Diffusers




Diffusers, registers and grilles are at the end of ducts and serve as the distribution equipment to the conditioned space. The purpose of these mechanical pieces of equipment is to provide thermal comfort for the occupants of the space or to provide proper thermal conditions suitable for the equipment in the space.

Diffusers are defined as air terminal devices that distribute conditioned air in various directions through the use of its deflecting vanes. It is designed to promote the mixing of conditioned air with the air already in the space. It is important to properly mix the conditioned air into the space, in order to provide cooling/heating and to distribute fresh air to the entire space and to avoid stagnant air in the space. However, not all types of diffusers have the same performance in mixing the conditioned air in the space. Each diffuser will be provided with a table describing its performance similar to the one on the following page.

Diffuser Throw for the Mechanical PE Exam

The values shown in tables similar to the one above are specific to a certain manufacturer’s type of diffuser and size. The third row indicates the total amount of CFM that is distributed through this diffuser. This term is the value with which the designer will begin. From this value, the velocity and pressure drop through the diffuser can be determined. It is also important to note that at higher velocities, the pressure drop increases and the NC or noise criteria increases. The NC rating corresponds to a curve of DB levels at various frequencies. This NC rating is used to rate the sound levels of air conditioning equipment and also used to rate the sound requirements of rooms. For example, a typical classroom will require a NC rating of 25-30. Using the table above, this corresponds to a maximum airflow somewhere between 85 and 95 CFM.

Throw is defined as the horizontal distance from a diffuser at a specified velocity. For example, T50 = 15’, indicates that at a distance of 15’ from the diffuser, the velocity of the air will be 50 feet per minute. T100 = 10’, indicates the distance at which the air velocity is 100 feet per minute and T150 = 5’, indicates the distance for 150 feet per minute. Often times throw is shown simply in the following format, [T150 -T100 - T50]. For example, in the table above an airflow of 60 CFM results in a velocity of 150 fpm at 7’ from the diffuser, a velocity of 100 fpm at 9’ from the diffuser and a velocity of 50 fpm at 12’ from the diffuser. Refer to the following figure for a graphical explanation.

Diffuser Throw for the Mechanical PE Exam

Typically in diffuser layout design for occupied areas, it is required to locate diffusers so that the T50 length is nearly equivalent to the characteristic length. The characteristic length is defined as the distance by one of the following:

1. Perpendicular distance between the center line of the diffuser and the wall.

2. Midpoint between the centerline of two diffusers.

Grilles are defined as air devices that consist of an opening with a covered grating or screen. Grilles are often used to return air back to the fan or to exhaust air from a space. Grilles are not typically used to supply air because there is an inability to accurately control the amount of air being supplied.

Registers are simply grilles with a damper that is used to restrict the amount of air flow required to be returned, supplied or exhausted.