Screw Compressor Air Intake Filter: Signs It Is Restricted and What It Costs You

A screw compressor intake filter is restricted when the service indicator shows red or the controller raises an air filter warning, when the machine takes longer to reach its unload pressure, or when it can no longer hold pressure at all on a demand it used to cover. The cost is air you pay for and never get. A blocked element lowers the pressure at the air end inlet, so every turn of the rotors draws less air and the compressor works against a higher compression ratio. The US Department of Energy puts the loss from a dirty intake filter at 1% to 3% of compressor efficiency. Change the element at the indicator or at the maker's interval, whichever comes first, and never run the machine without one.

What the intake filter does

The intake filter is the first part the air touches. On an oil injected screw compressor, room air passes through the intake filter, then the inlet valve (suction regulator), then into the air end, where it mixes with oil and is compressed. The air and oil then go to the separator tank, and the oil is cleaned by the oil filter before it is injected again.

Anything the intake filter lets through ends up in the oil. The Gardner Denver manual for its L15 to L22 screw compressors says the dust and gas content of the intake air has a strong influence on the service life of the air filter, the oil filter and the separator, and ties the oil change interval to how dirty the circulating oil gets. A torn or badly seated intake element is not just an air filter problem. It shortens the life of every filter downstream and wears the rotors.

The same manual is blunt about running without one: never operate the compressor without an air filter, because even removing it briefly can cause considerable damage to the machine.

Signs the intake filter is restricted

• The service indicator is red. Many screw compressors carry a restriction indicator on the filter housing. On Atlas Copco GA machines the instruction is to replace the element when the red band is fully extended, then push the knob to reset it. Machines with an electronic controller show the same thing as an air filter service message.
• Longer load times. The compressor runs loaded for longer to fill the same receiver, and unloads less often, because it is delivering less air per minute.
• Pressure falls at peak demand. A machine that used to cover the shift now sags. Gardner Denver's troubleshooting table lists a clogged air filter as one cause of "unit does not reach the set mains pressure", with the remedy "replace air filter cartridge".
• A heavy dust load on the dirty side when you open the housing, well before the interval is due. That tells you the room air needs attention.
• Dust on the clean side of the housing or in the inlet valve. This is the opposite problem: the element is torn or not sealing, and dust is getting past it.

Oil inside the air filter housing is a different fault. It points to the inlet valve not closing properly when the machine stops, not to the filter. Replacing the element will not fix it.

What a restricted filter costs you

A screw air end moves a fixed volume of air per revolution. The mass of air in that volume depends on the absolute pressure at the inlet. Room air at sea level is about 1,013 mbar absolute. If the filter is restricting the flow by 50 mbar, the air end sees 963 mbar, and in an ideal positive displacement machine it draws about 5% less air per turn. As a working figure, every 10 mbar of restriction costs about 1% of the air drawn in.

The air end also has to compress that thinner air up to the same discharge pressure. At 7 bar gauge discharge, the compression ratio rises from 7.9 with a clean filter to 8.3 with the 50 mbar restriction. The motor is still turning the rotors at full speed. You are paying for the same revolutions and getting less air out of them.

The DOE Compressed Air Tip Sheet #14 works an example through: a 200 hp compressor running two shifts, five days a week (4,160 hours a year) loses 1% to 3% of its efficiency to a dirty intake filter. At the electricity rate used in the tip sheet, that is roughly 6,500 to 19,600 kWh a year of wasted energy, far more than the cost of the element.

The second cost is the usual fix. When delivery drops, the common response is to turn the pressure setting up. The Atlas Copco Compressed Air Manual gives a rule of thumb of about 8% more power for every 1 bar higher working pressure, and names clogged filters as one of the problems people try to cover this way. Raising the pressure does not cure the restriction. It adds a second energy loss on top of the first.

When to change the element

Change it on whichever comes first: the restriction indicator or the maker's interval. Gardner Denver's schedule for the L15 to L22 is every 2,000 hours or 6 months, whichever occurs first, with the note that operating conditions decide the real frequency. The same manual asks for the filter to be checked for accumulated dust at least once a week, and daily in dusty conditions.

