Part 6: Reducing energy costs: focus on compressed air

Compressed air is a convenient and often essential utility, but it is very expensive to produce. Most of the energy used to compress air is turned into heat and then lost to the atmosphere.

Compressed air is not free. At the point of use, compressed air costs more than ten times the equivalent quantity of electrical power, i.e. an equivalent cost of around €1.0/kWh ($1.20/kWh). It should never be wasted and should only be used when absolutely necessary. Air also needs to be treated to remove moisture, oil, and dirt and the higher the quality required, the greater the energy consumed by the treatment system.

Over a typical 24-hour day, five-and-half-day week, a 100 kW compressor uses energy worth approximately €45,000 per year and the energy costs over a 10-year operating life are more than seven times the original capital cost. Compressors are the same as any capital equipment—they cost more to run than to buy! At these cost levels, an energy-efficient system is highly cost effective, even if it costs slightly more to install.

The cost of compressed air makes it an expensive resource and the way to achieve the best savings is to minimize the demand and then to optimize the supply. Up to 30% savings can be made by simple and no-cost measures such as making end users aware of the cost of compressed air and getting them to report leaks. A compressed air management program will save costs for very low investment in time or money. Have you got one?

Minimize the demand and then optimize the supply

A significant amount of energy is wasted through leakage, especially when leakage can account for up to 40% of the air generated, i.e. 40% of the energy is wasted in feeding leaks in the system. A 3-mm hole in a system at 7 bar will leak about 11 liters/sec and costs approximately €1500 per year.

Walk around the factory and listen to the sound of the compressed air escaping —it is actually the sound of your money escaping. If you can hear any leak then it is costing at least €150 per year—the ‘ssssss’ noise is your profits leaking away.

Simple leak surveys and maintenance can produce dramatic cost reductions and in some cases, leak reporting and repair have enabled companies to shut down some compressors for all or most of their operating time.

Tip: Estimate the losses due to leakage and set targets for demand reduction.

Tip: A simple walk-around survey, with leaks tagged and repaired as soon as possible, can greatly reduce leakage. The only tools needed are a good sense of hearing, some soapy water and a brush. Ultrasonic detectors are more accurate but start with your ears.

Tip: Isolate and seal off redundant pipe work: this is often a source of leakage.

Reduce usage

Compressed air is often wasted because everyone assumes it’s cheap. Check every application to see if it is essential or simply convenient.

Tip: Stop the using compressed air for ventilation or cooling—fans are cheaper and more effective.

Tip: Fit high efficiency air nozzles—the payback can be as short as four months.

Tip: Consider the use of electric tools instead of compressed air tools.

Tip: Do not use compressed air for conveying material or products.

Optimize the supply

Reduce generation costs

The higher the air pressure, the more expensive it is to provide the air. Compressing air to twice normal pressure uses four times as much energy. In many cases, the real pressure needed is less than the pressure being supplied. In some cases, a machine is rated as needing a 7-bar supply but pressure reducers are fitted inside the machine. Find out what the real needs are.

Tip: Check that compressed air is not being generated at a higher pressure than required.

Tip: Switch off compressors during non-productive hours. They are only feeding leaks or creating them.

Tip: Check that compressors are not idling when not needed—they can draw up to 40% of full power when idling.

Tip: Position air inlets outside if possible—it is easier to compress cold air.

Tip: If there is a machine or area that requires compressed air for a greater time than the rest of the factory, consider zoning or a dedicated compressor so that others can be switched off.

Tip: Investigate electronic sequencing to minimize compressors going on- and off-load without good reason. These are more energy efficient than simple cascade controls.

Tip: Maintain the system—missing a maintenance check increases costs.

Improve distribution

The longer the compressed air pipeline, the greater the pressure loss over the pipeline and the greater the cost to operate the system—air is a fluid and suffers from transport losses.

Tip: Make sure that pipe work is not undersized: this increases the resistance to air flow and causes unnecessary pressure drops.

Tip: Use a ring main arrangement in each building—air can then converge from two directions. This reduces the pressure drops and makes changes to the system easier.

Tip: Avoid sharp corners and elbows in pipe work as they cause turbulence and pressure drops.

Reduce treatment costs

Tip: Treat the bulk of air to the minimum quality necessary, e.g. 40-µm filters are usually sufficient. Specifying 5-µm filters will increase the filter purchase cost, the replacement frequency, and the pressure drop.

Tip: Test filters regularly to make sure that the pressure drop does not exceed 0.4 bar—if the pressure drop is higher than 0.4 bar, replace the filters as the cost of power to overcome the drop is usually greater than the cost of a filter.

Tip: Manual condensate traps are often left open and act as leaks. Fit electronic traps to replace any manual condensate traps.

Dr. Robin Kent is managing director of Tangram Technology—specialist consulting engineers for energy management in plastics processing. www.tangram.co.uk