Genset exhaust outlet panel manufacturing: cut, weld, coat

A genset exhaust outlet panel is the one painted part on a canopy that has 400 °C passing through it. The panel itself is ordinary work — 2 mm CRCA, four flanges, a louvre or a hood — but the collar where the flue leaves the enclosure sits at the meeting point of a heat path, a vibration path and a water path. Genset exhaust outlet panel manufacturing is decided by three dimensions on that collar, and a drawing that omits them sends the problem to site.

The first is the aperture. A 100 mm flue cut through a 100 mm hole is a shrink fit at ambient and a buckled panel at running temperature, because the stainless pipe grows 6.4 mm over a 1,000 mm run from 30 °C to 400 °C. The second is the coating. A standard polyester powder is comfortable at 120 °C and chalks above 150 °C, and the panel within 100 mm of the collar is hotter than that. The third is the support. A silencer hung from the panel puts a 12 kg mass on a 2 mm skin at the firing frequency of the engine, and the collar weld becomes the fatigue specimen.

None of those three is expensive to fix at the drawing stage. All three are expensive after the canopy is coated, because the fix is a re-cut panel, a re-mask and a second pass through the booth.

What genset exhaust outlet panel manufacturing has to survive

The exhaust gas leaving the manifold of a diesel genset runs at 450–650 °C, and a silencer brings the outlet gas down to roughly 250–400 °C. The flue surface itself sits at 350–450 °C near the silencer, and the panel does not see that temperature directly: it sees what conducts along the collar.

A worked heat path makes the coating zone concrete. Take a 3 mm stainless collar, 60 mm long, carrying heat from a 400 °C flue to a panel at 100 °C. Stainless steel conducts at about 16 W/m·K, the collar's cross-section is π × 0.1 × 0.003 = 9.4 × 10⁻⁴ m², and the gradient is 300 K over 0.06 m. The conduction is Q = kAΔT/L = 16 × 9.4 × 10⁻⁴ × 300 / 0.06 = 75 W.

That 75 W spreads into perhaps 0.2 m² of panel, which loses heat to still air at about 10 W/m²·K. The rise is 75 / (10 × 0.2) = 37 K, so the panel near the collar sits around 70 °C when the ambient is 33 °C: hot to the touch and well inside what powder tolerates. Shorten the collar to 25 mm and the same calculation gives 181 W and a 90 K rise, which puts the panel at 120 °C and the inner 50 mm into the range where a standard powder starts to chalk. The collar length is a coating decision, not only a fabrication one.

The second load is acoustic. The exhaust is the dominant noise path on a canopy at low frequency, and no louvre fixes 125 Hz. A louvre with 45° blades at 50 mm pitch gives roughly 5–8 dB of insertion loss around 500 Hz and 2–3 dB at 125 Hz, so it protects the mid-band and barely touches the low end. Where the canopy has an acoustic target, the exhaust silencer and the panel aperture are sized together; a panel designed after the silencer is a hole in an acoustic wall.

The third load is water. A standby set can stand for a month between runs, and a flue that faces up collects rain. Wind-driven rain at 12 m/s arriving at 15° to the horizontal will travel up a vertical pipe, which is why the outlet needs a hood with a 50 mm overhang and blades that face down, not a flat louvre set in the panel plane.

The dimensions that go on the panel drawing

The table is the specification we work to for a canopy exhaust panel, with the reason for each line. It is written for a 2 mm CRCA panel with a stainless collar; an aluminium canopy changes the coating lines and not the geometry.

Two lines deserve a note. The 10 mm radial clearance is not a fit tolerance; it is the space the flue needs to move while the panel stays where the canopy frame holds it. And the coating exclusion zone is measured from the pipe centre, not from the collar edge, because heat spreads radially through the panel and the 150 °C contour lands about 120–150 mm out on a 2 mm skin.

Cutting and forming the panel

The panel is nested with its outer profile, the aperture, the louvre openings and the fastener holes in one programme. Cutting the aperture in a second operation is the common mistake: the outer profile is located from the blank edge, the aperture from a different datum, and the two errors add. On a 900 mm wide panel a 0.5 mm locating error at each stage is a 1 mm offset between the flue and the louvre, which is visible from outside the canopy.

The 12 kW fibre laser cuts 2 mm CRCA at 8–12 m/min with a kerf under 0.3 mm and a square edge that needs no deburring on the outer profile. The aperture edge is different: it is a heat-affected edge that will sit inside the collar weld, so it is cleaned to bright metal before welding rather than coated over. Laser oxide on a cut edge is a coating adhesion problem on any panel, and on this one it is also a weld inclusion problem.

Forming is done on the 200 T, 5-axis brake with the 4,450 mm bed. The panel gets 25 mm flanges on four sides, which is what keeps a 2 mm skin from drumming, and the flange height is set by the bend allowance rather than by appearance. A 25 mm flange on 2 mm CRCA needs a minimum bend radius of about 2 mm and a die opening of 16 mm; anything tighter marks the outside face and the mark shows through a 90 µm powder film.

The louvre is either a set of formed blades welded into a frame or a separate louvre panel bolted over the aperture. The bolted version is better for service because the louvre can be removed to inspect the silencer, and it lets the louvre be coated separately from the panel. The welded version is stiffer and cheaper, and it leaves the painter working into inside corners where the film thins.

Welding the collar without pulling the aperture oval

The collar is a 3 mm stainless sleeve with a 5 mm flange, set into the aperture and welded to the panel on the outside face. A 3 mm fillet around a 320 mm circumference shrinks 0.3–0.9 mm in the transverse direction, and if the weld runs continuously around the circle the shrinkage accumulates in the last quadrant and pulls the aperture into an oval.

