Customers often ask me, 'Just how big a fan does this hood of mine need?' There is no single answer, but the calculation logic is clear. I once dealt with a buffet eatery that bought a very large fan yet still could not pull the fume cleanly, and the reason was that no make-up air had been provided. The kitchen went negative-pressure, and the moment a door was pushed open all the air poured outward—the hood was effectively fighting the atmosphere for air. Airflow is a study in balance; bigger is not always better.
The Basic Airflow Formula
The core airflow conversion is a single line: Q = A × V × 3600. Here Q is the airflow (m³/h, i.e. CMH), A is the cross-sectional area (m²), V is the velocity (m/s), and multiplying by 3600 converts seconds to hours. As long as you know the area of a duct or opening and the design velocity, you can back out how many CMH you need.
In practice there are two entry points. One is to work back from the exhaust-duct velocity: to keep grease vapor from settling inside the duct and grease from building up easily, duct velocity is usually designed at roughly 8 to 12 m/s. For example, a square duct with a cross-sectional area of 0.09 m² (about 30×30 cm) at 10 m/s gives an airflow of about 0.09 × 10 × 3600 ≈ 3,240 CMH.
Thinking from Hood Face Velocity
The other angle is to look at the hood's open face. To gather the fume steadily into the hood, the lower edge of the hood opening has to maintain a certain inward capture velocity, and the fiercer the flame and the higher the hood is hung, the more airflow is needed. Multiply the hood's open face area by the design face velocity, convert to CMH with the same formula above, then take the larger of the two demand figures—that way neither section becomes the bottleneck first.
Two reminders here. First, airflow has to be paired with make-up air: roughly whatever you exhaust, you have to supply back, otherwise the kitchen stays under negative pressure, air conditioning leaks out, gas burns incompletely, and even the hood's capture gets worse. Second, the calculated airflow is only a starting point; the rated airflow of the downstream purification equipment has to match, or the equipment gets overloaded and its efficiency falls off. The actual figures must come from an engineering calculation based on the on-site cooking layout and duct length; the numbers above are only to give you a feel for the concept.
TingHao is deeply rooted in Kaohsiung, Tainan, Pingtung, and across southern Taiwan, handling everything from on-site airflow measurement and capture hoods to purification-equipment selection and scheduled cleaning and maintenance. To find out which setup your kitchen needs, call 07-363-0802 or add us on LINE, and we will visit in person before making a recommendation.