The problem of dirt collecting around ceiling diffusers has been a concern in the air distribution industry for a long time. Some people have asked about this dirt and the impact on indoor air quality (IAQ).

This dirt on and around the diffusers accumulates from two sources:

1. Primary air.

2. Secondary air.

The primary airstream can carry dirt through the duct and deposit it on or near the supply air outlet. To ensure proper building hygiene, the primary air supply should be filtered. This will also require regularly scheduled filter maintenance. A source for outdoor air should be filtered where it enters the building to minimize exposure of the primary airstream from these contaminates.

The secondary or induced room air is a more likely source for contaminates. As the high velocity supply air jet discharges from the outlet, room air is induced into the air stream carrying with it particles of lightweight dirt. The source of this dirt is commonly materials that fall from the clothing of occupants in the space. These pollutants can also be transmitted from an adjacent space through open windows and doorways.

The amount of smudging that occurs from secondary air sources may be relative to the type of space being served. For example, in airports or hotel lobbies where occupants enter the space from outdoors, you may observe higher concentrations of contaminates on ceiling areas around the entrance and progressively get cleaner away from the doorway. In break room areas, smudging concentrations may be higher above areas where food is prepared, with lighter concentrations above the dining and seating areas.

Induction of dirty room air at the ceiling or on the ceiling is the key! In most buildings, smudging is the result of secondary room air being induced or drawn into the primary air stream along the ceiling. Airflow characteristics of the outlet will be a factor in the location and intensity of dirt particles in the secondary air.

An examination of the smudging pattern along with knowledge of the air distribution pattern will reveal the source of the smudging. Figure 1 shows a pictorial view or isovel for an outlet discharging air in four directions across the ceiling. A cross flow or directional pattern can also discharge air in three, two, or one direction from the outlet with the same result. Room air will be induced along the edge of this jet with the highest concentration of eddy currents occurring at the corners of the diffuser. This will also be revealed as the highest concentration of contaminates as shown in Figure 2.

When air is discharged into the room at an angle of up to 30 degrees from the ceiling as shown in Figure 3, another low pressure area will form between the discharge air and the ceiling. This negative pressure region will draw the primary air up and attach it to the ceiling surface. In a similar manner, a dropped or beveled frame on the diffuser or an obstruction in the ceiling material will cause a void to occur between the air jet and the flat ceiling surface allowing contaminates to gather as shown in Figure 4.

Therefore, dirt in the primary air system is not normally the problem that causes dirty black smudge areas on our ceilings. As stated before, the black spots around various diffusers are the result of dirt in the room air being deposited on the ceiling next to the diffuser where induction occurs. The darker areas indicate the areas of maximum induction on the ceiling.

The key to keeping a ceiling clean is to avoid induction of room air on the ceiling next to the supply diffuser.

Various diffusers have much more dirt or more pronounced black areas around them than others. The design of the diffuser and the path of induction of the room air to the diffuser are critical.

Diffusers with the least amount of smudge are those which have a smooth, low velocity circular flow pattern, and very little or no induction of room air is on the ceiling next to the diffuser. For best results, the air stream should be in a 360-degree (circular) air pattern around the diffuser as shown in Figure 5.

Some diffusers with the best isovel and the least smudging tendency are designed to distribute the airflow out of its cones in a circular pattern with very low turbulence at the ceiling. The smooth velocity next to the ceiling reduces the soiling of the ceiling surface. The room air is induced into the lower portion of the diffuser jet and the dirt is not deposited on the ceiling. The isovel for this type of diffuser is shown in Figure 5. Voids and low pressure induction areas along the ceiling and adjacent to the diffuser have been reduced to a minimum. The airflow from this diffuser is smooth with little turbulence. Because of this product superiority in design, these types of diffusers will produce less smudging and streaking than any other type of ceiling diffuser.

This makes these types of diffusers a great choice to use in restaurants, airports, and heavy traffic areas with a great amount of dirt in the secondary room air.

In conclusion, to minimize smudging around ceiling diffusers, install and service filters for all primary air sources. Periodically clean supply air ducts as required. Select outlets with a circular air pattern. Install ceiling with smooth, obstruction-free surfaces. Select outlets with moderated neck velocities and moderate air quantities.

Publication date: 12/3/2012