Two things control it. Weld in four opposite quadrants, 80 mm at a time, allowing the panel to cool between runs, and clamp the panel to a flat fixture with the collar piloted on a mandrel so the aperture cannot close. The mandrel is a turned disc 0.2 mm under the aperture diameter, and it stays in until the panel is cool. On a 2 mm panel the difference between a mandrelled collar and a free one is 1–1.5 mm of ovality, which is the difference between a flue that fits with a 10 mm gap and one that touches on two sides.

Stainless to mild steel is a dissimilar joint, and it is welded with a 308L filler rather than a mild steel wire. A 309L or 308L deposit tolerates the dilution better and does not crack at the toe the way a matching carbon steel wire can on a 3 mm section. The panel side of the weld is ground flush where the coating has to run over it, because a 0.5 mm proud weld toe is a sharp edge under a 90 µm film.

Heat, noise and water at the finished panel

Once the geometry is right, the panel has to be detailed for the three loads it carries. For heat, the collar length and the exclusion zone do the work, and the fasteners around the collar are stainless so they do not lose tension at 90 °C. For noise, the louvre blades overlap by at least 25 mm in the line of sight so that direct sound has to turn twice, and the aperture is kept as small as the flue allows because every 0.01 m² of open area is a leak in the acoustic envelope.

For water, the hood is a separate 1.5 mm pressing with a 50 mm overhang and a 15° fall, and it is fitted after coating so its fixing holes are sealed with a silicone washer rather than left as bare metal in the rain path. The bottom edge of the hood is turned down 10 mm to form a drip, so water that runs off does not track back along the panel by capillary action.

The gasket at the flue is the detail that gets specified last and fails first. EPDM is rated to about 120 °C and hardens in a season at the collar; silicone at 200–250 °C or a graphite-based rope is the correct choice, and it should be called out with the flue diameter on the drawing rather than left to the assembly team.

Where exhaust panels fail in service

Coating burnt back from the collar. A powder film that stops 60 mm from the pipe instead of 150 mm chalks within weeks, then loses adhesion, then rusts. The rust stain runs down the panel and reads as a coating failure, though the cause is a heat zone that was never defined.

Aperture pulled oval. The collar was welded without a mandrel and the flue no longer fits with clearance. The panel is scrapped after coating, which is the most expensive time to find it.

Cracks at the collar toe. The silencer was supported from the panel, or the collar was welded with a single pass and no throat. The crack is visible as a rust line before it is visible as a crack.

Water in the silencer. The outlet faces up or the louvre is flush. On a standby set the water sits for weeks and the silencer fails from the inside, which is not a panel defect but is always blamed on the canopy.

Drumming at 50 Hz. A 2 mm panel with 25 mm flanges and no mid-span stiffener resonates near the firing frequency. A swaged stiffener or a 30 mm hat section across the panel moves the mode up and takes the boom out.

Sharp aperture edge after service. The flue is removed for a silencer change and the bare laser edge inside the collar cuts a glove. A 1 mm radius or a fitted trim ring is worth specifying.

The line that builds canopy panels

Exhaust panels for genset canopies run the same route as the rest of the enclosure at Ganesh: nested and cut on the 12 kW fibre laser with its 40 ft × 16 ft bed, formed on the 200 T, 5-axis brake with the 4,450 mm bed, and welded in fixtures built in our own tool room on three CLOOS robotic MIG cells or one of more than 30 manual stations. The panel then goes through the 16 ft × 16 ft blast chamber, seven-tank pretreatment and the 18 ft × 14 ft powder booth, with the heat zone masked before it enters the line.

We build canopies, acoustic enclosures, base frames and skid bases for Mahindra Powerol, and the exhaust panel is quoted as part of the canopy kit rather than as a loose pressing, because the aperture position and the louvre are set by the silencer that goes inside. The parts are catalogued under genset parts.

The panel that surrounds this one is covered in genset acoustic enclosure design, the forming detail in bending for genset canopies and panels, and the enclosure-level weld practice in welded sheet metal enclosures. Canopy, louvre and base frame guides are collected under genset, canopy and acoustic enclosures. The canopy RFQ checklist lists the fields an enquiry needs before a panel can be priced.

What to put on the drawing

1. Flue outside diameter, silencer mass and the point where the flue is supported.
2. Aperture diameter as flue OD plus 20 mm, and its position from the canopy datum.
3. Collar material, length and weld size, with the instruction to weld in opposing quadrants on a mandrel.
4. Coating exclusion zone, measured from the pipe centre, and the coating on the collar.
5. Louvre type, blade angle, free area and whether it is removable.
6. Hood overhang, fall and drip detail.
7. Gasket material at the flue and at the louvre.
8. Finish system, DFT and the colour of the internal face.

When this panel is not the right answer

A flue that leaves through a roof rather than a wall does not need a canopy panel at all: it needs a factory-made roof penetration with a flashing and a storm collar, and a fabricated panel around it usually leaks. A set with a remotely mounted silencer and an insulated flue has different surface temperatures, and the 150 mm exclusion zone can shrink, but only after the insulation thickness is known.

Where the exhaust is hot enough to need refractory lining, or the canopy is a walk-in acoustic room with a rated door, the panel is part of a tested assembly and the penetrations have to follow the acoustic supplier's detail rather than a sheet metal drawing. And where the canopy is aluminium, the collar cannot be mild steel; the joint needs an insulating washer to keep the dissimilar metals apart in a wet enclosure.

Send the flue diameter, the silencer mass and the louvre requirement with the canopy drawing. Request a quote