Do not change it early just because it looks dirty. Donaldson's guidance on servicing by restriction makes two points that apply to compressors as much as engines. A loaded element filters better than a new one, because the dust cake catches finer particles. And every time the housing is opened there is a chance of dirt getting onto the clean side or the new element being seated badly. Read the indicator, not the colour of the paper.

Shorter life than expected usually comes from the room, not the filter:

• Dusty intake air. Woodworking, cement, foundry and grinding shops load elements fast. The Atlas Copco manual recommends a pre-filter (cyclone, panel or rotary band) where the surrounding air carries a lot of dust.
• Hot or contaminated air. The DOE tip sheet recommends drawing intake air from a clean, cool location, away from rain, dirt and cooling tower discharge. Hot air is less dense. The tip sheet notes the effect is smaller on oil injected screw machines than on centrifugal compressors, but it is still delivery you lose.
• A housing that does not seal. A cracked cover gasket or a missing wing nut lets unfiltered air bypass the element entirely.

Change the intake filter together with the oil filter where the intervals line up. If the intake element was torn or missing for any length of time, change the oil filter and check the oil as well; our guide on when to change an industrial oil filter covers the oil side. Dust in the oil also loads the separator element; if you are already seeing oil carryover, work through how to diagnose a failed separator before buying one.

Changing it without letting dirt in

• Stop the compressor, depressurise it and lock out the power.
• Wipe the outside of the housing and cover before you open it, so loose dust does not fall into the clean side.
• Remove the old element and look at the clean side of the housing. Dust there means the old element was leaking.
• Wipe the inside of the housing with a clean, damp cloth. Do not blow it out with compressed air toward the inlet.
• Fit the new element, check the end seal is seated all the way round, refit the cover and reset the indicator or the controller's service timer.

Finding the right replacement

We list 144 compressor intake filter cartridges in our air filter cartridge range. Across the 128 with recorded dimensions, outer diameter runs from 60 to 550 mm and overall height from 36 to 628 mm. The median diameter is 166 mm. Where the media is recorded it is cellulose paper rated at 5 µm.

The part number on the old element is the fastest route. One cartridge often carries a long list of codes from different compressor and filter brands. Our SPM-AFC-1074 cartridge (194 mm OD, 114 mm bore, 399 mm tall) has 107 recorded cross-references and is fitted to machines from 16 compressor brands. It is listed as Atlas Copco 2914830800, and the same cartridge also carries Kaeser 6.2085.0, Ingersoll Rand 54672522, CompAir 11323374, Mann C 20 500 and Donaldson P778994. The median across the range is 18 cross-references per cartridge. Search any of those codes on our compatibility page to land on the right one.

If the label is gone, measure. Diameter alone does not identify an intake element:

Three of our cartridges share a 281 mm outer diameter. The Oxfil AF.4093 is 420 mm tall with a 170 mm bore. The Atlas Copco 1621737600 is 563 mm tall and the Atlas Copco 1621510700 is 628 mm, both with a 200 mm bore. Measure the outer diameter, the inner bore at the sealing end, the overall height including end caps, and note whether the closed end has a hole or a stud. Those four facts plus the compressor make and model are usually enough for us to match it.

Can't find your code? Send the old part number, the compressor model or the four measurements through our quote request form and we will match it.

Frequently asked questions

Can I clean a paper intake filter element with compressed air and reuse it?
Some manuals allow it; Gardner Denver's says a clogged air filter must be cleaned or replaced. Every cleaning means opening the housing, which Donaldson identifies as the moment dirt reaches the clean side, and any damaged element has to be replaced. We recommend replacing the element at the indicator rather than cleaning it.

How often should a screw compressor air filter be changed?
At the restriction indicator or the maker's interval, whichever comes first. A typical published interval is 2,000 hours or 6 months. In dusty rooms it will be sooner.

Will a blocked air filter damage the compressor?
A blocked element mainly costs air and energy. A damaged or missing element does the real damage, because unfiltered dust wears the rotors and loads the oil, oil filter and separator.

Why does my compressor reach pressure more slowly than it used to?
Check the intake filter indicator first. After that, look at leaks in the network, a slipping belt, a separator with a high pressure drop, and the inlet valve opening fully.