Kitchen exhaust duct calculator — grease duct size and velocity

Sized to a velocity window, not a friction rate

Every other duct on a project is sized to keep pressure loss and noise down. Grease duct is different: it carries hot, grease-laden vapour that condenses on cold surfaces, and a duct coated in grease is a fire load running through the building. The velocity requirement exists to keep that vapour moving and to keep the duct small enough to clean.

For most of the twentieth century the rule was a flat 1500 fpm minimum. Nobody is entirely sure where the number came from — one widely repeated explanation is that it was carried over from an old requirement for coal furnace flues. Research on grease transport in the late 1990s led NFPA 96 to drop the minimum to 500 fpm (2.54 m/s) in 2002, and ASHRAE Standard 154 followed the same direction.

In practice both numbers are still in use. Many authorities, local amendments and hood test forms continue to require 1500 fpm minimum and 2500 fpm maximum for shop-built hoods, while listed hoods follow their listing. A grease duct is therefore sized to a window, and the first job is to find out which window applies.

How to use the kitchen exhaust duct calculator

1. Enter the hood exhaust airflow. The total exhaust the hood is listed or designed for — from the hood schedule or the manufacturer selection, not an estimate from the cooking line.
2. Choose the velocity requirement. Listed hoods follow the manufacturer listing. Shop-built hoods are commonly held to 1500–2500 fpm by the authority having jurisdiction. If only the NFPA 96 minimum applies, choose that — but check locally first.
3. Set the design velocity. The tool picks the size closest to this figure while staying inside the window. Designing near the middle of the window leaves room for the airflow to drift on commissioning.
4. Pick round or rectangular. For rectangular, fix the side the route allows and the tool solves the other. Round is easier to weld and clean and has the least surface for grease to collect on.
5. Check the velocity and the run loss. Velocity is reported on the real duct area. Add the straight-run loss to the hood, filter and fitting losses before selecting the fan.

Velocity requirements in use

Always confirm the edition of NFPA 96 or the mechanical code adopted by the authority having jurisdiction, and any local amendment, before fixing the duct size.

Formulas and a worked example

• Required area: A = Q ÷ V
• Round duct: D = √(4A ÷ π)
• Velocity check: V = Q ÷ A_actual, on the real section area

A hood exhausting 3000 CFM, shop-built, designed to 1800 fpm inside a 1500–2500 fpm window. Required area = 3000 ÷ 1800 = 1.67 ft² (240 in²), an exact round diameter of 17.5 in. A 16 inch duct would run at 2149 fpm and an 18 inch at 1698 fpm — both inside the window, and the 18 inch is closer to the design velocity, so it is selected. Under the NFPA 96 minimum alone a much larger duct would be legal, but the 1500 fpm local requirement would rule out a 20 inch duct or anything larger.

Things to keep in mind

• Velocity on the real area. Never check grease duct velocity on the circular equivalent diameter — it overstates the velocity of a rectangular duct.
• Construction comes with the size. Continuous liquid-tight welds, cleanout openings at changes of direction, grease-draining slopes and the clearance to combustibles — 18 in for an unprotected duct under NFPA 96 — all follow from the code, not from this calculation.
• Make-up air is half the system. Every cubic foot exhausted has to be replaced. An exhaust duct sized correctly but starved of make-up air will not deliver its design airflow.
• The duct is only part of the loss. Hood, filters, elbows, the fan inlet and the discharge all add pressure. Total them before selecting the fan.
• Hot air is lighter. The temperature you enter sets the density used for the friction and velocity pressure figures; exhaust from heavy cooking lines runs well above room temperature.