What Kind of Pneumatic Filter Do I Need?

A variety of contaminants may be introduced to the compressed air supply at different points in the system. These contaminants may already be present in the atmospheric environment from which the air compressor draws upon (e.g. water vapour). Alternatively, they may be from the fabric of the compressor or air receiver, themselves. Or, they may be introduced at exposure points along the tubing or around fittings. Over time, with ordinary use, dirt and even microorganisms may enter the system through various means. Contaminants harmful to pneumatic systems include water vapour, oil in aerosol or vapour form, dirt, micro-organisms, pipe scale and rust.

Pneumatic filters reduce these sorts of contaminants that are present in the air line. However, there are a number of different filters, each for a different purpose, and, moreover, their placement or sequence in the air supply also makes a difference. In this article, we will outline the kinds of pneumatic filters that are needed for different applications: e.g. air tool usage, general manufacturing, food preparation, semiconductors and electronics.

1. Why Do I Need a Pneumatic Filter

There are three major reasons why contaminants ought to be eliminated:

• decreased performance (e.g. sticking actuation, pressure drop)
• decreased reliability (e.g. broken seals, jamming of spools)
• increased operational costs.

Regarding the first, contaminants can interfere with the ordinary function of various pneumatic components; for example, contaminants can cause additional side loading in a pneumatic cylinder if it interferes with the piston movement. One of the most common effects of a contaminant-heavy system is that seals in both actuators and valves can become dislodged and extruded. This causes pressure leaks, resulting in decreased force output and loss of compressed air, which over a years’ time, can contribute to significant energy costs. Particles may also eventually make its way into many valve bodies. In the case of spool valves, such as most pneumatic directional control valves, particles may interfere with the movement of the aluminium spool within valve. Water vapour may cause unnecessarily corrosion and increase friction along many internal moving parts. Unwanted oils which contain hydrocarbons can further clog up the system and leak to contaminate sensitive treatment objects such as food or semiconductors.

2. Different Types of Pneumatic Filters

2.1. Particulate Filters

As their name suggests, particulate filters are used to prevent dust, dirt, and other particulates from entering the airline (or, at the very least, from passing through the filter). Their most common made of construction are with the use of a filter element that physically blocks particulates of a certain size from passing through. Common particulate filters in pneumatics are 5 microns or 40 microns, meaning, they filter out particulates that are larger than 5 or 40 microns. (In the past, it was common to speak of particulates being up to 5 or 40 micrometres instead).

However, as the filter element becomes clogged up, there are less pores for compressed air to pass through. This will result in a pressure drop. So, one tell-tale sign of a filter that has been well-used is that it presents a significant pressure drop at the output compared to the input. But this is somewhat good news as it means that the filter has prevented many contaminants from entering the air line.

Importantly, it is recommended that a 40-micron filter in sequence, upstream before a 5 micron filter. Whilst a 5-micron filter will filter out particulates that are larger than 40 microns, it could result in premature usage of the 5-micron filter element and unwanted pressure drops. Pneumatic system engineers often place a coarse-grained filter before a fine-grained one. One benefit of this of this two-phase filtration process (aka cascading filters) is that the more expensive 5-micron filter will last longer, and there is less unwanted pressure drop.

2.2 Water Separators

Water separators are the primary type of coalescing filters. Water vapour can be harmful to pneumatic components by causing corrosion. Water separators operate by causing the flow to occur in a rotary motion within a specialised chamber. As this process occurs, water molecules get larger and then drain into a bowl that is either self-draining or auto draining.

Due to laws of physics, water vapour is best removed when temperature is low and pressure is highest (optimal dew points).

2.3. Coalescing Filters

Coalescing filters are defined by its design and the method in which contaminants are filtered out. In this case, unwanted molecules coalesce, or physically stick together, and then, due to their increase in mass, fall to the base of the drawn bowl. Particles coalesce physically in three manners: (i) diffusion through a filter and then agglomeration into larger molecules, (ii) interception (0.2 to 2 microns) or when particles pass through the filter where the inertia forces are overcome and interception by a larger molecule occurs, and (iii) direct impact, or when a particle (2 microns or above) with sufficient mass directly impact on fibres.  (Barber, 1989; The Pneumatic Handbook).

