Flex duct calculator — pressure drop as installed

The most misjudged duct on the drawing

Flexible duct is drawn as a straight line between a branch take-off and a diffuser, and sized on the same chart as everything else. On site it goes in a metre too long, slumps between supports and gets pushed back on itself to make the connection, and the inner core ends up bunched like a concertina. Nothing about that looks dramatic, and nothing about it shows on a drawing — but it changes the pressure drop by multiples, not percentages.

The best measurements of this come from a study at Lawrence Berkeley National Laboratory, which tested 6, 8 and 10 inch wire-helix flex to ASHRAE Standard 120 at controlled compression ratios. Moderate compression of around 15 per cent raised the pressure drop roughly fourfold; compression approaching 30 per cent raised it by close to ten. The single size-independent correction curve in older editions of ASHRAE Fundamentals under-predicted those results by about a third, and the gap was largest on the smallest ducts — the ones used most.

That is the reason this calculator exists separately from the ductulator. The ductulator treats flex as a rough pipe. This tool treats it as what it is on site: a rough pipe that is almost never fully stretched.

How to use the flex duct calculator

1. Choose check or size. Check a flex size you already have on the drawing, or let the tool find the smallest standard diameter that meets your friction rate once the duct is installed the way it really will be.
2. Enter the airflow and the installed length. The airflow carried by this runout and the length of the duct as it will hang between the two fittings — not the length printed on the box.
3. Set the compression honestly. Zero means the inner core is pulled completely tight, which is rare on site. Around 15 per cent is a careful installation; 25 to 30 per cent is what a duct cut a metre too long ends up as.
4. Pick the core type. Typical wire-helix flex sits around 3.0 mm absolute roughness when stretched. Use the low or high end of the ASHRAE range if the manufacturer data points that way.
5. Read the as-installed figure, not the stretched one. The headline result is the friction rate after compression. The table underneath shows how fast it climbs, which is the argument for cutting flex to length on site.

The equations used

• Fully stretched friction: Darcy-Weisbach, Δp/L = f × (1/D) × (ρV²/2), with the Altshul-Tsal friction factor f′ = 0.11 × (ε/D + 68/Re)^0.25
• Compression ratio: r_c = 1 − L ÷ L_{fully stretched}
• Pressure drop correction factor: PDCF = 1 + a × r_c
• As installed: Δp/L_installed = PDCF × Δp/L_stretched

Measured compression coefficients

Coefficients from Abushakra, Walker and Sherman, Compression Effects on Pressure Loss in Flexible HVAC Ducts, LBNL, HVAC&R Research 10(3), 2004. Between the tested sizes the coefficient is interpolated; outside them it is extrapolated on a log-linear fit through all three points and the result is flagged.

Worked example

150 CFM through an 8 inch flex duct, ε = 3.0 mm, standard air. Velocity is 430 fpm and the fully stretched friction rate works out at 0.069 in.wg per 100 ft — comfortably under a 0.08 design rate. Install it 15 per cent compressed and the correction factor is 1 + 21.61 × 0.15 = 4.24, so the real friction rate is 0.29 in.wg per 100 ft. Sizing for 0.08 at that compression needs a calculated 9.9 inch duct: a 10 inch flex instead of an 8. The same airflow in 8 inch spiral steel would run at about 0.044.

Things to keep in mind

• Measure the core, not the jacket. The outer jacket and insulation can look fully stretched while the inner core is still several per cent compressed. In the LBNL tests one sample that looked taut from outside turned out to be 4 per cent compressed inside.
• Sag is a separate penalty. A duct that droops between widely spaced supports loses more than a straight one at the same compression. Support flex at the spacing the manufacturer gives and keep it as straight as the route allows.
• Bends are not included. A turn formed in the flex itself is an additional loss on top of the figures here — often a large one. Use a rigid elbow where the route turns sharply.
• Short is the whole strategy. Every figure on this page scales with length. A flex connection of a metre or two to the diffuser is defensible; a long flex run feeding several outlets is the system that never reaches setpoint.
• Cut to length on site. A duct that is too long can only be compressed. Specifying that flex is cut to the measured length, not installed off the roll, is the cheapest pressure saving available.