Common Terminology

ADPI (Air Diffusion Performance Index)

Statically relates to the space conditions of local temperatures and velocities to occupant’s thermal comfort. In an office environment, the design goal is to maintain high comfort levels by obtaining high ADPI values.

Air Diffusion

Distribution of the air in a space, called the treated space, by means of devices, called air terminal devices, in a manner so as to meet certain specified conditions, such as air change rate, pressure, cleanliness, temperature, humidity, air velocity and noise level.


A leading HVAC/R Association – American Society of Heating, Refrigerating and Air Conditioning Engineers.

Coanda effect

The Coanda effect occurs when airflow attaches itself to a nearby surface, such as a ceiling, or the walls of a duct system. When this occurs, the airflow is affected by the surface it is flowing with and remains attached to the surface direction temporarily after separation from the surface. In simpler terms, air clings to nearby surfaces as it moves.


The movement of heat by fluid flow (e.g. air, water).


A device that is located in ductwork to adjust air flow.


The drop of cooled total air is the result of the vertical spread of air and the buoyancy effect due to the density differences between the total air and the surrounding primary room air. The drop can often be minimised by spreading air uniformly across the ceiling surface.


The air flow forced out of the treated space.


A device for restricting dust particles passing through supply air.


Process by which the primary air sets into motion an air volume, called secondary air, into the room.

l/s (Litres per Second)

A standard measurement of airflow that indicates how many litres of air pass by a stationary point per second. The higher the number, the more air is being forced through the system.

Occupied Zone

The occupied zone is the area, generally considered to be within 1800mm above the floor and not within 300mm of the boundaries of the walls. Air diffusion designs should avoid drafts and high temperature differences within this area.

Pascals (Pa)

A unit measure of air pressure.


An air flow passage.

Primary Air

Primary Air is the conditioned air discharged through the supply terminal. This air provides the driving force for room air motion.


The spread is often defined as the separation of the air stream in a horizontal and/or vertical plane after it leaves the supply air terminal.

Static Pressure

The difference in air pressure between the suction side and pressure side. Unit of measure is Pascals (Pa).


The distance measured from the face of the terminal diffuser to the point at which the maximum velocity is reduced to a specific value (typically 0.25m/s).

Total Air

Total Air is the mixture of primary air and entrained room air (air supplied under specific conditions from the supply terminal). The temperature differences between total air and room air creates a buoyancy effect which causes cold air to drop and warm air to rise.