Filters;
Filters of all kinds are found everywhere in the world, and most of us are familiar with some of them.
On this site we're most interested in filters for compressed air systems as it's all about air compressors, compressing air, and using the compressed air.
The proper selection and use of compressed air filters will prevent many short and long term problems with your equipment and systems, and save you substantially in down-time and component replacement over the life of that compressed air system.
The standard compressed air filter will contain the following components. The number on the picture has a description beside that same number in the text.
1) An 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 is called "cyclonic action" and it 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. The filter requires that it be installed in the correct direction of flow, and this will almost always be identified on the cap with an arrow. The arrow points in the desired direction of flow, and the unit is to be installed that way. It will not work properly if you reverse the air flow.
3) The dotted line shows the flight path of the compressed air as it exits the filter. In order to move through this path, the air must have passed through the filter element, further purifying it.
4) This is the egress point of the air from the air filter. This fitting size will usually be the same as the inlet size. In most cases there are adapters available to allow you to change the "port" fitting size.
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, don't attempt the reverse until you've checked to ensure that the flow of the smaller air filter has sufficient flow capacity.
This will be a choke point if you mis-size the unit.
5) This is the filter element. It is usually a consumable, and quite often, problems encountered downstream from your filter are caused by this element becoming plugged and choking your air supply.
These elements have a flow capacity, and an orifice size measured in Microns.
The chart below tells you the actual size particle that a specfic 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 to use a general purpose filter upstream from the unit with the 5 Micron element, to increase it's life.
So too, when you move into even finer elements such as coalescing filter elements, you'll want to have a general purpose first, then a 5 Micron unit, and then the coalescing filter.
6) The bowl of your air filter may thread into the cap housing, or more likely use a "bayonet" type mount.
The bayonet style of mount can be installed by pushing the bowl up against the cap, rotating it a short distance, and letting the lugs on the bowl slide down into the receptacles in the cap.
To remove the bowl, you reverse the process, firstmaking very careful that there isn't air in the line.
Remember, force equals pressure times area.
You've got at least 3 or 4 square inches of "piston" area inside the bowl housing, and if your air-line is "charged" with 100 PSI,then there's three or four hundred pounds of force pushing down on the bowl.
If you manage to get the bowl un-bayoneted or unscrewed, it will likely and unfortunately "blow" itself right out of your hands, smashing itself into bits and pieces on the floor, and possibly injuring you or a colleague with flying debris from the impact!
Does this sound like something we're really familiar with? You bet! (Insert rueful chuckle here!)
Sometimes the bowls of the air filter are plastic with a metal shroud, or are completely metal construction. That being the case, why use why metal or metal shrouded bowls?
7) Inside of almost every air filter bowl there will be a device that separates the bowl into an 'above' and 'below' section. This barrier is usually made of a plastic or plastic composite and is usually installed hanging from the bottom of the element. This barrier blocks the cyclonic incoming air, preventing it from reaching the "soup" of debris, water and oil that's collecting in the bottom of the filter bowl. This barrier creates a "quiet" zone, allowing the contamination that collects onto the sides of the bowl to flow down, ultimately out of the cyclonic air, and to remain - without getting entrained or re-entrained back into the air stream, until it can be expelled from the drain at the bottom of the bowl8) All industrial compressed air filters will have a drain in the bottom of the bowl.
These drains may be manual, float type, or electronic auto drains. They need to be opened regularly to allow collected water and debris to escape from the filter bowl.
Failure to drain the filter bowls often enough will mean that the water and debris in the "quiet zone" will rise past the barrier referred to above, and once there, be entrained into the "cyclonic" air, and onto the element.
In some cases debris and water from a full bowl will flood the element so badly that it seems to become almost no barrier to the contamination at all, and your air filter is actually contributing a steady stream of "crud" downstream to damage the components in your application, while it chokes your air supply to death.
So, how do you figure out what size of compressed air filter you'll need? Here's information on filter sizes.
Here are the generally accepted symbols for drawing compressed air filters in your circuit schematic.
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