Nowadays many existing CASs lack an updated overall design. The implementation of additional compressors and various applications in several stages along the installation lifetime without a parallel redesign from the original system have frequently resulted in a suboptimal performance of a CAS.
One fundamental parameter in a CAS is the pressure value. A number of pressure demands, depending on the application, usually sets up a trade-off between low pressures giving a higher energy efficiency and high pressures where smaller and cheaper devices can be used. The majority of consumers use a pressure of about 6 bar(g), but there are requirements for pressures of up to 13 bar(g). Often the pressure is chosen to meet the maximum pressure needed for all devices.
It is important to consider that too low a pressure will cause malfunctioning of some machines, while a pressure higher than necessary will not, but will result in reduced efficiency. In many cases, there is an 8 or 10 bar(g) system pressure, but most of the air is throttled to 6 bar(g) by pressure reducing valves.
It is state-of-the-art to choose a pressure which satisfies 95 % of all needs and uses a small pressure-increasing device for the rest. Operators try to eliminate the devices needing more than 6 bar(g), or having two systems with different pressures, one with a higher pressure and one for 6.5 bar(g).
Another basic parameter is the choice of the storage volume. As compressed air demand typically comes from many different devices, mostly working intermittently, there are fluctuations in air demand. A storage volume helps to reduce the pressure demand fluctuations and to fill short-timing peak demands (see Section 3.7.10).
Smoothed demand allows a steadier running of smaller compressors, with less idling time and thus less electric energy is needed. Systems may have more than one air receiver. Strategically locating air receivers near sources of high short-timing demands can also be effective, meeting peak demand of devices and making it possible to lower system pressures.
A third fundamental design issue for a compressed air system is dimensioning the pipework and positioning the compressors. Any type of obstruction, restriction or roughness in the system will cause resistance to the airflow and will cause the pressure to drop, as will long pipe runs. In the distribution system, the highest pressure drops are usually found at the points of use, including undersized hoses, tubes, push-fit connectors, filters, re gulators and lubricators. Also, the use of welded pipework may reduce frictional losses.
Sometimes the air demand has grown 'organically' over the years and a former side branch of the pipework – with a small diameter – has to transfer a higher volume flow, resulting in pressure loss. In some cases, plant equipment is no longer used. The airflow to this unused equipment should be stopped as far back in the distribution system as possible without affecting operating equipment.
A properly designed system should have a pressure loss of less than 10 % of the compressor’s discharge pressure to the point of use. This can be reached by: regular pressure loss monitoring, selecting dryers, filters, hoses and push-fit connectors having a low pressure drop for the rated conditions, reducing the distance the air travels through the distribution system and recalculating the pipe diameters if there are new air demands.
What is often summed up under the point 'overall system design' is actually the design function of the use of compressed air. This can lead to inappropriate use, for example, over- pressurisation followed by expansion to reach the proper pressure, but these situations are rare. In industry nowadays, most people are aware of compressed air as a significant cost factor.
No data submitted.
There are many compressed air systems, with estimates as high as 50 % of all systems, that could be improved by a revision of their overall design, with a gain of 9 % by lowering the pressure and with better tank dimensioning (in 50 % of systems) and 3 % by lowering pipework pressure losses (in 50 % of systems) resulting in 6 % = 0.5 x (0.09 + 0.03) energy savings.
System design may also include the optimisation of certain end use devices, typically in 5 % of all systems it is possible to lower the demand by some 40 %, resulting in 2 % (i.e. 0.05 x 0.4) energy savings.
估計大約有50%的壓縮空氣系統可以經修改其整體設計而得到改善，其中9%來自將低壓力即將加的儲氣筒的大小，3%來自減少管路壓損，而得到6%的節能[ 50% x( 9% +3%)=6%]
系統設計也包括優化一些終端使用設備，在所有系統的5%可以減少40%的需用量，這可以有2%的節能[ 5% x 40% =2%]。
The costs of revising a compressed air system with consequent readjustment of pressure and renewing pipework is not easy to calculate and depends very much on the circumstances of the particular plant. The savings in a medium size system of 50 kW can be estimated to be:
50 kW x 3000 h/yr x EUR 0.08/kW x 10 % = EUR 1200/yr
The costs for a major revision in such a system, adding a 90 litre tank near a critical consumer and a shut-off valve for a sparsely used branch, replacing 20 metres of pipework, 10 hoses and disconnectors is about E UR 2000, so the payback period is a profitable 1.7 years. Often the costs are lower, when only some pressure readjustment needs to be done, but in every case there has to be thorough considerations about the lowest tolerable pressure meeting the needs.
Economics are a driving force to revise compressed air systems. A major obstacle is a lack of knowledge and/or of skilled staff responsible for compressed air systems. Technical staff may be aware that the compressed air is expensive, but the inefficiencies are not readily obvious, and the operator may lack staff with sufficient in-depth experience.
Initiatives in many countries of the EU for spreading compressed air knowledge strongly promoted the implementation, creating a 'win-win-win' situation: the owner of the compressed air systems wins lower overall costs, the supplier of compressors and other devices wins higher revenues and the environment wins lower power station emissions.
Energy Efficiency (2009) 3.7.1