High-Flow Coalescing Filter Selection Guide: Sizing a Compressed Air Coalescer and Oil Removal Filter for 1,000 to 6,000 SCFM (6 Inch and 8 Inch Flanged)

A high-flow coalescing filter, also sold as a compressed air coalescer or oil removal filter, is chosen on flow at your minimum pressure, not on pipe size. This guide sizes and compares flanged coalescing filters for roughly 1,000 to 6,000 SCFM using the published data sheets of Walker Filtration, Parker (domnick hunter OIL-X), Kaeser, Atlas Copco and Hankison, read side by side in October 2026. There is no single best filter: the right one depends on your flow, minimum pressure, oil target, pressure drop, drain and vessel code.

What does a compressed air coalescing filter remove, and what does it leave behind?

A compressed air coalescing filter captures oil and water aerosols and fine particles in its element, and the collected liquid drains from the housing. It does not deal with oil vapor: makers list vapor removal as a separate grade installed after the coalescer, and Atlas Copco states that its coalescer oil figures cover “oil aerosol and liquid.”

The data sheets show where each stage stops. Walker’s fabricated flanged sheet (WFL1043 Rev-C-UK 0326) lists maximum oil carryover at 20°C of 0.1 mg/m³ for X1, 0.01 mg/m³ for XA and 0.003 mg/m³ for its AC activated carbon grade. Parker’s OIL-X catalog (PISOILX-03-EN) gives 0.5 mg/m³ for AO, 0.01 for AA and 0.003 for its ACS vapor grade, which it places after AA.

Failure-mode note: Walker states that “Activated Carbon Filters must not operate in oil saturated conditions” and changes carbon elements at least every 6 months, against 12 months or 8,000 hours for its other elements. A saturated coalescer or a shut drain sends liquid oil on to a stage that was specified for vapor.

How do you size a coalescing filter (coalescer) for 1,000 to 6,000 SCFM?

Size on the maximum flow at the filter inlet and the minimum pressure at that point, never on pipe size. Every maker rates at one reference pressure (7 bar gauge or 100 psig), and four of the five publish a pressure correction factor: below that pressure, the same housing is rated for less flow. Kaeser puts it plainly: “Do not select filters by pipe size.”

• Walker (flanged and PRO XF) and Atlas Copco: multiply rated flow by 0.92 at 6 bar gauge (87 psig) and 1 at 7 bar gauge. Walker’s factor reaches 1.51 at 232 psig.
• Kaeser: multiply by 0.83 at 80 psig and 1.00 at 100 psig, with no point in between.
• Parker: the method runs the other way. Multiply your flow by 1.08 at 6 bar g to get the minimum rating, and round the pressure down (“for 5.3 bar, use 5 bar correction factor”).
• Hankison: the brochure publishes no correction factors; ask before sizing below 100 psig.

Worked example: 4,500 SCFM at a minimum of 87 psig

Our arithmetic from published flows and factors, rounded down; the duty is an illustration, not a customer case. For Kaeser we used the 80 psig factor, rounding the pressure down as Parker instructs; ask Kaeser whether interpolation is allowed.

• Walker A686, 6 in: 3,824 SCFM at 7 barg x 0.92 = 3,518 SCFM. Too small.
• Walker A688, 6 in: 5,130 SCFM x 0.92 = 4,719 SCFM. Covers the duty.
• Atlas Copco 2100+, DN 150: 4,450 cfm at 7 bar(e) x 0.92 = 4,094 cfm. Too small; the 2500+ (5,297 cfm) gives 4,873 cfm.
• Kaeser F1410, 8 in: 5,000 scfm at 100 psig x 0.83 = 4,150 scfm. Too small; the F1940 (6,875 scfm) gives 5,706 scfm.
• Parker: the duty needs 4,500 x 1.08 = 4,860 scfm at 7 bar g, while the largest die-cast model, P060K, is rated 2,119 scfm. We could not obtain data for Parker’s fabricated range.
• Hankison HF-72, 6 in: 5,000 scfm at 100 psig, with no published factor, so it cannot be checked this way.

The takeaway: a 5,000 SCFM filter at 100 psig (Kaeser F1410 here) is rated by its own maker’s table for 4,150 scfm at 80 psig, short of a 4,500 SCFM duty.

What do “7 barg”, “100 psig” and an 8 inch flange tell you about a coalescing filter rating?

A flow figure is valid only at the reference pressure printed with it, and a bigger flange does not raise it. Walker, Parker and Atlas Copco rate at 7 bar gauge (labelled 100 psig by Walker, 102 psig by Parker and Atlas Copco); Kaeser and Hankison rate at 100 psig, which Hankison prints as 6.7 barg. Pick the model on flow at your pressure first, then match flange size and standard to your header.

Reference conditions differ too. Walker states “1 bar (a) 20°C (68°F)” and Parker 20°C, 1 bar(a) and 0% relative water vapor pressure, while Walker’s PRO XF sheet prints “1 barg (a)”. Atlas Copco, Kaeser and Hankison print a reference pressure but no reference temperature with their flow tables (Kaeser’s 68°F, 14.5 psia note belongs to its ISO 8573-1 table), so ask for it.

