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Quotes - Engineering - SalesWhat double wall means inside a brazed plate
In a standard brazed plate exchanger a single 316L plate separates the two fluids. If that plate pits through or a braze line cracks, the higher-pressure fluid crosses into the other. On most industrial duties that is a tolerable failure caught at the next pressure test. For drinking water it is not: a glycol loop, a boiler circuit carrying treatment chemicals or a lube oil cooler must not be able to leak into potable water without warning. Plumbing codes answer that with the double-wall requirement, and the brazed plate version is built to satisfy it.
A double-wall pack replaces each separating plate with two plates pressed to the same pattern and laid face to face. They are brazed at the ports and along the edge, except at open vent slots, but not across the pattern, so a thin gap remains between them, and that gap is open to atmosphere through vent slots along the plate edge. A leak through either plate shows at the slots as a weep or a drip before it reaches the other fluid; to mix, a leak has to pass two walls. The second plate and the air film between the two add thermal resistance, so the pack needs more surface for the same duty: more plates than the single-wall selection, and a lower overall coefficient to size against.
The right place for it is any duty where the code or your own risk assessment says a single wall is not enough and the fluids are clean enough for a 1–3 mm channel: potable hot water from a boiler or heat-pump loop, process water that must stay apart from a heat-transfer oil, food-plant water against a refrigerant. Where the code calls for it, the unit also carries a drinking-water material listing and a lead-free certification, as specified on the order. What the second wall does not do is change the plate chemistry: a 316L plate on 1,500 ppm chloride well water can pit whether there are one or two of them.
Double-wall packs use 316L for both plates of each pair, copper brazed for potable water against glycol or boiler water, nickel brazed when the heating fluid is copper-sensitive. See Material & Construction for the grade comparison.
What the second wall gives and takes
What it protects
- Potable water from glycol, boiler water, oil or refrigerant without a leak-detection system
- Compliance where the code calls for a vented double wall between potable and non-potable fluids
- A visible leak warning at the edge slots, no instrumentation required
- The same 316L plate chemistry as the single-wall pack on both wetted sides
What it costs
- More plates for the same duty: the second plate and the air film slow the heat flow
- A lower coefficient per plate, so a longer pack or a steeper chevron that raises pressure drop
- A larger footprint and a heavier block to hang
- A rating that can sit below the single-wall figure; confirm it at your design temperature
What to specify
- Which fluid is potable, and which code or inspector calls for the double wall
- Both flows, both temperature pairs, and the allowable pressure drop on the potable side
- Drinking-water material listing and lead-free requirement, as specified
- Mounting with the vent slots visible and a drip path to a drain
Double-wall build
Standard build figures; the quotation fixes the plate count and the rating for your duty.
| Feature | Typical build |
| Separating wall | Two 316L plates per pair, brazed at the ports and edge, vent slots left open |
| Leak vent | Slots at the plate edge open to atmosphere; a weep shows the first failure |
| Braze filler | Copper for potable water against glycol or boiler water; nickel where the heating fluid attacks copper |
| Design pressure | 435–650 psig on single-wall plates; the double-wall rating is design-specific and may be lower |
| Channel gap | 1–3 mm; strainer on both sides, filtered or softened potable supply |
| Surface penalty | More plates than the single-wall selection for the same duty |
| Potable listing | Drinking-water contact and lead-free material certification where the code requires it |
| Mounting | Vertical, slots unobstructed, no insulation over the vent edge, drip tray or drain below |
Questions before you specify double wall
Most U.S. plumbing codes require double wall wherever potable water is heated or cooled by a fluid that is not itself fit to drink. A single wall may be allowed when the transfer fluid is classed as essentially non-toxic, such as food-grade propylene glycol, usually with pressure and labeling conditions; the inspector having jurisdiction decides which case you are in. If the heating fluid is boiler water with treatment chemicals, ethylene glycol or oil, assume double wall.
Enough that you must not size it from a single-wall table and add a margin. The second plate and the air gap between the pair cut the overall coefficient, and the plate count rises to make up the surface. We size double-wall packs on their own coefficient, with the vent gap in the thermal path, and the quotation shows the resulting plate count and pressure drop against the single-wall alternative.
Hiding the vent. The vent slots run along the plate edge, and an installer wraps the block in insulation or mounts it tight to a wall with the slots facing in. A leak then weeps in to the insulation for months with nobody the wiser, which is exactly what the double wall was bought to prevent. Leave the edge exposed, face the slots where a drip can be seen, and put a drain or tray under it.
Poorly. The gap drains to the slots by gravity, and a block laid flat holds leaked fluid between the plates instead of weeping it out, so the warning comes late or not at all. Mount the pack vertical with the ports facing out, which is also the orientation that keeps air out of the top channels. If the space forces a horizontal mount, tell us before the quote rather than after.
No. The double wall addresses leakage, not corrosion. A 316L plate can pit in hot or high-chloride water at the same rate whether it stands alone or in a pair, and the second plate only buys time before the leak reaches the other fluid. On water past the 316L limit the answer is a titanium or higher-alloy plate, usually in a gasketed frame, and the double-wall question is asked separately on top of that.
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