Pre-filter for a sterile air system (the coalescing and particulate stages that protect the sterile compressed-air filter)
The compressed-air filters that sit ahead of a sterile air filter · coalescing grades down to 1 µm and 0.01 µm, remaining oil 0.5 and 0.01 mg/m³, initial differential pressure, 12-month element change, float drains, and where they fall in a three-stage sterile air manifold
A sterile air filter is the last stage on compressed air going to product or a tank vent. Prefilters upstream remove bulk water, oil aerosol and particles from the compressed air before it reaches that element. Which grades, in what order, how they drain and when they are changed are what a pre-filter specification covers.
Coalescing and particulate compressed-air filters installed ahead of sterile air filters in pharmaceutical, food and beverage plants. The sterile filter itself is covered on the Sterile air and vent filter page; bulk water removal on the Moisture separator (compressed-air water separator) page; dryers on the Desiccant compressed air dryers page; aftercoolers on the Compressed air aftercoolers page; machine air supply and ISO 8573-1 classes at valve manifolds on the Pneumatic system for production machines (air supply to cylinders) page.
CNFX editorial research · Sources checked October 13, 2026 · 2 sources
Coalescing grades and their order ahead of the sterile filter
Parker's OIL-X catalogue (PISOILX-03-EN) gives grade AO as removing particles including water and oil aerosols "Down to 1 micron" with a maximum remaining oil content of 0.5 mg/m³ at 21 °C, and grade AA "Down to 0.01 micron" with 0.01 mg/m³; AA is preceded by AO, and AO by a WS water separator for bulk liquid. [1]
Parker lists filtration efficiency of 99.925% for AO and 99.9999% for AA, tested to ISO 8573-2, ISO 8573-4 and ISO 12500-1 at inlet challenges of 40 mg/m³ and 10 mg/m³. A further ACS oil vapour grade, preceded by AA, leaves 0.003 mg/m³. [1]
Donaldson's process filtration catalogue (effective August 2026) builds its sterile air manifolds in three stages: stage 1 P-SMF, stage 2 P-AK and stage 3 the LifeTec P-SRF V sterile element, for line sizes from 3/4 in. (100 scfm) to 4 in. (3,800 scfm). Its recommended sterile air system places cyclone separation, coalescing filters, dryers, particulate and carbon adsorption filters before the PG-EG sanitary housing with the P-SRF V sterile filter. [2]
Parker OIL-X grade
Particle removal
Remaining oil at 21 °C
Efficiency
Precede with
WS water separator
Bulk liquid
N/A
> 92%
—
AO
Down to 1 micron
0.5 mg/m³
99.925%
WS
AA
Down to 0.01 micron
0.01 mg/m³
99.9999%
AO
ACS
Oil vapour
0.003 mg/m³
N/A
AA
Give the supplier the compressor type (oil-lubricated or oil-free), flow, minimum pressure and dryer, and ask for the prefilter grades, their order and the air quality class reached at the sterile filter inlet.
02
Pressure drop, drains, flow correction and change-out
Parker gives AO and AA an initial dry differential pressure below 70 mbar (1.0 psi) and an initial saturated differential pressure below 125 mbar (1.8 psi), with the element changed every 12 months; AO/AA coalescing filters come with a float drain and a differential pressure indicator. [1]
Rated flows are at 7 bar g. Parker's sizing multiplies the flow by a correction factor for the minimum operating pressure, rounded down, for example 1.18 at 5 bar g for its coalescing and particulate filters, and selects a filter whose flow is at least that figure. Coalescing versions with float drains run from 1 to 16 bar g and 2 to 80 °C; dry particulate versions with manual drains to 20 bar g and 100 °C. [1]
Donaldson's sterile air manifold is rated to 150 psig (10.3 barg) at 150 °C and refers maximum differential pressure to the element data sheets. [2]
Ask for the differential pressure at which elements are changed, the drain type on each stage, and the minimum line pressure used to size them.
03
Sterile air pre-filter details
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Compiled and source-reviewed by the CNFX editorial team.
Each manufacturer above links to the page on that company's own website where the product is listed, with the date we checked it. How we source and review
· Page last updated 13 October 2026
Buyer questions
What does a 1 micron coalescing grade leave in the air?
Parker's AO grade leaves a maximum of 0.5 mg/m³ oil at 21 °C. [1]
Can a 0.01 micron filter be used alone?
Parker says grade AA should be preceded by AO. [1]
How many stages does a sterile air manifold have?
Donaldson's has three: P-SMF, P-AK and the P-SRF V sterile element. [2]
Sources checked 2026-10-13. Parker Hannifin publishes the OIL-X compressed air filter catalogue PISOILX-03-EN (2017, copy hosted on a distributor site); Donaldson publishes its process filtration catalogue.