• Around 5,000 SCFM, Walker A688 is 6 in (DN150) at 5,130 SCFM, Atlas Copco 2500+ is DN 150 at 5,297 cfm, Hankison HF-72 is 6 in at 5,000 scfm and Kaeser F1410 is 8 in at 5,000 scfm.
• Walker’s only 8 inch model in its UK sheet, the A8810, is rated 6,470 SCFM; Hankison’s smallest 8 inch, the HF-84, is rated 11,875 scfm.
• Flange standards: Walker’s UK sheet supplies BS4504 PN16 mating flanges, PRO XF offers PN16 or ANSI B16.5, and Kaeser uses ASME pattern Class 150.

Walker’s public sheet uses A model codes. The NA codes sold in the US do not appear in the public Walker documents we reviewed, so ask for an NA model’s rated flow and reference pressure in writing.

Why put a 1 micron coalescing filter ahead of a 0.01 micron coalescer?

The trains the makers publish put a general purpose coalescing grade (1 micron for Walker X1 and Parker AO) ahead of a high efficiency 0.01 micron grade, with a water separator first where there is bulk liquid. The first stage takes the heavy load so the fine element is not the only barrier. Do not over-specify: in Walker’s words, “The larger the micron rating, the less pressure loss that occurs within the filter.”

• Parker: precede AA with AO, and AO with a WS water separator for bulk liquid.
• Atlas Copco: precede PD+ with water separation and DD+.
• Hankison: use a Grade 7 as a prefilter if heavy liquid loads are present.
• Kaeser: its KBE combination pairs KB and KE for higher assurance of air quality.

The pressure cost, from published figures: Walker X1 plus XA, both saturated, adds up to 2.2 + 4.4 = 6.6 psi. Atlas Copco DD+ plus PD+ is 119 + 132 = 251 mbar average wet, against 220 mbar for its single-housing UD+, which it says combines both stages.

Failure-mode note: without a first stage, the fine element carries the whole liquid and particle load, and its pressure drop climbs toward the change point sooner. On Walker’s flanged XA the margin between saturated (4.4 psi) and the change point (6 psi) is only 1.6 psi.

How do mg/m³, ppm w/w, ISO 8573-1 classes and ISO 12500-1 tests describe oil removal filter performance?

ISO 8573-1:2010 sets total oil classes: class 1 is at most 0.01 mg/m³, class 2 at most 0.1, class 3 at most 1.0 and class 4 at most 5.0 (as printed by Kaeser). ISO 12500-1 is the test method for oil aerosol removal, and its result depends on the inlet challenge: Parker, Atlas Copco and Kaeser state 10 or 40 mg/m³, Hankison states CAGI ADF400 at a 100°F inlet, and the Walker documents we reviewed give no test standard or inlet concentration.

• Kaeser’s ISO table pairs class 1 (0.01 mg/m³) with 0.008 ppm w/w and class 2 (0.1 mg/m³) with 0.08 ppm w/w, while Parker prints identical numbers in both units (0.01 mg/m³ and 0.01 ppm(w) for AA). Do not convert yourself; ask for the unit your specification uses.
• Hankison’s Grade 5 is 0.008 ppm w/w on one page and below 0.08 ppm/w on another: the first sits at the class 1 limit in Kaeser’s table, the second only guarantees class 2.
• Atlas Copco publishes PD+ at 0.008 mg/m³ yet claims oil class 2, and says the figure covers aerosol and liquid. ISO 8573-1 classes are for total oil, so ask whether a class claim assumes a separate vapor stage.

Before comparing oil figures, ask for the test standard, the inlet challenge concentration, the inlet temperature and any independent validation (Parker cites Lloyd’s Register; Kaeser cites the German testing agency IUTA).

What should you check on coalescer pressure drop, drains and limits before you order?

Published wet or saturated pressure drop for fine grades runs from below 1.8 psi to 4.4 psi, depending on the maker and its definition, and every psi lost across the filters is pressure the compressor has to make up. Change points and standard drains differ by maker, and the large flanged filters reviewed are rated 225 to 232 psig with maximum temperatures from 140°F to 248°F.

• Walker flanged XA: 1.5 psi clean and dry, 4.4 psi saturated, change at 6 psi (400 mbar) or 12 months / 8,000 hours, read on the 65DPIG indicator Walker fits to its flanged models.
• Element change: Parker every 12 months; Atlas Copco 8,000 hours or 1 year in its brochure, but 4,000 hours, 1 year or 350 mbar for flanged filters on its US web page; Kaeser and Hankison publish no interval.
• Drains: Walker fabricated flanged filters come with manual drain valves, PRO XF with float drains, Kaeser flanged KB and KE with the electronic ECO-DRAIN 31 Vario, and Hankison ASME housings with a drain plug or an internal automatic drain. Walker recommends electronic drains where high liquid quantities are expected.
• Temperature and vessel code: PRO XF is rated 176°F with its automatic drain and 248°F with a manual drain. Kaeser states ASME stamped and registered vessels, Hankison ASME vessels (HF-54 to HF-92), and Walker’s UK sheet BS EN 286 Part 1.

