Filters For Compressed Air

Filters of all kinds are found everywhere in the world, and most of us are familiar with some of them. On www.about-air-compressors.com we're most interested in filters for compressed air systems, items that are essential for most compressed air applications.

The proper selection and use of compressed air filters will prevent many short and long term problems with your compressed air equipment and systems and save you substantially in down-time and component replacement costs over the life of your compressed air system.

The standard compressed air filter will contain the following components. The numbers on the picture of the compressed air filter picture correspond to the description in the text.

1) Air inlet; in North America usually a female NPT thread, although lately it could be a metric thread, into which the air line is connected, using the fitting of choice.

Air flows through the inlet. The cap is plumbed internally to force the air to flow downwards and spiral into to the filter bowl. This "cyclonic action" will "throw" free water and debris that may be in the air against the walls of the bowl, where it will flow down into the bottom.

2) The filter cap; may be a composite plastic or metal depending on the manufacturer.

Correct air filter operation depends on the air flowing through the unit in the correct direction from the supply line out the filter discharge. The correct air flow direction will almost always be identified on the cap of the filter, usually with an arrow. The arrow points in the desired direction of air flow from the supply line to the filter discharge. The air filter will not work properly if you inadvertently reverse the air flow.

3) The dotted line shows the flight path of the compressed air as flows toward the filter discharge. In order to exit the filter through this path, the compressed air must have undergone the cyclonic action phase, and then passed through the filter element, further purifying the airstream.

4) This is the filter discharge; allowing the flow of the filtered compressed air from the filter and down the air line to your applications.

The fitting size of the discharge port will usually be the same as the inlet port. Most filter manufacturers offer adapters to allow you to change the "port" fitting size to one that best suits your application.

In order to reduce inventory of spare parts, some companies will keep a standard 3/8" NPT filter as their base unit, and add adapters to the inlet / outlet ports to fit them to the smaller NPT sizes.

While it's feasible to use a larger filter unit on the small air line, attempting the reverse may impede compressed air flow. Do not do this unless you've checked to ensure that the flow of the smaller air filter has sufficient flow capacity for your application.

This will be a choke point if you under size the air filter.

5) This is the filter element; Insufficient air supply problems encountered downstream from your compressed air filter are often caused by the element becoming plugged and choking your air supply. If your air tool or applications isn't getting enough air, check the element. Clean it or replace it, depending on the type.

Some manufacturers offer a Delta P gauge as part of their filter assembly. The Delta P will indicate pressure drop across the filter, and the increase of pressure drop indicates that the filter element is doing it's job, and trapping debris and removing it from the compressed air stream. The dirtier your air, the sooner your elements will plug.

Filter elements have a specific flow capacity measured in CFM, and a particulate size rating measured in Microns.

The chart below tells you the actual size particle that a specific Micron rating represents.

General purpose filter elements are 30 or 40 Micron sized. For some applications, you'll want a 5 Micron element, however, depending on your air quality, an element that 'fine' will clog quickly. It's common, then, to use a general purpose filter upstream from the unit with the 5 Micron element, to increase it's life.

Here's more info on compressed air filters.