Compressed air systems: Efficiency matters

Pneumatic systems have become critical to automation motion and control applications in almost every industry sector. Clean, versatile and powerful, compressed air is piped around a ring main in a factory much in the same way as electricity and is in effect another utility. Air is an abundant and benign – therefore safe - raw material, and when compressed is easily transported from a compressor to the point of use at a machine. In a well-designed and maintained system, compressed air can be accurately controlled and adjusted to meet the varying needs of different processes and machines. 

However, compressed air systems are often neglected, and if poorly maintained, become inefficient due to leakage and other factors, resulting in breakdowns with implications such as costly line stops and extended down times while unscheduled maintenance takes place.

Wastage

Compressed air systems are in most cases operated by electricity from the national grid. It’s estimated that around 3% of the energy consumption of the European industrial sector as a whole is used on compressed air. However, companies are rarely aware of the cost of compressed air, and this is understandable since it is usually produced on site and not by an external supplier. Monitoring the costs involved is therefore not an easy task.

Alarmingly, studies have indicated that in an average manufacturing plant, only around half of the compressed air generated is actually consumed by the normal production processes for which it is provided. Up to 30% is lost in leaks, with another 15 to 20% consumed in non-manufacturing uses such as handheld blowguns for cleaning, and even cooling for workers in the plant or factory. The remaining air is accounted for by what is often termed ‘artificial demand,’ caused by operating the system at excessively high pressure. Use of equipment such as Parker’s Air Saver Unit can reduce air consumption by up to 50% in such applications through pulsed air technology to produce high speed on and off pulses resulting in fewer tank recharging cycles.

Looking at the issue from a cost point-of-view makes the issue even more of a concern for business owners with an eye on costs. For every pound spent on electricity to generate compressed air – for example, powering the compressor – only about 12 to 17 pence is actually used productively. These figures only take into account direct energy costs, and do not reflect the cost of capital equipment investment and maintenance.

A closer look

It is natural, and usually the case, that people tend to consider air systems in relation to the direction the air flows; that is from the compressor out to the end piece of pneumatically operated equipment. However, it is important to understand that the demand side of any system determines what needs to happen on the supply side.

Stopping leaks is the obvious first step in improving the efficiency of a compressed air system and enjoying immediate cost savings. A managed and effective leak identification and correction program should be an integral part of any compressed air energy management program. However, while leak detection and management is important, there are numerous other areas to address; it is a fact that many compressed air audit programs only take a cursory look beyond leaks.

After leaks, the next biggest potential improvements are to be yielded from reducing or eliminating inappropriate uses of compressed air and identifying and stopping the artificial demand caused by operating the system at excessive pressure. 

Reverse engineering

Taking a reverse engineering approach to highlight and deal with inefficiencies in a compressed air system is a good approach once an ongoing leak detection program has been implemented and the work to deal with inappropriate use and over-pressurizing of the system is done. Reverse engineering can help highlight, check and then allow the addressing of the following factors: 

1.  Correct sizing of pneumatic piping and connections from the main header to the inlet of the equipment to minimise pressure drop. Best design practices include minimising the number of angle fittings and connections to allow an uninterrupted flow and reduce the opportunity for leaks and pressure drops.

2.  Use of appropriately sized air treatment components, such as filters, regulators and lubricators, can also help reduce the pressure drop. Cutting the total cost of ownership over the life of the equipment may far outweigh the acquisition cost of properly sized equipment.

3.  In automation applications using valves, cylinders and actuators, the proper use of reverse flow regulators and dual pressure circuits can reduce air consumption while also increasing efficiency. Most actuators such as cylinders perform work in one direction only, and the return stroke is merely to allow repositioning for the next cycle. Performing work extending a cylinder or proper operation, and then retracting the cylinder at a reduced pressure is a strategy that can result in significant energy savings and therefore reduced cost of operation. By reducing from 6 to 4 bar, the pressure for retracting the cylinder can save 30% of air consumption for this movement (and 45% reducing from 6 to 3 bar).

4.  Appropriate control and regulation of air consumption devices such as air knives, pneumatic motors, diaphragm pumps and compressed air venturi-type vacuum generators. When operated in an unregulated or uncontrolled manner, these devices can be large consumers of high-pressure compressed air. As well as being wasteful, this places additional stress on the system that may result in premature failure or reliability issues. Installing simple sensing circuits that turn the air off when the machine is idle or parts are not present can deliver significant savings.

5.  The design and implementation of directional control valves, with wear compensated seal technology on the spools and solenoid-controlled pilot-operated technology, instead of lapped-spool and sleeve design using direct-solenoid operators, can reduce energy consumption and increase reliability. In a large manufacturing plant utilising thousands of directional valves, the energy savings can be very significant indeed and the risk of breakdown and costly downtime significantly reduced.

After compressed air ‘demand side’ opportunities have been identified and acted upon, then a similar analytical look can be taken at the supply side to optimise potential savings there.

Example:

In a machine with a 100mm bore cylinder and a 160mm stroke with a 25mm diameter rod, the system operating pressure extends and retracts the cylinder at 7 bar. To go through the reverse-engineering steps above, the first improvement that could be made would be following step two, by installing a regulator in the inlet supply line and reducing the operating pressure to 5 bar, on both extend and retract. Step three would involve installing reverse flow regulators in the lines between the directional valve and the cylinder, thereby reducing the retract pressure from 5 bar to 4 bar. 

On the fourth step, we can bring the retract pressure down from 4 bar to 3 bar. By implementing the fifth step in a trial and error process, the extend pressure could be further reduced from 5 bar to 4 bar. The resultant combined savings could exceed 30% on the plant’s annual electricity cost associated with running air compressors.

Taking time and allocating resources to optimise the design and performance of compressed air systems is becoming increasingly important as energy costs continue to rise.  The impact on profitability of not taking steps can be significant and have a direct effect on the bottom line of any business. Achieving long-term energy savings and high system efficiency with reliability will be enjoyed if the steps described above are implemented and sustained. An ongoing plan as opposed to a one-time, short-term approach will normally achieve excellent payback.