8-Inch Long-Radius 90° Airflow Duct Elbow

Description

8-inch long-radius 90-degree duct elbow — the gentle turn

This 8-inch long-radius 90-degree duct elbow is built for one thing: turning a full-size exhaust run through a right angle while giving up as little as possible in the process. The bend radius is 175 mm, an R/D of 0.858, and both ends are the line’s standard 8-inch rigid sockets, so it drops into an existing run in place of a sharp corner.

Bend loss is one of the few things in duct design that is genuinely settled engineering. Published duct-loss data puts a long-radius round elbow of this proportion at a loss coefficient of about 0.40, against 1.04 for a compact, tightly mitred 90 and 0.92 for two 45-degree elbows stacked in series. Those numbers come from the published literature on duct fittings, not from a claim of ours — the physics is that a sweeping turn keeps the airstream attached to the outer wall, while a sharp turn separates the flow and leaves a stalled, turbulent core doing no useful work.

The cost is space. This elbow occupies roughly 304 × 209 × 304 mm, which is real volume behind a machine. Give it that room and it is the best turn in the line; where the duct has to fold into a tight shelf gap or hug a wall, the compact 90 is the part that fits.

What it fits — 8-inch rigid pipe, both ends

• 8-inch rigid snap-lock pipe, seated in a socket at each end.
• Any other 8-inch rigid fitting in the line — couplers, reducers, window ports.
• Flex duct, once an 8-Inch Flex Duct Adapter — Rigid Socket to Flex Collar is seated in the socket. A flex core cannot sit square inside a socket on its own.

Working in 6-inch duct? The 6″ 90° Duct Elbow (Compact) is the 6-inch right angle in the line. Short of space at 8 inches? Take the 8″ 90° Duct Elbow (Compact) instead and keep the run tight to the wall.

How it installs

1. Plan the corner first — mark out the 304 × 209 × 304 mm envelope where the run turns, so the sweep has somewhere to go.
2. Seat rigid 8-inch pipe into the socket at each end and secure with foil tape or sheet-metal screws as your run is built.
3. Where the run continues in flex, put the 8-inch flex adapter into the socket first and clamp the flex over its collar.
4. Support the elbow near the corner. It is the heaviest fitting in the 8-inch family, and a supported bend keeps the pipe joints square.

Why a long sweep costs less than a sharp corner

Air moving down a duct has momentum, and momentum does not like corners. At a tight bend the stream cannot follow the inside wall, so it separates: a recirculating pocket forms on the inner radius, the effective cross-section shrinks, and the fan behind it has to push against pressure it never sees on a straight run. Lengthening the radius keeps the flow attached the whole way round, so the section stays open through the turn and the pressure penalty falls to a fraction of what the sharp version costs — which is exactly what the published coefficients describe. Two 45-degree elbows in series get part of the way there, because they split the turn into two smaller ones with a straight in between, but they still cost more than one continuous sweep and take two joints to do it.

• Sweep the turn on a 175 mm radius, R/D 0.858, instead of folding it.
• Sit at 0.40 on the published loss-coefficient scale, against 1.04 for a compact 90 and 0.92 for two 45s in series.
• Replace two elbows plus a coupler with one purpose-shaped bend and two joints instead of four.
• Drop into any 8-inch run on the line’s standard rigid sockets.
• Continue in flex through the 8-inch flex adapter when the run has to leave rigid pipe.
• Hold its shape in PETG at ASIC exhaust temperatures, where PLA softens.

Specifications

Open source — print it yourself, or buy it finished

Long-radius bends are the fitting hardware stores are worst at stocking at this size, so D-Central Technologies drew one — the 175 mm radius, the sockets, the wall sections — and gave it away. The design is ours, released under CC BY-SA 4.0 through the 3D models download center, free to download, modify, remix, print and sell, radius and all. The model card is the 8-inch long-radius 90° elbow.

The file is elbow_90_8_v2.zip, with the STL, the CC BY-SA 4.0 licence and the installation and safety guide inside. Making a 304 mm sweep yourself is a serious job, which is why this listing exists: the finished elbow, made to order in PETG in Montreal, Quebec, black or orange, with sockets matched to the rest of the 8-inch family. The file costs nothing today and nothing later — the part you buy, or D-Central’s fund page, is what keeps the open design work going.

Works with

• 8″ 90° Duct Elbow (Compact)
• 6″ 90° Duct Elbow (Compact)
• 8″ 45° Duct Elbow
• 8″ Duct Coupler
• 8-Inch Flex Duct Adapter — Rigid Socket to Flex Collar

Build the full run from the D-Central shroud and ducting system.

Frequently asked questions