Why be energy efficient?iStock_000003156313Large

Approximately 10% of the electricity supplied to Australian industry is used to compress air. Many industrial businesses use compressed air to operate equipment such as hand tools, pumps, valve actuators, pistons and large-scale processes.

There is significant potential to save energy and improve the bottom line in this area. It can be achieved by reducing the need for compressed air services, optimising equipment and upgrading old systems. Significant energy savings are possible.

For information on types of air compressors, see Technology Background – Compressed Air.

Ways to save

A good air compressor energy efficiency strategy relies on an integrated approach. It should consider multiple initiatives such as those outlined below.

For information on types of air compressors, see Technology background – Compressed air.
 

Reduce demand for compressed air services

Compressed air is often used as an energy source because it is clean, readily available and simple to use. But compressed air is also the most expensive energy source in most industrial plants. So, as a first step, it is important to review and reduce demand for compressor air services. Switching to other energy sources, or using alternative strategies where appropriate, will reduce costs.

Examples of potentially inappropriate uses of compressed air include: open blowing, sparging, aspirating, atomising, padding, dilute-phase transport, dense-phase transport, vacuum generation, personnel cooling, open hand-held blowguns or lances, diaphragm pumps, cabinet cooling and vacuum venturis. Electrically powered tools can, in many cases, replace compressed air power tools, saving energy and improving the bottom line. This may allow branches of the compressed air network to be shut down permanently or for operating hours to be reduced. Alternative equipment with lower running costs is outlined in Table 1.

Table 1: Compressed Air Use and Substitution

Compressed Air UseEquipment UsedSolutions/Alternatives
Blowing or cleaningNozzle/gunAir knife, induction nozzle, low pressure blower, broom/brush
CoolingCooling induction systemAir conditioning systems, chilled water, fresh air ventilation, fans
Drying of water on productNozzle/gunSolenoid control, air knife, induction nozzle
Screwdriving or drillingScrewdriver or drillBattery-electric portable drill or screwdriver

Source: Sustainability Victoria (2009)

Some plants contain equipment that is no longer used but is still connected to the compressed air system, resulting in unnecessary costs and energy consumption. This equipment should be disconnected or their air supply should be cut off as far up the distribution system as practical without interrupting functioning equipment.

  1. Sustainability Victoria (2009) Energy Efficiency Best Practice Guide: Compressed Air Systems Sustainability Victoria (Opens in a new window) PDF 1.6 MB

Optimise the use of existing air compressor systems

The energy efficiency of existing compressed air systems can be improved through using the minimum pressure for the required task, eliminating leaks, improving air compressor controls, fixing pressure drops, and utilising air receivers and heat recovery units.

Opportunities in this area are outlined below.

Use the minimum appropriate air pressure for the required task

Use of the minimum appropriate air pressure for the required task can save energy as the work required to compress air is dependent on pressure as well as volume. Most procedures do not require air at the maximum pressure that the compressed air system can produce. Delivering a higher than necessary pressure results in excessive energy use, excessive equipment wear, high maintenance costs and low availability of compressed air for other procedures.

The system should be designed and maintained to provide the minimum pressure required to achieve the task.  Control systems can be used to ensure pressure levels match the requirements of the system.

Detect and eliminate leaks

Some compressed air systems lose up to 50% of their air through leaks. Leaks not only increase energy costs, they also cause other problems such as a drop in air pressure. This means the compressed air system must run longer or harder to deliver the expected airflow and pressure, shortening equipment life, increasing maintenance requirements and potentially increasing unscheduled downtime. If a system is already running at full capacity, the performance of air tools at end use deteriorates. In these cases, unidentified leaks may lead to unwanted expenditure in additional compressor capacity.

A proactive leak repair and maintenance program will help to maintain system efficiency and can reduce leakage to less than 10% in most cases.  Such a program involves:

  • regular inspection of compressed air equipment
  • regular inspection of air pipes, bends and valves
  • ensuring that all air channels have proper physical supports to prevent leaks through excess stress
  • disconnecting or isolating any unused parts of the air distribution network or unused pressure regulators
  • consulting with staff who are on the plant floor and are most likely to notice changes in system performance.

Improve air compressor controls

Air compressor controls turn equipment on and off or to part-load to meet the variable demand for compressed air. These controls, together with electronic control equipment, help to minimise energy costs by selecting and engaging a combination of compressors that must operate near full load. Controls are applied directly to compressors, as well as to other parts of the system.

The most appropriate controls for a particular compressor depend on the compressor type and the typical operating load.  Compressors should be managed to work primarily in their ranges of maximum operating efficiency, which vary among compressor models.

Types of controls that can be used to do this, include:

  • start/stop controls to turn compressors off when not in use
  • load/unload controls that close the compressor intake valve when not in use
  • modulating controls to adjust the intake valve to control the output
  • variable speed drives to enable efficient part load operation. 

Note, some controls cannot be used with certain types of compressors.

Controls for other parts of a unit include time-operated valves that zone off parts of the distribution system, interlocks that open a zone only when the end use is operating, and sensors that detect that an end-use piece of machinery is operating. These controls can be integrated into a site Supervisory Control and Data Aquistion (SCADA) system or building management system.

