400 mm Spiral Duct Through a 240 mm Void in an Occupied Kitchen
A staff kitchen canopy is served by a 400 mm diameter galvanised spiral duct that rises 3.5 m and then runs horizontally 4.2 m through a ceiling void before rising through a slate roof. You are replacing the horizontal section. The drawing shows a 300 mm deep void; a site measurement taken through the access hatch gives 240 mm clear between the joist soffit and the ceiling lath. The existing duct is hung on plumber's tape from ceiling joists at roughly 3 m centres, passes about 150 mm from a 100 mm cast-iron soil stack, and goes through a 60-minute fire compartment wall with no damper fitted. The kitchen is in use and must reopen in nine days. The client wants the ceiling closed and decorated the moment the duct is in. The workshop can fold any rectangular galvanised section from 1.2 mm sheet within four days; the existing fan curve is on file. Anything bought in is a two-week lead.
1. The 400 mm circular duct clearly cannot be fitted in a 240 mm void. Compare the realistic rectangular substitutes and recommend one, showing your reasoning and the checks you would make before committing.
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Why circular is out: 400 mm diameter plus the clearance needed for slipping it in and for supports needs roughly 450–500 mm of void. At 240 mm it cannot be done without chasing joists (never acceptable without structural approval) or dropping the ceiling line.
Comparing rectangular options - use the equal-friction equivalent diameter, De = 1.30(ab)^0.625/(a+b)^0.25:
| Section | De | Velocity at Q = 0.5 m³/s |
|---|---|---|
| 500 × 200 | ≈ 337 mm | 5.0 m/s |
| 700 × 200 | ≈ 391 mm | 3.6 m/s |
| existing Ø400 | 400 mm | 4.0 m/s |
Both 200 mm deep sections fit the 240 mm void with 40 mm to spare. But 500 × 200 behaves like a 337 mm duct: velocity rises by about 40% and pressure drop per metre roughly doubles, plus you add two taper losses at each transition. The fan has to find that extra resistance, and the kitchen will lose extract exactly when it is busiest.
700 × 200 is the better judgement call: equivalent diameter ≈ 391 mm, velocity essentially unchanged, aspect ratio 3.5:1 (within the usual 4:1 limit), and it is makeable in the workshop inside four days.
Before committing: pull the fan curve and check available external static pressure against the revised system resistance including both transitions; check the section actually fits round the joist hangers and the soil stack; check noise at the higher velocity if 500 × 200 is forced; confirm with the client in writing that the ceiling line will not change.
2. List what must be resolved before the ceiling is closed and decorated, and explain the consequence of getting each wrong.
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1. Fire compartment wall. The 60-minute wall has no damper. A tested fire damper (or a tested penetration seal system appropriate to the wall) must be installed, sized to the new rectangular section, and it must be independently supported - not hung off the duct. It also needs an access door on the accessible side for inspection, testing and reset, because an inaccessible damper is a damper nobody ever tests. Consequence of skipping this: the compartment is breached, the building's fire strategy is invalid, and the ceiling has to be opened again later at your cost.
2. Supports. Plumber's tape off ceiling joists is not a duct support system. Rigid duct is normally supported at 3 m maximum centres with proper hangers, and the joists were never designed to carry the duct. Fix independent hangers to structure, and remember the damper needs its own. Consequence: sag, joint separation, leakage, and a ceiling that cracks.
3. Clearance and services. Confirm the clearance to the cast-iron soil stack is adequate for maintenance and that nothing is trapped between duct and joist. Consequence: no access for cleaning or repair.
4. Grease and condensate duty. Even a light-duty kitchen extract carries grease and moisture. Confirm the specification accepts a galvanised duct with access doors at changes of direction and a low-point drain; if the canopy is heavier duty, the duct should be a purpose-made grease duct. Consequence: grease build-up and a fire risk inside the duct.
5. Commission before closing. Air-flow test and a photo record of the installation while it is still open. Once the ceiling is decorated, every one of these becomes a demolition job.