Historically when energy costs and environmental awareness were lower, close attention wasn’t always paid to the air consumption on machinery. Designers typically selected pneumatic actuators based on a range of ad hoc criteria: what they had used before, what was on the shelf, “it looks about right”, looking at a simple pressure/force graph and up sizing by one diameter “just to be safe”. This means the actuators on many older machines are over specified and it is perfectly possible and correct to turn down the operating pressure and reduce the redundancy and wastage.
Compressed air on machines is not only consumed for driving pneumatic cylinders. Other typical areas include:
Each of these require individual consideration for optimised performance and air consumption.
Pneumatic vacuum generators utilise a venturi nozzle based on the Bernoulli principle to create vacuum by accelerating the air flow through a constricting nozzle. Nozzles within the generators are designed for individual operation or as elements within a multi-stage generator. Nozzles are specifically designed to achieve high levels of vacuum, high flows for short evacuation times or a general-purpose combination of both. Passing compressed air through a nozzle and straight to atmosphere is potentially a very high consumption element. Catalogue data will identify the optimum supply pressure to achieve the flow and negative pressure required. Often vacuum generators are designed to provide the highest vacuum level at only 4 bar. Increasing the pressure to 6 bar does not draw a deeper vacuum level although the flow will increase.
Air jets are used for ejecting parts from conveyor belts and housings. There are different ways to optimise the energy usage – either by concentrating the flow through nozzles or using a reservoir to increase the stored energy but only ‘blipping’ the pulse for a very short time.
Air blow guns are commonly used for cleaning down and removing dust and particles. As well as being potentially dangerous they are also another high consumption element. Modern air blow guns are equipped with an in-built regulator dropping pressure to only 2 bar and have a safety tip with relief ports.
Leaks need to be eliminated wherever possible but clearly the higher the pressure, the bigger the losses.
Dead volumes in pneumatic systems such as filling and exhausting long or large diameter pipe runs for every cylinder cycle are another cause of energy wastage. Mounting the valves closer to the actuators shortens pipe run lengths. Reducing pipe diameters can also help but will also restrict pressurisation and to a greater extent exhaust times, therefore they should be simulated or calculated to achieve the required performance.
Energy losses also occur due to non-laminar and turbulent flows. Pipe friction and eddies due to tight radii or kinks or supersonic flows can all create further efficiency losses within pneumatic systems and are affected by the operating pressure.
On new machine designs and builds reducing the operating pressure from say 6 bar to 4 bar may not actually reduce air consumption. If parts are correctly sized for the application – and this includes not only the actuator but also the tubing runs, fittings and air supply unit etc. then it may be better to stay at a higher pressure. For example, if you design a machine to operate at 4 bar pressure operation it will require larger diameter cylinders to obtain the same force as a 6 bar machine, the tubing will be a larger bore to achieve the flow at a lower pressure differential. Larger actuators, mountings, tubing and fittings will usually cost more. In addition, larger cylinders will add weight to the machine (critical if it is adding to the payload of other actuators), and take up more space.
Hmm… maybe this doesn’t sound such a good thing?
If all the machines and devices connected to a compressor system can operate at a reduced air pressure, not only can the local pressures be reduced but it’s possible to reduce the whole network pressure. Compressor systems may be optimised for a given pressure/flow demand and this must be considered. Lower network pressures mean less losses from leaks.
Air networks need to be able to cope with peak demands and fluctuations, educing pressures and the peak flow capacity whilst potentially saving energy could cause pressure drops and lost production.
The best approach is to engage with specialist applications engineers that can support machine optimisation according to all your priorities – machine performance, build costs, lifetime costs, energy efficiency etc. We would also strongly recommend looking at the whole solution not just one function at a time. Even narrowing the discussion down to replacing air actuators with electric drives can be a false generalisation. For example, does it make sense to use a sophisticated electric gripper on the end of a pick and place system, if a pneumatic one can do the job? Unlike electric grippers, pneumatic grippers do not consume energy when exerting a constant force gripping a component. In most cases you can save 50-60% of the mass. This is critical at the ‘end-of-arm as it enables you to reduce the size of all the supporting actuators – rotary, X-Y and Z. The savings on the overall system can be dramatic in purchase and operating costs.
The recommendation is to focus on what you want to achieve and let the Festo specialists support you in the configuration of your machine. They will do this with engineering software that will provide you with the tools, data and documentation to make informed decisions on a component and system level
One of the most effective ways to save money on compressed air is to ensure the correct sizing of the complete ‘pneumatic chain’ from compressor and dryer/receiver to distribution and the working elements such as the air preparation units, valves, tubing, fittings and actuators. Savings of 50% in energy usage/energy consumption are achievable and can be modelled using free of charge software like the Festo Pneumatic Sizing tool Pneumatic sizing | Festo GB
Typical compressed air systems include:
1) Pneumatic valves switching pneumatic actuators such as cylinders, gripper and clamps
2) Vacuum generators based on the Venturi and Bernoulli principles produce a vacuum by passing compressed air flow through a nozzle. This is then used with vacuum cups for picking and placing applications or aspirating liquids.
3) Air eject systems for moving or ejecting parts.
4) Air jets for drying
5) Air for providing a ‘head’ pressure over liquids for dispensing or pumping
Air pressure can be reduced throughout the network, meaning the compressors will generate a lower pressure or cut out when a lower pressure is reached. The air receiver and distribution network can be set to operate at a lower pressure, typically reducing from 7.5 bar to 6 or even 5 bar. If a higher pressure is required at one or more individual points of use, then a pressure booster such as the Festo type DPA can be used locally. Where redundancy has previously been designed into a compressed air system, reducing the operating air pressure directly reduces the energy consumption.
Pneumatic systems utilising compressed air have several key advantages:
The quick maintenance wins with pneumatic systems are to maintain or replace air filters within air preparation sets regularly, to test or listen (either audibly or preferably ultra-sonically) for leaks and fix immediately, and to check there are no kinks or restriction in tubing runs. These fast and easy measures can reduce costs typically by 5-10%.