What is in this article?:
- Opportunities abound for Saving Energy
- Artificial demand
Looking to slash your electric bill? The more pneumatics your plant uses, the more opportunities exist to save money.
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.