Pneumatic products, systems, and machines require clean compressed air to operate efficiently and reliably.
By Michael Guelker, Festo Product Manager, Pneumatic Actuators and Air Supply products
Particles, water and oils in compressed air reduce the service life and functionality of components and systems. They also impair productivity and energy efficiency. Being aware of how to achieve proper compressed air quality—and the filters which are needed to achieve the recommended air quality for common industrial applications—is critical to a well-run plant.
There are three variables to air preparation: compressed air purity, compressed air quantity (flow) and air pressure. Depending on the system requirements, coordinating these three variables ensures a high compressed air quality and forms the basis for selecting the proper service unit components.
The required compressed air purity increases both the running performance and the efficiency of pneumatic systems and can ensure compliance with legal specifications in industries, such as the food and beverage industry. The flow quantity is largely determined by flow cross sections and the design dimensions of the machine.
In general, provided the design is the same, larger components have higher flow rates. Optimized operating pressure increases the efficiency, minimizes wear and reduces power consumption. To coordinate the compressed air purity, compressed air quantity, and pressure for the specifications of a system, the correct individual components must be selected. These components include on-off valves, pressure build-up valves, pressure regulators, water separators, filters and drying units.
Why filtering is necessary
Atmospheric Air contains dust and dirt particles, not to mention considerable amounts of moisture in the form of humidity. The air produced by a compressor will be hot, wet and dirty. So the first step is to filter out these contaminants, starting with removing the moisture. This is typically achieved with a refrigerated dryer next to the compressor, which simply cools the air to just above freezing (~3˚ C) and removes the liquid that condenses.
If the contaminanats are not filtered out, trouble-free operation of the system components, like valves and cylinders, cannot be guaranteed in the long term. Poorly-prepared compressed air can cause seals to swell and wear prematurely, and contaminate control valves. As a result, the right compressed air preparation is essential for reducing machine downtime and idle periods, and for reducing maintenance and energy costs.
To help everyone communicate in the same language regarding air quality, international standard ISO 8573 was established in 2010 with definitions for compressed air quality. The air quality is defined with Class ratings for three types of contaminants: solid particles, water condensate content, and oil content. The Classes range from 1-9, with the lower numbers representing higher air purity. It specifies the maximum permissible levels of contamination and particle sizes for the respective quality classes. The Air Quality Class will help you identify what type of filter, or combination of filters, is needed.
Best Practices—filtration for common applications
Following are some common applications with the corresponding Air Quality Class, and the type of filtration.
Standard automation components such as valves and cylinders
• Quality Class 7.4.4—40 µm filter
Proportional valves, compact valves, pneumatic tools
• Quality Class 6.4.4—5 µm filter
Primary packaging, reduction of odors and oil vapors
• Quality Class 1.4.1—5, 1 and 0.01 µm, and active carbon filters
Semiconductor industry, pharmaceutical products
• Quality Class 1.3.1—5, 1 and 0.01 µm, and active carbon filters, membrane dryer
The required Air Quality Class or level of filtration is often defined by standard Industry guidelines or best practices. Most manufacturers of air preparation equipment can provide reference documentation that define the air quality requirements for different applications and industries. Once you have identified the ideal air purity class for the application, then choose the filters and dryers that are most appropriate. The following provides filter and dryer guidelines.
Filter types and when to use them
There are different types of filter components for removing contaminants such as solid particles, liquid water and water vapor, and oil vapors, as well as odorants and even bacteria and viruses. For most automation applications, the focus is to remove solid particles and water.
Water separators remove condensate, either with a centrifugal design or a coalescing principle.
• A centrifugal separator causes rotary motion in the air, forcing particles to accelerate in a radial outward movement. Once they reach the outside, they drain into the bowl. Centrifugal separators are effective for removing water droplets and dust and dirt particles >5 microns. No maintenance is required for this process.
• A coalescing separator flows the air from the inside to the outside of the filter element. These filter cartridges must be replaced regularly.
Filters are used to remove particles, condensate and oil.