Fix pressure drop

Pressure drop refers to the drop in pressure between the compressor and end uses. It represents the accumulated losses, mainly due to leaks and friction, in the distribution system. An efficient system typically has a pressure drop of less than 10% of the compressor’s discharge pressure. A 100kPa reduction in operating pressure can save about 8% of energy costs.

Some amount of friction is unavoidable, such as that generated by filters, dryers and separators as they clean and dry the air, as well as valves and pipe components. Typically, the higher quality of air required, the greater the flow losses from these components. Therefore, components should be selected to meet the required air quality, as determined by the application. Over-specifying these components will result in higher component costs and excess energy consumption.

The distribution layout is also a source of friction, especially when it requires many bends in order to supply air to end uses. In these cases, friction can be minimised by using a ring system layout or, if more practical, a single main system layout, both of which have relatively few bends. Small-diameter pipes have high friction due to increased boundary layer effects. If compressed air demand has increased since a distribution system was installed, larger-diameter pipes may now be more suitable than the original pipes. Maintaining filters and drying equipment prevents pipe corrosion and the friction caused by it.
 

Energy Efficiency Best Practice Guide: Compressed Air Systems 2009 (Opens in a new window)

Review air receivers

Air receivers are units that typically store 5–10% of compressor capacity.  They store the excess air produced by a compressor, allowing that compressor to turn off while the receiver empties. The less time a compressor is on, the less wear it sustains and the less energy it consumes. Compressed air systems usually have one primary receiver and possibly a few secondary receivers near intermittent end uses.

Upgrading to a larger primary receiver as base demand increases may prevent the compressor from running too long. Adding secondary receivers as more intermittent end uses come online can help to buffer new peaks. Receivers can also avoid the need for the installation of more compressed air capacity.

Undertake preventative maintenance

Regular preventative maintenance of compressed air system equipment should be conducted as per the equipment manufacturer’s instructions. Some tasks for maintenance staff include:

  • checking for leaks (see Detect and eliminate leaks)
  • apply lubrication, including grease, and top-up and/or replace oil
  • clean and replace filters
  • correct any anomalies identified by indicator readings and set points
  • correct the tension of drive belts and replace worn belts
  • maintain the operation of valves, oil coolers, intercoolers, and aftercoolers
  • close condensate traps
  • check that compressor intake air is drawn from a cool space (e.g. outside) rather than a hot space (e.g. boiler room).  Every 3°C reduction in intake air temperature saves about 1% of energy required to run the compressor
  • schedule or conduct major overhauls of compressors

For more information

Upgrade air compressor systems

There are several measures that can be implemented to help ensure air compressor upgrades lead to energy savings.

Upgrading an air compressor involves some upfront costs, so this measure is best applied after opportunities to reduce demand for compressed air services and optimise the use of existing air compressor systems have been addressed. Failing to address these opportunities will likely result in selecting an oversized compressor, leading to excessive energy and maintenance costs.

Opportunities in this area are outlined below.

Selecting and designing the right air compressor system

There are many types of compressors, including reciprocating, vane, screw, centrifugal, scroll, and rotary tooth. For more information on types of compressors, see Technology background – Compressed air.

Upgrading and improving your air compressor system can take time. Compressed air service providers can assist in assessing, upgrading and installing new systems. To assist selection, resources are also available with information on different compressor types and which type is best suited to specific applications.

System design is also important in maximising efficiency and reducing energy use. Some considerations include:

  • Using air compressor controls, together with electronic control equipment, to ensure the combination of compressors chosen  operates near full load. See Improve Air Compressor Controls
  • Using air receivers to store compressed air which can meet rapid increases in demand. This enables compressors to run at their optimum load position and minimise the number of load/unload cycles. See Review Air Receivers
  • Optimising the layout of piping to minimise pressure drop through reducing bends and optimising pipe diameter and length. See Fix Pressure Drop.
  • Choosing components such as filters, dryers and separators to meet the required air quality, as determined by the application. Over-specifying these components will result in higher pressure drop and a less efficient system.
  • Locating inlet and discharge air outlets to provide cooler air at intake and discharging waste heat away from inlet air sources

For more information

Future developments

Emerging innovations related to air compressor technology include:

Increased portability of electric tools

As the power-to-weight ratio for small electric motors, lightweight gearboxes, more powerful and lighter batteries and smart controls improves, portable electric tools (both mains-powered and battery) are beginning to compete with compressed air equipment, even in vehicle assembly.

Smaller decentralised air compressors

Some firms historically have left their large centralised compressed air systems on overnight and over weekends because part of the factory or a laboratory would still function out of hours. Many businesses are finding that it pays to invest in a smaller compressed air system to use for those specific after-hours tasks, rather than running the large centralised compressed air system constantly. This is a recent development that is saving businesses money.

Control technologies

It is advisable to review the effectiveness of variable speed drives on air compressor systems. Some companies leave their air compressors on, assuming that because they have a variable speed drive the compressors don’t not use much energy. However, even when on standby mode with a variable speed drive, air compressors are typically using around half of their full-capacity power.