Every bag filtration system starts with the housing — and the configuration you choose determines your flow capacity, maintenance schedule, and whether your process can tolerate any downtime at all. This guide breaks down the three main bag filter housing configurations — single-bag, multi-bag, and duplex — so you can match the right setup to your facility's requirements. For a broader look at filter bag selection, see our complete industrial filter bag guide.
Bag Filter Housing Configurations at a Glance
Before diving into the details, here is a side-by-side comparison of the three housing types:
| Configuration | Flow Range | # of Bags | Relative Footprint | Bag Change Downtime | Typical Applications |
|---|---|---|---|---|---|
| Single-Bag | 5 – 180 GPM | 1 | Smallest | 5 – 15 min (process stops) | Parts washing, small water treatment, coolant loops, batch processing |
| Multi-Bag | 200 – 1,000+ GPM | 2 – 24+ | Medium to large | 15 – 30 min (process stops) | Municipal water, large-scale chemical, high-flow cooling towers, food & beverage |
| Duplex | 5 – 500+ GPM | 2 (one per chamber) | Medium (twin vessels) | Zero (transfer valve switches flow) | Continuous chemical processes, pharma, power generation, 24/7 manufacturing |
Single-Bag Filter Housings: The Standard Configuration
A single-bag filter housing holds one filter bag inside a pressure vessel with a removable lid. Process fluid enters the housing, flows through the bag, and exits clean through an outlet port. When the bag loads up with contaminant and differential pressure rises, you shut down flow, open the lid, swap the bag, and restart.
Single-bag housings are the most widely installed configuration in Canadian industrial facilities — and for good reason. They are compact, affordable, and handle a broad range of flow rates and pressures.
When a Single-Bag Housing Is Enough
- Flow rates under 180 GPM. A standard #2 bag in a properly sized housing handles up to 180 GPM. Most small-to-mid-size applications (parts washing, coolant filtration, small water treatment skids, batch chemical processing) fall well within this range.
- Intermittent or batch processes. If your process runs in batches or can tolerate a 5-to-15-minute interruption for bag changes, a single-bag setup keeps costs low.
- Space-constrained installs. Single-bag housings have the smallest footprint of any bag filtration option. Floor space, headroom for lid removal, and piping are all minimal.
Sizing a Single-Bag Housing to Flow
Bag size and housing inlet/outlet diameter determine maximum flow. The relationship is straightforward:
- #1 bag (7" x 17"): up to 100 GPM in a standard housing
- #2 bag (7" x 32"): up to 180 GPM — the most common industrial size
- #4 bag (4" x 15"): up to 50 GPM — compact option for low-flow polishing
For more on bag sizing and ring compatibility, see Filter Bag Sizes, Dimensions & Ring Compatibility.
ERE Single-Bag Housing Options
ERE stocks Sampson single-bag housings in 304 and 316L stainless steel with threaded or flanged connections. For corrosive environments where stainless is not chemically compatible (concentrated acids, chlorine, caustics), the Sampson HX Plastics line offers uPVC and CPVC construction. The A.P. Buck BF Series covers standard industrial applications at competitive price points.
Multi-Bag Filter Housings: Scaling Up for High Flow
A multi-bag filter housing arranges multiple bags in parallel inside a single large vessel. Flow divides across all bags simultaneously, which does two things: increases total throughput and reduces the loading rate on each individual bag.
When You Need a Multi-Bag Housing
- Flow rates above 200 GPM. Once you exceed single-bag capacity, the choice is either multiple standalone housings plumbed in parallel or one multi-bag unit. The multi-bag housing is almost always the better option — one vessel, one lid, one set of connections, lower installed cost.
- Extended service intervals. Distributing flow across 4, 8, or 12 bags means each bag loads more slowly. If your single-bag setup requires weekly changes, a multi-bag housing handling the same total flow might stretch to monthly changes — reducing labour cost and consumable spend.
- High dirt-load applications. Municipal water intake, cooling tower blowdown, and industrial wastewater carry heavy particulate loads. Multi-bag housings provide the dirt-holding capacity to handle it without constant intervention.
Bag Change Logistics in Multi-Bag Systems
Multi-bag housings require the same shutdown-and-open procedure as single-bag units — but you are changing multiple bags at once. The larger lid (bolted closure on most units) takes more time to open, and you need adequate overhead clearance and floor space for the operation. Plan for 15 to 30 minutes per change-out depending on the number of bags and housing size.
Some facilities stagger bag changes by installing isolation valves on multi-bag housings, but this adds complexity and is not standard practice. If uninterrupted flow is critical, a duplex configuration is the correct answer.
ERE Multi-Bag Housing Options
The Sampson Multi-Bag Filter Housing (EMC) handles 200 to 1,000+ GPM depending on bag count and size. The BFS Series from Shelco is another high-flow multi-bag option available through ERE. Both are built for continuous industrial service with carbon steel or stainless construction.
Duplex Bag Filter Housings: Zero-Downtime Filtration
A duplex bag filter housing pairs two independent filter chambers with a transfer (diverter) valve between them. At any given time, one chamber is active and filtering while the other is on standby with a fresh bag installed. When the active bag needs changing, you turn the transfer valve to route flow through the standby chamber — instantly, without interrupting the process. Then you change the bag in the now-idle chamber at your convenience.
