Part 1 introduced the link between power consumption and greenhouse gas emission. Improved efficiencies both save power and cut emissions. We ended by discussing air supply circuit design and continue with vacuum ejectors:
A popular trend in air blowing systems is the use of a rubber pad connected to a vacuum ejector for the pick up and transfer of work pieces. (A vacuum ejector uses a flow of compressed air through a venturi to generate a vacuum.) Either this method or a system using a vacuum pump can be adopted. Comparing the energy cost of each system can be difficult. Comparative tests indicate that the vacuum pump approach can exhibit up to three times the energy conversion efficiency of a vacuum ejector in continuous operating conditions. However, in the vacuum pump system, `energy minimisation' becomes a problem when deciding how to adapt electric power to the fluctuating load cycles. On the other hand, in a vacuum ejector system it is easy to achieve `energy minimisation' since air-blow occurs only when necessary.
Pneumatic cylinder systems
The energy efficiency of a pneumatic driving system compared with that of hydraulic or electric driving systems may appear very poor on the surface, but it must be borne in mind that such comparisons are based on power conversion efficiency under continuous operation. It must be remembered that cylinder operations are intermittent and power is only consumed during movement and is therefore essentially 'energy minimising' in practice. From the point of view of LCA (life cycle assessment), attention to total energy consumption is essential in establishing an evaluation method. The following section gives samples of approaches to model selection and the setting of operating conditions.
Minimising energy needs: the concept
The load, stroke and operation time (application of mechanical energy) that is demanded of a cylinder determines its power consumption. The operating energy in LCA is made up of the air consumption of the cylinder and associated components in the pneumatic circuit (Yr). Energy used in production, the costs for repair and disposal of the mechanical components, is equivalent to the total equipment cost (Yi). Total costs, including energy, can be expressed as the sum total (Yr + Yi) and the graphs of these two values taken over a range of pressures will add up to a minimum total value at a certain supply air pressure. This point corresponds to the minimum energy requirements and gives the optimum index of the cylinder driving systems.
An example of optimum index
For simplification, the initial cost of the driving circuit may be expressed as conductance for solenoid valves and running cost expressed as air consumption. With any given cylinder bore size, air consumption increases with supply pressure and conductance descends with increasing supply pressure. In low-pressure conditions, air consumption is less but a larger conductance is required. The reverse applies in high-pressure conditions. It can be seen that the optimum driving condition occurs around the intersection of the two curves.
Obtaining the optimum index
In a typical example, with air supplied at pressure is 0,4 MPa to a 63 mm cylinder, the system is at the optimum index. For a 50 mm cylinder with 0,5 MPa air pressure, the system is also at the optimum index. As supply pressure rises, the cylinders used become smaller, valve conductance and air consumption decreases while load ratios increase.
Model selection
To enable correct selection of the components for a pneumatic system, an accurate model selection program has been developed. It allows the user to simulate the dynamic characteristics of components within a pneumatic system including tube and fittings. The input screen comprises of the circuit configuration and application data, for example a 50 kg load moving a distance of 400 mm stroke in a time of 1 second. The output screen would show a combination of suitable cylinder models and solenoid valves for the task. This is a combination of minimum units corresponding to the most effective use of energy. The software also displays the response curve and main characteristics of the system.
Air delivery efficiency
Equally important to the overall system are the tubes and fittings that connect the solenoid valves to the cylinders. When tubing becomes larger and longer, its running cost increases. Since tubing has both conductance and capacitance, there is an optimum tube diameter for a given cylinder response.
The right pressure in the right place
With a pneumatic system possessing constant pressure source it is easy to set the optimum power level for each unit by using pressure and booster regulators. It is possible to have a dual pressure driving system for each actuator. In such a system, air consumption can be reduced by 25% through setting the return stroke to a low pressure.
In an example of an energy saving lifter that raises and lowers a heavy load, compressed air in the lower chamber of the cylinder is balanced with the load and will not be exhausted - it merely extends the piston rod to tilt the load and then returns to the tank where pressure is applied to the retraction side of the cylinder. By using a regulator to supply a reduced pressure to the upper chamber of the cylinder, air consumption can be reduced by 75%, due to exhausting occurring just once in a single cycle.
A reduction in air consumption can be achieved by using a mains air shut-off driving system. Exhaust flow from a cylinder may be used to provide a low-pressure air source. In these systems, the study of the actuator's performance and evaluation of the total energy will play an important role.
Conclusion
Essentially, pneumatic systems used in the production lines of industry are energy transmission systems. Therefore, important tasks for the future will be firstly, the study of the system from an environmental and LCA perspective and secondly, the development of pneumatic components and circuitry which realise energy savings. Another aspect of the future direction of pneumatics will be the change from the development of flexible actuator controls to more focused agile actuator controls, to meet the specific needs of industry applications and customer requirements.
For more information contact Hyflo SA, +27 (0) 11 386 5878, [email protected]
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