Wednesday, August 16, 2017

What is Throw? What is Terminal Velocity? How Are The Two Linked?

Throw value means how well air moves across a room from a vent, or diffuser. A major factor in the throw value is the terminal velocity of the air coming from the diffuser. When air flows out of a supply, we’d like to know the result. Since we cannot see what is happening, we use throw as one indicator of a register’s performance abilities.

Throw is measured in feet from the face of the register along the primary direction of flow. However, a throw distance is meaningless unless given a point of reference. 

We use the term terminal velocity in conjunction with throw to describe what the air is doing at the end (or terminus) of the designated throw. A typical terminal velocity is 100 feet per minute (FPM). This means that no matter how fast the air is blown out of the register, the throw tells us, at that distance, the air has slowed to 100 FPM. Titus throw values are presented using three industry standard terminal velocities: 150 FPM, 100 FPM, and 50 FPM. All throw values are obtained utilizing isothermal air (ASHRAE Standard 70-2006). Isothermal air is the same temperature as the room air allowing test data to be repeatable and predictable.

The supply air velocity measured at the register face determines how far the throw will be. The faster the air exits the face, the farther the air will travel into the room. The resistance of room air to the supplied air will cause the supply air to slow down. 

Eventually, the supply air will slow enough to become ineffective in mixing with room air. The point that air velocity becomes ineffective is called the terminal velocity. Generally terminal velocity ranges from 150 down to 50 FPM.

The distance from the face to where this terminal velocity occurs is the throw.

Throw patterns of a sidewall grille that illustrates the air velocity becoming gradually less the farther away it moves from the grille

EXAMPLE: The performance data for a sidewall supply register states that all throws are at a terminal velocity of 100 FPM. No matter what the face velocity is or how much air is being delivered, each throw is measured at the point where the supply air stream has slowed down to 100 FPM.

If we use 50 FPM as the terminal velocity, the throws are longer (farther from the face). At the register face where the throw is "0," the velocity of the supplied air is highest. No matter what distance we choose to stop moving away from the face, there will always be a corresponding velocity that becomes less and less the farther away we move.

For more information on this topic, please contact our GRD department at or Titus Communications at