How the Transfer Valve Works
The transfer valve is the key component that separates a duplex housing from two standalone housings plumbed in parallel. It provides a single-handle switchover that redirects flow from one chamber to the other. No isolation valves to sequence, no risk of accidental dead-heading, no process interruption. The switchover takes seconds.
When Duplex Is the Right Configuration
- Continuous-flow processes that cannot stop. Chemical reactors, pharmaceutical production lines, power plant cooling loops, and 24/7 manufacturing operations all depend on uninterrupted fluid flow. Any stoppage — even 10 minutes for a bag change — means lost production, off-spec product, or safety risk.
- High dirt-loading with frequent bag changes. If your application consumes bags quickly, a duplex housing lets you change bags on your schedule instead of shutting down every time pressure rises.
- Regulatory or quality requirements. Some processes (pharma, food-grade, boiler feedwater) cannot tolerate even brief periods of unfiltered flow. Duplex housings guarantee continuous filtration coverage.
Duplex vs. Parallel Standalone Housings
You can achieve similar functionality by installing two single-bag housings with manual isolation valves and a bypass arrangement. But the duplex housing is purpose-built for the job: the integrated transfer valve is simpler to operate, reduces the chance of operator error, and takes up less floor space than two standalone units plus piping and valves.
Decision Framework: Choosing the Right Configuration
The selection process comes down to two questions asked in sequence:
Step 1: What Is Your Required Flow Rate?
- Under 180 GPM: single-bag housing is sufficient. Size the housing and bag to your flow — no need to oversize.
- 180 – 200 GPM: borderline. A large single-bag housing with a #2 bag may work, but you are at the upper limit. Consider a small multi-bag (2-bag) housing for headroom.
- Over 200 GPM: multi-bag housing. Select the bag count based on total flow divided by per-bag capacity, with some margin for dirt loading.
Step 2: Can Your Process Tolerate Downtime for Bag Changes?
- Yes — batch process, intermittent operation, or redundant lines exist: standard (non-duplex) housing. This is the majority of installations.
- No — continuous process, zero tolerance for flow interruption: duplex housing. Available in single-bag-per-side (for flows under 180 GPM) or multi-bag-per-side (for high-flow continuous applications).
Don't Forget Pressure
Standard bag filter housings are rated for 150 PSI or less. If your system operates above that — high-pressure chemical injection, boiler feedwater, certain oil and gas applications — you need a housing rated for the actual working pressure. The Sampson High Pressure Bag Filter Housing (EMC) is rated for 150+ PSI service and available through ERE.
Real-World Examples
Small Machine Shop — Single-Bag
A metalworking shop running a parts washer at 40 GPM installs a single-bag housing with a #1 polypropylene felt bag at 25 microns. Bag changes happen every two weeks during routine maintenance downtime. Total filtration cost: one housing, one bag every 14 days. Simple, effective, low overhead.
Municipal Water Treatment — Multi-Bag
A water treatment facility filtering 800 GPM of raw intake water uses a 12-bag multi-bag housing. Each #2 bag handles roughly 67 GPM — well within capacity — which extends bag life and keeps differential pressure low. The facility changes all 12 bags during a scheduled weekly maintenance window. The alternative (five standalone single-bag housings in parallel) would cost more to install, take up more floor space, and require five separate change-out procedures.
Continuous Chemical Process — Duplex
A chemical plant running a continuous reactor at 120 GPM cannot tolerate any interruption to the cooling water filtration loop. A duplex bag filter housing with a transfer valve provides uninterrupted filtration. The operator monitors differential pressure on the active side, switches flow to the standby side when pressure rises, and changes the spent bag without ever stopping the reactor. Total downtime: zero.
Need help selecting a bag filter housing configuration?
ERE's filtration team can size and spec the right housing for your flow rate, pressure, and process requirements. We stock single-bag, multi-bag, and duplex configurations from Sampson, Shelco, and A.P. Buck — with fast shipping across Canada.
Request a Quote | Call 1-888-287-3732 | Email sales@ereinc.com
Frequently Asked Questions
What is the difference between a single-bag and multi-bag filter housing?
A single-bag housing holds one filter bag and handles flows up to approximately 180 GPM. A multi-bag housing holds 2 to 24+ bags in parallel inside a single vessel, handling 200 to 1,000+ GPM. Multi-bag housings also reduce loading per bag, which extends bag life and stretches change-out intervals.
What is a duplex filter housing?
A duplex filter housing consists of two independent filter chambers connected by a transfer valve. One chamber filters while the other stands by with a fresh bag. When the active bag needs changing, you switch flow to the standby side — providing continuous, uninterrupted filtration with zero process downtime.
When should I choose a duplex housing over a standard housing?
Choose duplex when your process cannot tolerate any flow interruption — continuous chemical reactions, pharmaceutical production, 24/7 manufacturing, or any application where even a 10-minute shutdown causes lost product, off-spec material, or safety concerns.
Can I use a multi-bag housing in a duplex arrangement?
Yes. For high-flow continuous processes, duplex configurations are available with multi-bag chambers on each side. This combines the flow capacity of multi-bag filtration with the zero-downtime advantage of duplex operation. These are typically custom-engineered for the specific application — contact ERE for sizing assistance.
Related Articles
- Industrial Filter Bags: Complete Selection Guide for Canada
- Bag Filter Housings: Sizing & Selection Guide
- Filter Bag Sizes, Dimensions & Ring Compatibility
- Polypropylene vs. Nylon vs. Polyester Filter Bags
- Filter Bag Micron Ratings: How to Choose
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