Bypass Fume Hood manufacturer-Accumax India

Thu Oct 09, 2025

Bypass Fume Hood

A Bypass Fume Hood is a type of ducted chemical fume hood designed to regulate the air velocity at the hood opening, especially when the sash is adjusted. It is an improved version of the conventional Constant Air Volume (CAV) fume hood.

Here is a simple explanation of its function and how it works:

Function and Design

The main purpose of a bypass hood is to maintain a relatively constant volume of exhaust air being pulled from the laboratory, regardless of the sash position. This is achieved through a built-in feature called the bypass.

Feature	Conventional CAV Hood	Bypass Fume Hood
Sash Movement	As the sash is lowered, the face opening shrinks.	As the sash is lowered, the face opening shrinks.
Airflow Path	Air can only enter through the face opening.	Air enters through both the face opening and the bypass grille.
Face Velocity	The velocity of air at the face increases dramatically when the sash is closed (like putting your thumb over a hose).	The bypass mechanism prevents a dramatic increase in face velocity, maintaining a safer, more consistent speed (usually within acceptable limits).
Air Volume	The total volume of air exhausted remains constant.	The total volume of air exhausted remains relatively constant.

How the Bypass Works

Bypass Grille: A bypass grille, or opening, is located above the sash.
Sash Open: When the sash is fully open, it typically covers the bypass grille. All the air flows through the face opening.
Sash Closed: As the user lowers the sash, the sash simultaneously exposes the bypass grille. The air that can no longer enter through the face opening is diverted to enter through the bypass grille instead.
Result: The sum of the air entering through the face and the air entering through the bypass remains nearly constant. This prevents the face velocity from becoming dangerously high when the sash is lowered, which could otherwise cause turbulence and draw contaminants out of the hood.

Note: While a bypass hood maintains a relatively constant volume of exhaust air, it does not offer the same energy-saving benefits as a Variable Air Volume (VAV) fume hood, which actively reduces the total volume of air exhausted when the sash is closed.