Most coalescing filters in pneumatics are designed to filter out oil, but, in principle, they can also filter out water molecules.

2.4 Adsorbing Filters (in contrast to Absorbing)

Oil separators can also be the adsorption type. Adsorption filters physically cause liquids to move to a region within the filter (usually, the base) and activated carbon material adsorbs the oil. These filters are designed to remove vapours from the air line that cannot be removed by a coalescing filter. Adsorption filters are critical for food and beverage industry, electronics, and semiconductors.

2.5 All-In-One Filter Assemblies

It is common for pneumatic component manufactures, like Knocks or Mindman, offer and all-in-one filter assembly that combines various filters together, and in the right sequence, for optimal results. Please do not confuse these with pneumatic ‘FRLs’, as they do not regulate pressure or lubricators, but are filter assemblies that has the primary function of removing contaminants from the air supply.

KNOCKS FDO.23 G MAN FRL SERVICE UNIT ASSEMBLY

 3. Different Filters for Different Applications

Strictly-speaking, the more contaminants you can remove or prevent from entering the system the better. However, there can be cost considerations in terms of setup or maintenance costs. So, it is advisable to use certain types of pneumatic filters that match your application requirements. In some cases, any additional filtration will bear little benefit to your system. Below is some general guidance on the sort of filtration solution you will need for an application.

An adsorbing filter should always be preceded by a particulate filter and a coalescing filter.

4. Port Sizes and Flow

Pneumatic filters will have an input and output port. Common port sizes for pneumatics are 1/4", 1/2”, and 3/4”.  Too small of a port will reduce flow rates. It is important to be aware that pneumatic fitters should not be chosen only according to its port size. Rather, the selection of pneumatic filters should be dependent on flow rate and acceptable pressure drop, both of which are correlated with port size. It is best to consult the manufacturers technical specifications.

5. Auto Drain vs Self Drain

There are a few drainage types when it comes to pneumatic filters.

Manual: Condensate is drained manually by an operator or technician by loosening the drain plug. If condensate is not removed, there could be a build-up of water/oil that can re-enter the pressure line.

Semi-Automatic (Normally Open): This is a type of drain that opens as compressed air is shut off.

Automatic (Normally Open): When compressed air is shut off, or when a specified level of condensate is reached in the bowl, there is drainage.

Automatic (Normally Closed): When compressed air is switched on, or when or specified level of compensate is reached in the bowl, there is drainage.

Electrically controlled drains: Upon electrification of a solenoid, there is drainage. These sorts of draining mechanisms allow for full integration with electronic systems.

6. Other selection criteria: differential pressure indicator, bowl material, and filter element

6.1 Differential Pressure Indicator

As the filter element of a pneumatic filter clogs up, there is greater differential pressure. An increase in differential pressure is an indicator that the filter element or filter needs to be exchanged. Some pneumatic filters are designed to have its internal filter element replaced (however, this is becoming increasingly uncommon as the cost benefits of replacing a filter element rather than a filter are becoming negligible).

6.2 Drainage Bowls

Drainage bowls can either be polycarbonate (see through) or metal. Metal bowls are more robust, suitable in lower and higher temperatures, and are also resistant to certain chemicals that polycarbonate bowls are not resistant too. However, polycarbonate bowls are usually cheaper and can be seen through by the operator to gauge condensate level.

6.3 Filter Elements

Most modern filter elements are made of paper, and cannot be washed. Paper filter ememtns normally result in a drop of 2.5 to 3.5 millibar (mbar).

Fabric filters are stronger than paper, and can usually be cleaned by backflow of compressed air or by distilled water.

Addendum 1. ISO8573 Air Quality Control Chart

In the late 90s, the International Standards Office produced quality standards that for assessing pneumatic air quality. ISO8573 is the most commonly cited standard for compressed air quality, and is given in three parts according to quality class. For example, quality class 2.2.1 is: maximum particulate size 1μm, water -40C, and max 0.01 mg/m³.

BCAS Food and Beverage Guidelines state that compressed air that is in direct contact with the product should meet or exceed quality class 2.2.1 or 2.4.2 for indirect contact.

Table 1. Compressed Air Quality ISO8573 Standards Chart