The defining of the causes of global warming, and the measures that have been taken to reduce it, have been issues of worldwide concern and interest for some years. Of all the causal factors, contributing to global warming, energy consumption is by far the largest. The greenhouse effect that leads to global warming is triggered by the release of gases into the atmosphere. 60% of these contributing gases is carbon dioxide (CO2). Reduction of CO2 emissions is now paramount in the worldwide effort to stem global warming.
Japan is the largest generator of CO2, being double that of transportation. Given this situation, Japan established its 'Regulations of increasing efficiency in energy consumption' laws. The laws prescribe energy savings that were set for automobiles, electrical appliances and office automation equipment, as well as the introduction of the world- class design method, with its associated obligation to for industry to improve energy efficiency. As they are proud of their performance, some companies have started to release environmental data such as life cycle assessment (LCA) together with an 'environmental account' which details the cost and effect of energy saving initiatives in monetary terms.
Energy savings in pneumatic systems
Taking a typical factory as an example, and examining the factors contributing to energy consumption it is found that electricity is by far the largest, with air compression a major component - accounting for 16% of the total energy consumption. Pneumatics has a significant impact on energy consumption and therefore can play a significant role in the reduction of energy consumption.
To achieve the best possible savings, efficiency should start with 'energy minimisation', followed by 'energy conservation'. Energy minimisation is based on constructing the production system in such a way that the required volume of energy is consumed only when required. Having achieved this, energy conservation is made possible by increasing the energy efficiency throughout each of the production processes. We can effectively review energy consumption by classifying it into the two categories of energy minimisation and energy conservation.
'Total energy minimisation' is the preferred method by which to evaluate energy saving measures. It is a concept of minimising energy consumption in all processes of manufacture, operation, repair and recycle/disposal through LCA. Energy consumed in manufacture, repair and recycle/disposal is directly related to the initial cost of acquisition and installation while energy consumed in operation is directly related to the running cost. It is therefore preferred to adopt measures whereby the total cost (initial cost + running costs,) becomes the minimum possible.
Key factors in energy saving
Key energy minimisation measures are those that should be addressed first, with priority given to operations that place significant demands on the air supply system. This could involve the reduction or elimination of leakage and the use of air blowing applications in a more efficient manner. The basic operations of all pneumatic cylinders and associated components must be critically examined in any energy minimisation program.
Compressed air supply - the cost of compressed air
While compressed air is universally recognised as a major energy source in industry, it is not well recognised as a serious energy cost. It is important therefore to highlight the real cost of compressed air in a system. Around 80% of this cost is directly attributable to power.
Air consumption in a factory fluctuates considerably over time. In a system with a wide load fluctuation, a single compressor installation may prove to be inefficient during periods of very low demand. A more efficient energy solution could be multiple compressors with sequencing control that matches compressor availability and capability with the specific demands of the system.
Reduction of compressed air energy consumption
The lowering of air system pressure will decrease energy usage. This is a basic means of energy saving in a factory, but the reduction of pressure from a control plant may not suit the pressure requirements of particular applications. A new factory, however, can be designed with all processes working at lower, more energy efficient pressures.
Supply piping
A pressure drop always occurs during the transmission of air from the compressor to the point of use. Cost of pressure drop fails as pipe diameter increases. Piping costs also rise as a pipe diameter is increased to reduce pressure drop. A compromise between pressure drop and pipe diameter costs needed for achieving 'cost efficient piping'.
Large and complex systems make it difficult to measure flow rate and pressure at any given point, but the addition of new equipment complicates the system even further. Piping network designs that address issues such as optimum pipe sizing and the selection, location and effective operation of air compressor plant - with provision for future expansion - will.
Air leakage accounts for between 10% and 20% of air consumption in a typical factory - and should be considered as energy wastage. Investigating the leakage in a factory may well reveal that leakage often exists in each component as well as fitting and piping. In a factory using 1000 cylinders, around 20 000 leak points would exist throughout the installation. Therefore, in addition to an efficient piping system, other measures, such as reducing the allowable leak rate of system components, together with detection and monitoring of leaks and flow requirements throughout the system, can provide significant savings.
Air blowing systems
Often overlooked is the fact that, in some compressed air applications, compressed air is run continuously - despite the air consumption in these applications exceeding the air consumption of the actuators. Air-blow applications are affected by supplying an air jet directly to the work piece using a nozzle attached to a machine, or a manual air gun. In some current production systems however, the air continues to blow regardless of the existence of work to be performed. It is also common for nozzles to be larger than necessary, leading to increased energy costs. The criteria for efficient air-blow are itemised in the following sections.
It is difficult however to quantitatively compare the characteristics of each type of air nozzle. Therefore, relative judging criteria has to be established based on the free jet from a single hole nozzle. The pressure distribution in a free jet can be established by a traversing measurement. As the pressure before the nozzle decreases, the delivered flow will increase. In the choked flow region, the reverse will occur. It can be seen that the airflow rate decreases if the nozzle pressure is set higher than the critical pressure when certain blowing work is performed. By decreasing the nozzle diameter, the flow rate can be reduced by 70% whilst maintaining the same impact pressure.
Air supply circuit design
Many instances exist where air consumption is several times greater than is necessary because the pressure before the nozzle is too low and the nozzle air supply circuit is incorrectly designed and installed. The placement of a pressure regulator is essential for setting the pressure before the nozzle to achieve optimum air blow effect and air consumption rate. To achieve 'energy minimisation', a 2-port valve should be used to control the flow of blow air to the nozzle. Air is supplied only when required by the production process and excess air consumption is prevented. In this tubing system, it is preferable that subsonic flow, providing a lower pressure drop, be used and that flow chokes at the nozzle. The relationship between the conductance ratio (area of flow path at pressure source (mm²) vs area of orifice at nozzle (mm²)) and the pressure ratio (pressure and supply/pressure at discharge orifice.) If system component sizing is selected by setting the total conductance (effective area mm²) of upstream tubing to more than twice the nozzle diameter, then the pressure drop ratio will be controlled to within 5%. (To be continued)
For more information contact Hyflo SA, +27 (0) 11 386 5878, [email protected]
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