Failure-mode note: a manual drain removes liquid only when someone opens it. On a large housing with a steady liquid load, a forgotten valve lets condensate build up inside the filter, so put the drain type in the specification.

How do Walker Filtration, Parker OIL-X, Kaeser, Atlas Copco and Hankison coalescing filters compare on published numbers?

On paper the fine grades are close, at 0.01 mg/m³ or lower where published in mg/m³, but they are tested differently. The practical differences are rating basis, connection size for a given flow, pressure drop, drain, vessel code and how consistent each maker’s documents are. Figures are as printed; “not published” means the documents we reviewed do not give the value.

Parker’s fabricated flanged range is not in the table because we could not obtain its data sheet.

Which coalescing filter data sheet figures should you confirm in writing?

The conflicts below are printed in the makers’ own documents. We have not resolved them: ask your supplier which figure applies to the exact model and element you are buying, and get the answer in writing.

• Walker US flanged page: “up to 16,100 SCFM” and, further down, eight models “up to 15,000 SCFM”; it lists four grades from 5 micron, while the UK sheet lists five from 25 micron.
• Walker X1: 0.1 mg/m³ (oil class 2) in the flanged sheet, 0.3 mg/m³ (class 3) in the Alpha sheet (WFL1064).
• Kaeser KB: below 0.1 mg/m³ in the literature, below 0.01 on the web page. Pressure drop: KB 2.0 and KE below 2.9 psi in the literature, below 2.2 and 3.0 psi on the web. Temperature: 150°F maximum inlet in the literature, 140°F on the web. The F1060’s 6 inch connection is read from a shared table cell.
• Atlas Copco: element service after 8,000 hours or 1 year in the brochure, after 4,000 hours, 1 year or 350 mbar for flanged filters on the US web page.
• Hankison: Grade 5 at 0.008 and below 0.08 ppm, a tenfold gap; HF-72 is 6 inch flanged in the brochure but 4 inch on the web page; Grade 3 dry pressure drop is printed as 2 psid beside 0.07 barg.

Documents compared: Walker WFL1043 Rev-C-UK 0326, WFL1010 Rev-A-UK 1017 and WFL1064 Rev-C-UK 0122; Parker PISOILX-03-EN and PISOILXDCAOAA-00-FR; Kaeser USKFILTER 09/24; the Atlas Copco filters brochure (2022); Hankison HK_HF_US/12-2023/CI; and the Walker, Kaeser, Atlas Copco and Hankison web pages, viewed October 2026.

Where can you buy a high-flow coalescing filter in Houston?

PE Energy Industrial Supply in Houston, TX supplies Walker Filtration products as an authorized distributor, including fabricated flanged coalescing filters with 3 to 12 inch connections, matching water separators, PRO XF flanged filters and Alpha threaded filters. Send your maximum flow and minimum pressure at the filter inlet, inlet temperature, connection size and flange standard, required ISO 8573-1 oil class, drain preference and vessel code, and ask us to confirm in writing the rated flow and reference pressure of the model we quote. The Parker, Kaeser, Atlas Copco and Hankison figures above are for comparison; the products below are Walker models.

• Fabricated flanged coalescing filters: Walker NA382, NA483, NA684, NA685, NA686, NA688, NA888 and NA8810, with larger sizes in the coalescing filters category.
• Water separators for bulk liquid ahead of the first stage: Walker NA888WS and the water separators category.
• PRO XF, 700 to 3,000 SCFM at 7 barg: XF371F (3 inch, 1,500 SCFM), XF462F (4 inch, 2,500 SCFM), XF472F (4 inch, 3,000 SCFM), the XFM072 filter module and the XF400FWS water separator.
• Alpha threaded: Walker A31500 (3 inch, 1,500 SCFM at 7 barg; NPT threads in North America).
• Accessories: zero-loss and float drains, the Walker 65DPIG indicator, differential pressure gauges and indicators, oil-water separators and the Walker Filtration brand page.

Summary: how to choose a high-flow coalescing filter

• Size on flow at your minimum pressure at the filter inlet, not on pipe size, and convert each data sheet rating with the maker’s correction factor for its reference pressure.
• Run the stages the makers publish: a water separator first where there is bulk liquid, then a general purpose coalescing grade, then a high efficiency grade.
• Compare remaining oil and ISO 8573-1 oil class only together with the test basis each maker states.
• Check saturated pressure drop, the drain, maximum pressure and temperature, and the vessel code before you order: a saturated coalescer or a shut drain sends liquid oil downstream.
• Ask for the rated flow and reference pressure in writing, above all where a data sheet is silent or contradicts itself.