How to Size a Bag Filter Housing by Flow Rate, Micron Rating and Pressure Drop

An undersized bag filter housing announces itself within weeks: bags that blind off days into a run, or a differential pressure gauge that lives at the top of its range. An oversized one only costs money quietly. Sizing one is not a catalogue lookup, because the three numbers that fix the size — the flow one bag may carry, the micron rating, and the pressure-drop budget — are not published on the same basis by every maker. This guide works through the three inputs using Eaton’s and Rosedale’s published data, shows where their figures differ, and ends with the checks to complete before ordering.

Start with the bag: standard sizes and rated flow

Bag housings are built around a small set of standard bag sizes, and the bag — not the vessel — sets the flow ceiling. Eaton’s published product data for its standard needlefelt bags gives the following maximum flow rates, for water-like liquids:

Bag size	Dimensions	Filter area	Maximum flow, standard felt (Eaton)
Size 01	7 × 17 in (180 × 430 mm)	0.24 m²	20 m³/h (about 88 GPM)
Size 02	7 × 32 in (180 × 810 mm)	0.48 m²	40 m³/h (about 176 GPM)
Size 03	4 × 9 in (100 × 230 mm)	0.08 m²	6 m³/h (about 26 GPM)

Eaton

Two cautions before using any maker’s table. First, even the filter area of the same nominal bag is not stated on a common basis: for a 7 × 32 in size 2 bag, Eaton states 0.48 m² of filter area, while Rosedale states 4.4 ft² (0.41 m²) for its 7.2 × 32 in bag. Rosedale Compare bags within one maker’s table, not across brands. Second, a maximum is a ceiling, not a design point. Rosedale publishes a typical flow range of 5 to 80 GPM (1 to 18 m³/h) for its size 2 bag and footnotes that flow rates are specific to the application. Rosedale A bag run at its ceiling starts life with little dirt-holding margin: as solids accumulate, pressure drop climbs from the clean value toward the change-out point, and the closer the starting point is to the ceiling, the shorter the run between bag changes.

The micron rating re-prices the same housing

Filter bags span a wide rating range: Eaton’s monofilament bags run to 800 µm, and Rosedale’s Bagwash line is offered from 1 to 1,200 µm. Rosedale The rating also carries an efficiency basis that buyers should ask about. Standard needlefelt bags are nominal-rated — Eaton lists the retention type of its standard bags as “Nominal” — and Rosedale states that its Bagwash bags are 90 percent efficient at the micron rating. Eaton Rosedale Two bags both labelled 25 µm are therefore not automatically equivalent, and if a specification genuinely requires near-absolute removal at the stated size, the answer is a different media family, not a bigger felt bag.

Changing the media changes the flow arithmetic directly. In the same size 02 envelope, Eaton’s ACCUGAF high-efficiency meltblown bag, rated 10 µm, carries a maximum flow of 15 m³/h (about 66 GPM) — against 40 m³/h for its standard felt bag of the same size. Eaton That is a reduction of more than 2.5 times from the rating and media alone. Any bag count worked out for a coarse nominal felt must be recomputed when the specification tightens, and finer media also blinds faster at the same solids load, so the change-out interval shrinks from both directions.

Budget the pressure drop, not just the flow

Total pressure drop across a bag filter is the housing drop plus the bag drop, and the two are set by different things. The housing’s contribution is governed mainly by its inlet and outlet size: Rosedale’s method is to read the housing pressure drop from the maker’s curve for the chosen pipe size and flow, multiply by a viscosity correction factor from the maker’s table, and then add the pressure drop of the bag. Rosedale

Three pressure figures discipline the design:

• Clean pressure drop. Rosedale’s published practice is that basket strainers and bag filters are usually selected so the pressure drop does not exceed 2 psi (about 0.14 bar) when clean, with higher clean drops tolerated only when contaminant loading is low. Rosedale
• Change-out point. The same Rosedale data sets bag change-out at 15 psid (about 1 bar). Running past it wastes pumping energy and raises the risk of bag damage and dirt unloading into the filtrate.
• The bag’s survival limit. Eaton rates its filter bags at a maximum differential pressure of 2.5 bar (about 36 psi). Eaton That figure is a limit the bag must never reach in service, not an operating target; the run should end at the change-out point far below it.

Viscosity rewrites water-based numbers. Rosedale’s correction table multiplies the clean-water pressure drop by a factor that grows with viscosity — its table runs to 2,000 cP — and grows further with finer basket linings. Rosedale A housing sized on water data for a viscous duty will show a higher clean drop and a shorter bag life than the calculation promised. The vessel’s pressure rating is a separate check again: Rosedale’s Model 6 housings, for example, are offered at 125 or 150 psi ratings, and the rating must cover the system pressure regardless of how the differential figures work out. Rosedale

From bag count to vessel

The bag count follows from the first three sections: divide the design flow by the allowable flow per bag for the media and micron rating actually specified, and round up. A worked example, using only the figures cited above: a 600 GPM water-like duty on size 02 bags needs 4 bags at Eaton’s 40 m³/h (176 GPM) ceiling for standard felt, 8 bags at an 80 GPM per-bag design point, and at least 10 bags if the specification is tightened to the 10 µm high-efficiency bag with its 66 GPM ceiling — before any safety margin in each case. Same duty, three defensible answers; the inputs, not the catalogue, decide.

Vessel format then follows the count and the process. Rosedale describes single-bag housings as typically handling up to 100 GPM, while its multi-bag vessels hold 2 to 23 bags and serve flows from several hundred to thousands of GPM. Rosedale Dirt load can overrule the flow calculation: Rosedale notes that fluid viscosity, the bag used and the expected dirt loading should all be considered when sizing a filter, and a high-solids stream may need its bag count set by the target change-out interval rather than by flow. Rosedale Where the process cannot stop for a bag change, duplex or parallel housings let one vessel be opened while the other carries the flow; Rosedale lists duplex units among its Model 6 options. Rosedale

Before you order

1. State the design flow and the fluid honestly: viscosity at operating temperature, not just “water-like”.
2. Fix the micron specification in writing — nominal or high-efficiency — and the efficiency expected at that size.
3. Ask for the allowable flow per bag for the exact media and micron rating quoted, not the product family’s headline maximum.
4. Require the clean pressure-drop calculation in the quotation: housing curve value at your connection size, viscosity correction applied, bag drop added — and test it against a clean target of about 2 psi or have the exception justified.
5. Agree the change-out differential pressure (15 psid is the maker figure cited here) and specify differential pressure gauges or gauge ports on the vessel so operators can see it.
6. Size the bag count for the change-out interval your dirt load implies; if the process cannot stop, quote duplex or parallel vessels instead of a single larger one.
7. Confirm the vessel pressure rating and any code requirement, such as an ASME stamp, separately from the differential-pressure figures.
8. Lock in the bag size standard you will stock as consumables; the housing and its baskets are built around one bag footprint.