Pneumatic Filter Basics

Pneumatic filters

by Justin Uhm

Many pneumatic components make up an entire air preparation system that can be used for automation. In this piece, I will explain what a pneumatic filter is, as well as the different types of pneumatic filters and what they are used for.

For a visual reference you can see the Air Preparation Units we make here at TPC Automation

Filter Structure and Operating Principle

So, what exactly is a pneumatic filter? Filters are devices that remove particles or impurities that pass through it by utilizing a compressed air stream. This article reviews the different types of pneumatic filters and explains which filters are best for each application in a pneumatics system

Types of Pneumatic Filters 

There are four different types of pneumatic filters:

1) Pneumatic Water Separators 

Mist separator
Mist separator

Water or moisture can wreak havoc on a pneumatic component. Without a water separator in place, water can make its way to the valve or cylinder and can cause them to deteriorate and stick, reducing the effectiveness of the product. By adding a water separator, a pneumatic filter can help remove water or water-borne contaminants. 

2) Pneumatic Particulate Filters 

Particulate filter
Particulate filter

Particles such as dust and debris can infiltrate the pneumatic components causing premature rust. The dust generally comes from old carbon pipes and air dyers. Particulate filters have a woven design, which is effective for trapping dust and particles. 

3) Pneumatic Coalescing Filter 

Piggyback Coalecing Filter from TPC Automation for Pneumatics systems
Piggyback Coalescing Filter from TPC Automation

Coalescing filters are good at removing water, rust, and oil from an air supply. A coalescing filter’s design is different than other filter types, but it works similarly to remove unwanted liquids. As air moves through the filter, sold particles are trapped inside, forming droplets that drain away. 

4) Absorbing Filter 

Adsorbing pneumatic filters are used to remove oil from compressed air streams. They are located close to the source of the oil to provide a more effective result. They are often used for breathing apparatuses and food and drug applications. 

Filter Structure and Operating Principle

The incoming air from the outside filter first passes through a deflector, which is a round system with several wings installed at angles. As air passes through the wings, it rotates and separates relatively large particles and impurities are separated by centrifugal force and accumulate at the bottom of the filter. Once through the deflector, the air passes through the filter element in the center to filter all the other remaining impurities to flow to the secondary area. The baffle on the bottom of the element prevents the accumulated drain from entering the secondary area with the flow of air.

Pneumatic filters are rated by the particle size of the impurities they remove. These sizes are defined in a unit of measure called microns, which is one-millionth of a meter. 

By way of comparison, a coffee maker filter can filter items as small as about 20 microns, preventing the coffee grains from ending up in your cup. Cigarettes can filter items as small as 0.3 to 0.5 microns, lessening the amount of tar and nicotine inhaled by the user.

Parts of a Filter

The components of a pneumatic filter generally use sintered metal, synthetic resin, or a metal screen. Sintered metal is compacted and formed into a solid mass. A metal element heats micro metal particles (bronze) with electricity to attach the connecting parts of particles to form a filtering layer with many small holes. 

Filtering occurs inside the element rather than just on the surface. This type of element has outstanding durability and strength and can be reused with simple cleaning. Also, drain material accumulated on the bottom of the case is exhausted through the hole on the drain valve with the pressure inside the case. If the drain is not exhausted and moisture reaches the element, the separated drain is blown into the secondary area. To prevent that from happening, it is convenient to use a filter with an auto-drain.

Filter Characteristics and Selection 

When selecting pneumatic filters, it is necessary to consider their flow characteristics and filtering capacities. 

Filtering capacity is determined by the minimum size of solid impurities that can be removed by the filter element and the moisture-separating coefficient. General pneumatic filters are set to 5 [μm] or greater in filtering capacity, and 0.8 [μm] or greater in the moisture-separating coefficient. 

If you need to remove more micro-sized impurities, consider using a mist separator or micro-mist separator. Coalescing filters are the most frequently used mist separator or micro-mist separator.

A pneumatic filter’s flow characteristics indicate the relationship between the flow of compressed air that passes through the filter and the increase or decrease in pressure. A flow characteristic is defined as when a valve opens, the flow characteristics allow some of the flow through the valve at a particular percentage of the stroke.  

When choosing a pneumatic filter, it is important to examine flow characteristics. Generally, there is a lubricator and solenoid valve (which I will write about in another post) installed at the end of the filter, and they contribute to the increase or decrease of pressure. Therefore, the range of maximum pressure increase or decrease at the filter can be decided considering the driver apparatus’s necessary pressure. In this respect, it is best to select a filter with a reasonable capacity, so that its range of pressure increase or decrease can include the actual pressure increase or decrease at the driver apparatus’s maximum flow. 

If you choose a filter with an extremely small range of pressure increase or decrease, its size becomes excessively large. 

Also, an ordinary filter case is mostly made out of synthetic resin and may develop cracks when exposed to thinner or other organic solvents. Because of this, it is more appropriate to use a metal case if there are organic solvents nearby or it is installed at a high temperature above 160-degree Fahrenheit.

Pneumatic Filters - table
Pneumatic Filters table – click to zoom

Table 2.11 lists various pneumatic filters and filtering capacities and uses. 

More about FRLs

This is one of three articles about pneumatics components called FRLs. See the other FTL posts here:

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