• Coarse/particulate filters have a pore size of 5 to 40 µm. The air flows past a centrifugal separator and then through the filter element. The filter elements are often a sintered material, like polyethylene or bronze.
• Fine and micro filters remove particles smaller than 1 µm, 0.01 µm. The air flows through the filter cartridges from the inside to the outside. Solid particles get stuck in the filter cartridge, clogging it up. Fluid particles, such as condensate or oil, coalesce or attach to larger droplets, which flow off and are caught in the filter bowl. It is important to cascade your filters to avoid premature clogging of the filter element. For example, if 1- µm filtration is needed, we recommend a 5 µm filter upstream so that the 1- µm filter does not become clogged with larger particles.
• Activated carbon filters bond hydrocarbon residue, odorants and oil vapors. They are used to remove odors, in applications such as food packaging and handling pharmaceutical products.
• Sterile filters ensure that the air is free of germs.
Dryers are used to remove water vapors beyond the capability of the fine and micro coalescing filters, and are classified according to the pressure dew point that can be achieved. The pressure dew point defines the temperature to which compressed air can be cooled without the water in it condensing. If the temperature is below the pressure dew point, condensate will form. Even if the temperature is subsequently increased, this condensate will remain and can lead to corrosion of components.
• Refrigeration dryers are commonly located downstream from the plant’s air compressor. The air is cooled to just above freezing in a cooling unit, and the condensate that falls off is drained away. The pressure dew point achieved is around 3° C. To avoid condensation, it is recommended that the pressure dew point needs to be 10° C below the ambient temperature. A refrigeration dryer is sufficient for systems whose operating temperature never drops below 13° C.
• Membrane dryers lower the pressure dew point, for example by 20° C. The air flows longitudinally through a bundle of parallel, hollow fibers. During this process, water vapor diffuses because of a partial pressure drop from the inside of the fibers to the outside. The vapor is drained out using purge air. Due to the purge air, the maintenance-free membrane dryer has a certain amount of constant bleed/air consumption.
• Adsorption dryers are used when pressure dew points of –40° to –70° C are required. The dryers use molecular forces to bond gas or vapor molecules to a drying agent, such as desiccant beads. Since the drying agent is regenerative, two chambers are required: while drying takes place in one, in the other the drying agent has time for cold or warm regeneration. In devices with cold regeneration, some of the dried air is used to dry the adhesion agent. When warm regeneration is used, the water evaporates as heat is applied. The drying agent must be replaced periodically, for example after 8,000 hours of service.
There are a few different types of drains available for filter units:
• Manual‑condensate—is drained manually by twisting the drain plug. These require a regular maintenance schedule, for example once per shift.
• Semi-automatic/normally open—opens as soon as the compressed air is shut off.
• Fully-automatic/normally open—opens as soon as the compressed air is shut off or a specified level is reached in the bowl.
• Fully-automatic/normally closed—opens as soon as the compressed air is switched on and a specified level is reached in the bowl.
• Electric drains—are available that can be opened/closed remotely with an electrical signal.
Images of a Centrifugal Water Separator, Particulate Filter, Coalescing Filter:
When to replace a filter element
To maintain efficient operation of filters, the filter elements need to be replaced periodically. How frequently this needs to be done depends on variables like the quality of the supply air and the hours of operation of the machine. You can identify a preventive maintenance schedule—for example, to replace filter elements every 6 months. A more reliable method is to use differential pressure sensors, which measure the pressure drop between a filter’s supply and output pressure, which indicate when the filter is becoming clogged. These can be electrical sensors that send a signal to a PLC, which can then alert the operator. They can also be visual indicators on the filter unit itself, where the indicator may show a green color when the filter element is clean and a red color when the filter element is clogging up and needs to be replaced.
Most pnuematics companies can offer completely assembled and tested air preparation assemblies, which makes ordering and installation of air quality systems fast and easy. Idenfity the required filtration or classes required and work with a trusted supplier to ensure higher air quailty and achieve lowest total cost of ownership.