Evaluate the efficiency of your compressed air system by scrutinizing material, labour, downtime and energy.
November 4, 2011
by Brent and Roark Patton
Compressed air is as ubiquitous a resource to industry as electricity, water, and natural gas and it’s a vital part of almost all industrial processes. As such, costs associated with operating compressed air equipment are often thought of as fixed, yet there are many simple ways to significantly reduce them.
Costs break down into two main categories; material, labour and downtime; and energy. Both should be scrutinized when evaluating the efficiency of your compressed air system.
The best way to cut parts, labour and downtime costs is to reduce the probability of breakdowns and that requires having a preventive maintenance plan. Many breakdown calls result in simple repairs that could be prevented. The most common calls are related to overheating caused by low oil levels or blocked cooling mediums. Regular, visual inspections of coolers and oil levels will eliminate a significant amount of equipment downtime. And locating equipment outdoors, independent from the facility, eliminates the contamination of intake air, one of the primary causes of failure. Additional benefits are more plant floor space and reduced equipment noise.
At an average efficiency of 5% to 15%, compressing air is very expensive. The electricity cost of 100 hp worth of compressed air rises from $17,120 for a single eight-hour shift, to $32,330 for 16 hours and $63,900 for 24 hours.
Electricity is charged based on peak demand and use. Reducing use also lowers overall peak demand. For example, if your plant is using between 600 and 800 kilowatt-hours (kwh), but for a short time consumes 1,000 kwh, the bill will reflect a peak charge based on 1,000 kwh. Therefore, a small reduction of 100 kwh will not only reduce the kilowatt-hour use portion of your energy charges, it will also reduce your peak by the same amount. Typically, the peak charges for power consumed range from $5 per kwh to $15 per kwh in some remote regions in Ontario.
The obvious solution is to reduce the amount of energy used by focusing on the following:
Leaks and pressure drops. During an air audit of a large stamping facility it was determined that more than 100 hp of compressed air was required during periods of zero-demand to merely bring the system pressure to 100 psi. Furthermore, although the pressure requirement for the stamping equipment was 90 psi, the customer was operating at 120 psi to compensate for line pressure drop.
Air leaks are sometimes overlooked as an energy saving opportunity because typically most plants will have many small ones. The 100 hp or so being consumed by the stamping facility was costing more than $32,000 annually in direct energy losses, excluding additional maintenance costs, equipment wear and tear, and the incumbent “peak” charges.
Line pressure loss is a common problem that’s overcome by raising the plant system pressure. The ideal solution is to install a loop system to eliminate line pressure drop and take a step further – use strategically placed air receivers to overcome pressure fluctuations at the point of use. Savings will vary from plant to plant, but generally every 5 psi pressure increase equals 5% more energy consumption.
Blocked filtration. As filters and automatic drains age, they become blocked with particulate, causing an immediate pressure drop in the entire distribution system. Systems with both pre- and coalescing-type filters will see a drop in plant pressure of up to 20%. Unfortunately, compressor pressures are often increased to compensate for the decline. Routinely changing line filter elements and inspecting automatic drains prevents blockages and eliminates the impulse to increase delivery pressure.
Appropriate use. Given the inefficiency of compressing air, consider looking at processes that can be switched to more energy efficient tools. For example, many high consumption processes use blowers and fans for open blowing, cleaning and drying. Further, if compressed air is still preferred, consider using an engineered nozzle rather than a crushed copper tube. These nozzles significantly reduce backpressure and increase the velocity of the compressed air, thus reducing the volume needed.
Automatic drains. Ensuring your automatic drains function correctly is an inexpensive repair. Often neglected and tucked underneath a receiver tank, they become damaged over time resulting in a constant stream of wasted air. Typical cost for an automatic drain is $150 to $300, which will pay for itself in less then a year.
Heat recovery. When operating a rotary screw compressor, consider reclaiming the warmed discharge air. In the summer hot air should be redirected outside the plant while proper ducting in the winter months will save you thousands of heating dollars. For example, heat recovery using a 100-hp compressor will save $3.70 per hour. Three shifts daily for six months will save $15,906.
Controls. Many rotary air compressors operate in “modulating” mode. When fully loaded, the compressor operates at peak efficiency but efficiency decreases drastically as demand lessens. A 100-hp compressor draws approximately 100 amps at 575 volts while fully loaded, and yet, the same compressor will draw 65 to 75 amps fully unloaded. Most manufacturers have abandoned the modulating system in favour of the “online – offline” automatic control system that operates at a much higher efficiency in compressors with a fluctuating or low air demand. Switching from modulating to automatic mode will reduce unit electrical costs by up to 40%.
The most efficient rotary compressors available for changing air requirements are variable speed, which are becoming the top choice for energy conscious consumers. An air audit by a qualified technician will determine the type of control system that best matches your consumption characteristics.
Finally, there are free tools such as The Compressed Air Challenge (www.compressedairchallenge.org) and RetScreen (www.retscreen.net) that will help you find ways to reduce costs and calculate precise savings and payback periods so you’re not blowing your compressed air budget.
Brent and Roark Patton operate AirCell, a supplier of compressor technology, and Comtract Air Compressors, a manufacturer of compressed air systems, in Mississauga, Ont. E-mail email@example.com and Roark@comtract.ca.