Exhaust Hoods

Exhaust hoods are essential in kitchens, laboratories and industrial application for removing

• fumes
• mists
• vapors
• aerosols
• particulates
• hazardous substances
• polluting contaminants

before they "escapes" to from the surrounding air.

In general for an exhaust hood to be efficient

• the height - y - should not exceed 1.20 m (4 ft)
• the distance - x - should not be less than 1/3 y
• the capture velocity - v₁ - should not be less than 0.15 - 0.20 m/s (30 - 40 ft/min)

Note! Potential hazardous and polluting applications requires special solutions. Always check local regulations before design.

Capture velocity - v₁ - for an exhaust hood can be estimated with the empirical equation

v₁ = q / 2 y² c                                     (1)

where

v₁ = capturing velocity (m/s)

q = air volume flow (m³/s)

y = distance between table and exhaust hood (m)

c = circumference of the hood (m)

(1) can be modified to calculate air volume flow

q = 2 v₁ y² c                                    (1a)

Example - Kitchen Exhaust Hood

Required air volume flow for a exhaust hood with circumference 3 m located 1.2 m above a stove can be calculated  as

q = 2  (0.2 m/s) (1.2 m²)²(3 m)

    = 1.7 m³/s

Note! The units don't match since the equation is empirical (a result of experiments).

Exhaust Hood Calculator

The air flow volume in the exhaust hood can be calculated below

Exhaust Hood with internal Plate

The efficiency of an exhaust hood can be improved by adding an internal plate.

The required air volume for an exhaust hood with an internal plate can in general be reduced to approximately 80% compared to an exhaust hood without a plate.

Exhaust Hood with Side Walls

The exhaust hood efficiency can be further improved by adding side walls.

Temperature ^oC
K
^oF

Length m
km
in
ft
yards
miles
naut miles

Area m²
km²
in²
ft²
miles²
acres

Volume m³
liters
in³
ft³
us gal

Weight kg_f
N
lbf

Velocity m/s
km/h
ft/min
ft/s
mph
knots

Pressure Pa
bar
mm H₂O
kg/cm²
psi
inches H₂O

Flow m³/s
m³/h
US gpm
cfm