# Air Terminal Design

The main objective of air terminal design is to find the right size and combination of diffusers to be used by an HVAC system to deliver the appropriate amount of air into the desired room.

There are mainly 2 methods to design such air terminals: using the ‘Formula/Equation Approach’ and using the ‘Manufacturer’s Catalog’.

Let’s now consider the formula approach. This method uses a simple equation of continuity of flow:

Q = A*V

Here ‘Q’ refers to the total volumetric flow rate of air into the desired room/lobby where the diffuser will be connected. It can be measured in liters/second (l/s) or cubic feet per minute (cfm).

The value of the parameter ‘Q’ is made available from the load calculations that are performed before the ‘Design of Air Terminal’.

The ‘V’ refers to the flow velocity of the air through the diffuser. And the velocity can be measured in feet per minute (fpm) or meters per second (m/s). In Air terminal Design the maximum velocity is usually capped based on the type of application of the HVAC system. The higher the velocity of air flow, the more noise it will naturally make. Hence, for instance in a residential setting the maximum allowed velocity is much lower than in a public setting like an Airport.

Once we have an idea of the load (Q) and the maximum allowed velocity (V) we can use the above mentioned formula to find the area of the Diffuser (A). This is the main parameter we are designing for. Lets say, at this point that our calculation says: the area should be 1 sq ft. We can now look at the available sizes of diffusers. Both Circular and Square Diffusers are available. In this situation for instance, we would select the 12″ * 12 ” square diffuser as it perfectly fits our area requirement of 1 sq ft. In a situation where the calculated area does not exactly match the areas of the available diffusers, we try to select the diffuser closest to the required area on the lower side. Selecting a larger than required diffuser is called ‘Overdesigning’ and should be avoided as far as possible because it is very inefficient.

In a more realistic scenario, a single diffuser may not suffice for a given application. For instance, in a large office setting, we might want to install a 2-D Array of diffusers to cover the entire air flow load requirements. In such situations, one must keep in mind certain limitations, such as keeping the individual diffusers at a distance of 15 feet or more, so that the individual terminals wouldn’t destructively interact with the other counterparts. It is a matter of experience, to learn how many diffusers to use in a given situation.

Let’s us now consider the ‘Manufacturer’s Catalog’ method. To use this method effectively, one must understand certain parameters and terminology. Lets enumerate some of these terms:

1. Flow Pattern: This refers to the actual flow configuration of air through a given diffuser. For instance, a square diffuser with 3 sides closed and one side open would be considered a ‘One-Way Diffuser’. Similarly, you can have 2-way, 3-way or a 4-way diffusers with varying ‘angles of flow’ and ‘shapes of diffusers’.
2. Throw: The throw refers to the distance travelled by the (hypothetically) fastest moving air molecule, after leaving the air diffuser, before it reached a speed of 50 feet per minute (0.25 meters/second). The throw is usually specified using a 3 or 2 number sequence. For instance, a throw of 7-9-12 means that at 7 feet from the diffuser the air speed is 150 fpm, at 9 feet it’s 100 fpm and at 12 feet it’s 50 fpm. If instead the throw is mentioned as 11-15, it refers to the distances for 100 fpm and 50 fpm respectively.
3. Noise Criterion: This number refers to the maximum amount of noise level allowable to the HVAC system. The lower the NC number, lower the noise and hence lower the flow velocity must be.

image source: ASHRAE 2001: Page 7.15

Once the above three parameters are determined, an appropriate diffuser can be selected while keeping in mind all the requirements, using a manufacturers catalog. A manufacturer’s catalog is essentially a look-up table relating all the above mentioned parameters, with the ‘Diffuser Size’ and expected ‘Velocity’.