In the previous article, we talked about terminology and the difference between a diffuser, a grille and a register. In this article, we are going to talk about dirt streaks on the ceiling, their causes and solutions.
We have all seen restaurants or office buildings where the ceiling has become smudged and dingy with dirt streaks running across the ceiling next clearly coming from the air outlet device. Most people assume the cause is dirty filters or dirty air within the space. While they are contributing factors, there are several others which most contractors will not think about. Fixing this problem when other contractors have not been able to could bring you a long-term customer! So what are those other factors?
The first thing to look at are the air outlets (assume they are diffusers) themselves. Have they been properly chosen for the application? Believe it or not, diffusers are more than just “hole covers.” Perhaps future articles will tackle the discussion of air inlets/outlets and their general applications, because the scope of this discussion is beyond this article. You would be well served however as a contractor to take an application class from a trusted industry source such Hart & Cooley or Titus. Bear in mind, proper diffuser selection cannot only impact dirt streaks but also sound levels within the space.
The second thing to keep in mind is the duct system leading to the diffuser. Are the duct air velocities appropriate or is there a restriction somewhere? It is not uncommon for example to find a situation where the round duct is kinked as it enters the top of the diffuser. If there are restrictions, outlet velocities will be much lower than they should be, causing induction with room air as soon as it leaves the diffuser. If you find this to be the case, there are companies that offer a brace designed to shape flexible ducts into highly efficient 90° elbows at the diffuser inlet. When room air mixes with supply air at the outlet of a diffuser, not only will the system perform inadequately but you could also smudge the ceiling.
The third thing to keep in mind are the jet characteristics in the four zones of expansion for a diffuser. As air leaves an outlet device, four distinct zones of expansion define the jet of air. Jet velocities from a ceiling diffuser can be measured outward from the discharge point of the device. The first zone which is closest to the outlet of the diffuser extends approximately 1 ½ duct diameters from the face of the diffuser, and is characterized by a constant velocity with minimal mixing of supply and room air. In other words, a properly designed air distribution system will not induce room air within 1 ½ duct diameters. For example, if the diffuser is being fed by a 7 inch round duct, there should be no room air mixing with the supply air within the first 10-12 inches. In the second zone, the jet of air begins to mix with room air, and the resultant induction causes the jet of air to expand. Velocities may well exceed 150 ft./m in this zone, depending on the design and application. The third zone is where most of the induction occurs, and is the most important zone because it has the most effect on room air velocities and room induction. Velocities at the edge of this zone may run between 50 ft./m(considered terminal velocity) and 100 ft./m. The fourth zone is the one with relatively low air velocities. Typically, air will reach terminal velocity in this zone.
Understanding the science and principles of air distribution can be a point of marketable differentiation between you and your competitors!
Photo courtesy of Titus.