Square Metal Substrates for Industrial Exhaust – Why Round Doesn't Always Fit

Most people think catalytic converters are round. That's what's under a car. Round can, round metal substrate, round everything.

But industrial exhaust? That's different. Sometimes the duct is square. Sometimes the only space in the thermal oxidizer is a rectangular opening. Round won't fit. You need a square metal substrate.

We make them. They're a pain compared to round ones, but when a customer needs square, they need square. Here's what we've learned about building square metal honeycomb for industrial exhaust treatment.

Why Square? Why Not Just Use a Round One?

Two reasons.

First, space. A thermal oxidizer or catalytic reactor is often built with square or rectangular ductwork. Putting a round catalytic substrate inside a square duct means wasted space in the corners. Exhaust can bypass around the round can. That kills conversion.

Second, flow distribution. Round metal honeycomb in a square housing creates uneven flow. The gas speeds up around the edges. The center might get less flow. That means some of the catalyst is overworked and some is underused. Overall efficiency drops.

Square metal substrate fills the whole duct. No bypass. Even flow. Higher conversion per volume.

We had a customer with a VOC oxidizer. Round catalytic carrier in a square housing. They were only getting 70% destruction. Switched to square metal honeycomb – same size housing – destruction went to 95%. No other changes.

How Square Is Different to Make

Round metal substrates are easy. You corrugate the foil, take two strips – one flat, one wavy – and wind them around a mandrel. Like rolling a sleeping bag. Easy. Consistent.

Square is different. You can't wind a square. You have to stack flat strips. Layer on layer, cut to length, stacked in a fixture. The cells have to line up perfectly. Any shift during stacking, and the channels are crooked.

Crooked cells mean bad flow distribution. The exhaust takes the path of least resistance – usually the gaps where cells don't line up. That kills conversion.

We use stacking fixtures with guide pins. The fixture holds the layers square while we build the stack. Then the whole fixture goes into the brazing furnace. Once brazed, the square metal honeycomb comes out as one piece.

It's slower than winding. Takes more labor. Costs more. But that's how you get a square part that works.

Brazing Square – Corners Are Weak

The corners of a square metal substrate are the hardest part. More foil layers meet at the corners. More brazed joints. More chance of a void or a weak spot.

We use extra brazing filler on the corners. Not a lot – just a thin strip of filler foil laid along the corner before brazing. That ensures the joints are filled.

We also do peel tests on samples cut from the corners. If the corner braze fails, the whole batch is bad.

I remember a batch where the corners looked fine from the outside. But when we peeled a corner sample, it came apart like a deck of cards. The brazing hadn't flowed to the center. We changed the filler placement and slowed the furnace ramp. Every batch after that was solid.

Coating Square – Edges and Corners

Coating a square catalytic substrate is also different from round.

When you dip a round part, the washcoat drains evenly. When you dip a square part, the corners hold more slurry. Dry too fast, and the corners crack. Or the washcoat pools in the corners and plugs cells.

We tilt the square substrate as it drains. Let one corner hang low, then the opposite corner. Helps the slurry run off evenly.

We also use multiple air knives – not just one. Top, bottom, sides. Lower pressure than round, but more angles. It took us a few tries to get it right.

The precious metal coating – platinum, palladium – same deal. Tilt, multiple blow‑offs, lower pressure. And we test distribution. Cut a square metal honeycomb into nine sections. Send each to the lab. The corners shouldn't have more than 10% higher loading than the center.

Mounting Square in the Housing

A square metal substrate needs a square can. That's fine. But sealing the edges is trickier than round.

Round cans use a mounting mat wrapped around the substrate – the mat expands with heat, fills the gap, holds everything tight.

Square cans have corners. The mat has to fill those corners too. If the mat is too thin in the corners, the substrate can shift. We use a denser mat for square applications, and we cut the mat with notched corners so it folds into the corner without bunching.

We also add corner retention – little metal clips welded into the can that hold the square substrate even if the mat loses tension. Overkill? Maybe. But industrial exhaust systems run for years without shutdown. You don't want a rattling catalytic carrier.

Industrial Applications We've Seen

RTO systems. Regenerative thermal oxidizers sometimes use a catalytic stage after the main burner. Square metal honeycomb fits the rectangular duct perfectly.

Chemical plant off‑gas. One customer had a rectangular exhaust stack, and needed to cut VOCs before emission. Round substrate would have left gaps. Square one filled the whole duct. Destruction efficiency went from 80% to 98%.

Printing and coating lines. Those ovens have wide, flat exhaust ducts. Round converters don't fit. We made square metal substrates, 2 feet wide, 1 foot tall, only 6 inches thick. Multiple units in parallel. Worked great.

Landfill gas treatment. A customer needed to remove siloxanes from landfill gas before burning. They had a rectangular vessel. Square catalytic carriers stacked in a grid. Hundreds of pieces. We made them all the same size – tight tolerance – so they stacked without gaps.

What Customers Should Know

If you're buying square metal substrates for industrial exhaust, here's what to ask.

Cell size. For VOCs, 200 to 400 cpsi is typical. If the gas is dirty, go lower – 100 or 150 cpsi – so cells don't plug.

Foil thickness. 0.05 mm for clean gas. 0.08 or 0.1 mm for dirty or high‑temperature.

Material. Stainless 304 or 316. Aluminum is too weak for most industrial applications.

Tolerances. Square substrates need to fit the housing. We hold +/- 1 mm on length and width. For stacked applications, we hold +/- 0.5 mm.

Testing. Ask for flow distribution data across the face. And a cut‑up sample to show corner washcoat.

A Real Square Job

We made a batch of square metal honeycomb for a chemical plant in Texas. The gas stream had chlorinated VOCs – tough stuff. Housing was 24 inches square, 12 inches deep. They needed 300 cpsi, 0.08 mm stainless 316L.

First batch, the corners had uneven washcoat – flaking off after firing. We adjusted the drying ramp – slower, and added rotation during drying. Second batch was clean.

They installed them in a two‑stage system. Inlet VOC was 500 ppm. Outlet dropped to 15 ppm. That's 97% destruction. They've reordered twice.

square metal substrate for industrial exhaust gas treatment are not just round ones flattened. The corners change everything – stacking, brazing, coating, mounting.

But when the duct is square, or the space is rectangular, square catalytic carriers are the only way to get full coverage and even flow.

We've made them for RTOs, chemical plants, printing lines, landfill gas treatment. Every one taught us something new.

If you have a square hole and need a square metal honeycomb to fill it, talk to us. Bring your duct dimensions, your gas composition, your temperature. We'll build one that fits – and stays sealed.