes, even when the regulator was selected correctly, the delay in signal time has the regulator opening at the end of the action, leaving only the feed line storage volume to handle the process. If, during that delay, adequate volume does not exist to hold the required pressure at the end use, the pressure drops, production slows, and the operator opens the regulator up to full line pressure. This negates the effectiveness of the regulator and immediately creates an artificial demand.
A fix for this problem is to size a small storage vessel between the regulator and the process to hold pressure until the regulator opens. To size this, measure the flow volume, time required, and allowable pressure drop.
Training, training, training
Working toward energy savings requires many individuals to contribute to the effort. However, to be effective they must understand the cost of compressed air and the interdependency of the components of an air system.
Companies that have trained their people on the importance of saving energy have earned the greatest payback. This type of training pays off quickly because energy savings go directly to the bottom line and make a big impact on profitability.
Even though compressed air costs seven to eight times that of electricity, it remains ignored or misunderstood, which results in millions of dollars of wasted energy every day. Control and management of this utility poses significant opportunities at both the supply and demand side to save money. In order for any program to optimize these opportunities, those responsible for short and long term implementation must focus on all the interrelated parts of the system and understand its working parts. This is often best accomplished by a professionally implemented compressed air system evaluation or general audit to develop an accurate profile the system.
More savings await
Even more energy- saving techniques for plant air systems are contained in the author’s book, Energy Savings in Compressed Air. The hard-bound textbook contains five chapters on compressors, plus individual chapters on capacity controls, determining air requirements, distribution systems, dryers, condensate removal, flow measurement, and more.