Stainless steel Duplex filter
Introduction
Duplex filter is a pair of stainless steel filter housings joined by one changeover valve. Liquid flows through one housing while the other sits in standby. When the working screen clogs, you turn the valve and flow shifts to the spare, so the line never stops. Plants that move milk, syrup, beverage, process water, or oil around the clock use it because a single filter would force a shutdown every time the element fouled.
This guide covers how the two-housing layout works, what sits inside, and the points that decide whether you spec a manual, semi-automatic, or automatic unit.
What a duplex filter is for
The duplex filter, also called a duplex changeover filter, removes solids you cannot see from liquids. In a dairy it cleans the milk before pasteurization. In a beverage plant it guards the syrup and juice lines. In water treatment it protects reverse osmosis membranes from suspended grit. The job is simple: trap rust, scale, sand, and debris before they reach a pump, valve, or membrane and wear it out. What makes the duplex version different from a plain single filter is that it does this without halting the run.
How the two housings keep flow going
Each housing holds a filter element. One is on duty, the other on standby. Fluid enters, passes through the mesh or basket in the active housing, and leaves clean. As solids pile up, the pressure drop across the element climbs. A gauge or differential-pressure indicator shows when the element is near its limit. At that point you turn the three-way valve. Flow moves to the standby housing in seconds, and the clogged side is now isolated. You open its cover, pull the basket, rinse or replace it, and close it up ready for next time. The product never stops moving.
Most suppliers trigger the swap at a pressure drop of about half a bar to one bar across the element. That window is wide enough to catch fouling early but narrow enough that you are not throwing away good screens. On an automatic unit the valve flips on its own the moment the threshold hits, which is the whole reason to pay for the controls.
What sits inside the housing
Inside each barrel is a stainless steel mesh or a support basket that holds the filter medium. Cheap setups use a cloth or bag. Food and pharma lines use a welded stainless screen that you can steam and scrub. The top cover is a quick-open type so you reach the basket without tools. A vent valve on top releases trapped air when you start up, which matters because air locks cut flow and throw off the pressure reading. The whole unit is mirror polished inside and out, and the pipe connections use clamps or live joints so you can break it down for cleaning.
Manual, semi-automatic, or automatic
Manual duplex filters switch with a lever or handle. They cost less and suit lines with an operator nearby who checks the gauge. Semi-automatic units add an assisted switch that cuts the effort. Automatic units fit actuators and sensors that read the differential pressure and flip the valve on their own, then log the event. For an unmanned or high-value line, automatic pays for itself the first time it swaps over at 2 am without anyone waking up. Most food plants start manual and move to automatic as volume grows.
Material: 304 vs 316L
Duplex filter housings are almost always stainless. Grade 304 covers water, milk, and most food lines and shrugs off the mild stuff. When the fluid is salty, acidic, or cleaned with strong chemistry, 316L is the safer call because the extra molybdenum resists pitting. One hard rule from the manufacturers: never wash these units or store liquid in them with anything containing chlorine, because chloride cracks stainless and the damage is permanent. Our sister mill stocks both grades, from 316L plate to 304L bar and tube, if you want the metallurgy before you specify.
How to choose the right size
Size the filter to the flow it must pass, not to the tank it feeds. Match the pressure rating to your line, pick a micron grade that catches what harms your downstream gear, and decide the element type (mesh, bag, or cartridge) by how often you can stop to change it. Note the port size, usually 32 or 40 mm on the smaller units, and leave room for a bypass line so you can service the whole assembly. A typical small unit runs at 0.2 MPa, filters five to ten tonnes an hour, and holds a screen of 60 to 400 mesh depending on the job. The Chinese industry standard that covers these units is QB/T 1166.
| Spec | Small unit | Mid unit | Slightly larger |
|---|---|---|---|
| Filter area | 0.15 m² | 0.30 m² | 0.75m² |
| Capacity | 5 t/h | 10 t/h | 30t/h |
| Working pressure | 0.2 MPa | 0.2 MPa | 0.2 MPa |
| Temperature | ＜100 ℃ | ≤100℃ | ≤100℃ |
| Screen mesh | 60 to 400 | 60 to 400 | 60 to 400 |
| Port size | 32 mm | 40 mm | G2＂ |
| Material | 304 or 316L | 304 or 316L | 304 or 316L |
Where a duplex filter fits
The filter sits inline ahead of the vessel it protects. In water systems it guards the pressure water tank and the pump that feeds it. In a mixing line it sits before the mixing tank so powder and scale never reach the batch. In dairy it protects the pasteurizer and the sterilization tank. In an emulsifying line it shields the high-shear emulsifying tank. Because it touches the same stainless the product lives in, the filter and the tank should share one material standard. Wanzhi builds the tanks these filters guard, so the two are designed to match. That also covers the 304 water tank and the jacketed mixing tank used across these lines. See the full stainless steel tank range for the vessels a duplex filter typically protects.
Conclusion and next step
Choosing a duplex filter comes down to three things: keep the line running, size to the flow, and match the material to the fluid. The two-housing layout is the whole point, so do not settle for a single filter on a line that cannot afford a stop. Pick manual until volume forces automatic, and go 316L the moment the chemistry turns aggressive. If you are specifying filtration for a new or upgraded line, look at the stainless steel tanks and pasteurizers the filter will protect and size them as one system. The steel behind those vessels comes from our sister mill, which you can read about on the Wanzhi Steel about page. Our team can help match a duplex filter to your flow and explain the grade choice in plain terms, so reach out before you commit to a size.
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