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One rule of thumb is that the filter inlet and outlet ports should be the same NPT size as the O.D. of air line on which you are installing a compressed air filter. If you're plumbing an application using 1/4" polyethylene tube, the filter port size should be 1/4" NPT.
That "rule" is not always valid, but is a reasonable guideline. In some cases, with a 1/4" NPT ported compressed air filter you'll be oversized for one application, and it's possible that with the same 1/4" NPT ported filter, you'll be undersized for another.
Each filter manufacturer will provide the flow characteristics of their filters yet the flow characteristic of a given compressed air filter will change depending on the Micron rating of the element inside it. A 1/2" NPT filter housing, therefore, can have a significant variety of flow ratings depending on the element that's installed inside, and you can't tell what that is from the outside!
It's also important to recognize that the flow through the filter will be determined by the application demand, not necessarily the compressor size. A 25 HP compressor can theoretically produce 100 CFM of flow at about 90 PSI, so with a 25 HP compressor it would be logical to think that you would need a filter with a capacity of 100 CFM to handle all of the air from that compressor.
But, what if you have a 200 gallon reservoir, and the downstream demand is for 150 CFM when a specific piece of equipment starts? Even though the 25 HP compressor can only produce 100 CFM at 90 PSI, if the receiver is big enough, and the downstream demand large enough, the flow of air to that application from the receiver means that you've immediately exceeded the capacity of the filter. True, you might exceed the capacity of that filter for a short time; just until the receiver is exhausted and the flow from it decreases as the compressor is unable to keep up with the demand.
For more information on compressor receivers, please click this link .
It's important to know what the maximum plant CFM needs could possibly be if all demand was on stream at one time, and the system needs to have enough filter capacity at the receiver for that total demand if there's a possibility that it could come 'on-stream' all at once.
As the plumbing reaches the compressed-air-consuming locations downstream from the compressor, you'll need to know what each application will consume in compressed air when it's running at capacity, and have an appropriately sized filter installed in the compressed air line there.
Even though the system will have an appropriately sized air filter at the receiver, the nature of compressed air is that there will likely be condensation of water in the air lines after the initial filter, therefore, you will need to install a filter at each air consumption point, to remove the water that's condensed in the air lines as the compressed air travels through them.
For more information about determining the consumption of air in your plant and to help size the appropriate filters, please click here , or to this link on air cylinder consumption .
One of the frustrating things about already-installed compressed air filters is that it's virtually impossible to tell from the outside what the flow rating of that filter is. You can't be expected to know just by looking at it.
If it's a filter housing that contains a "general purpose" element, and the NPT port size is the same inch size of the air line, usually you'll be OK.
If in doubt, locate the vendor / manufacturer / model number information on the filter head or housing, and contact the vendor for flow ratings for that unit. After you've "dumped" the compressed air from the line, and removed the filter bowl, by describing the element to the vendor or distributor, they should be able to identify the type of element and the flow rating of that compressed air filter with that element installed.
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