Tuesday, November 6, 2012

Room Design

The K-Select program for grilles and diffusers contains a feature called “Room Design”. This feature was designed 20 years ago to help optimize the selection of overhead diffusers in large spaces, to get the best arrangement of diffusers in the space. In practice, it a great tool for large open plan spaces, for which it was designed. It does not, however, work very well in small spaces with just a few diffusers, or along a perimeter. The problem lies in applications where the space being supplied by the diffuser results in asymmetrical throws. The “characteristic room length” is defined as half the distance to the adjacent diffuser or the distance to the wall. Unfortunately, there just isn’t any way to use the Room Design feature if the distance to the wall is not the same as half the distance to the adjacent outlet. In most cases, to get the 150fpm throw to comply with ASHRAE Standard 62.1 requirement that it make it half way down the window, the diffuser must be much closer to the window than half the space to the next diffuser.

Small rooms (such as a classroom) also have the same problem with a wall about 30 feet from the window. One diffuser has to be close to the window, but will likely overthrow the one towards the opposite wall. Overthrow results in drafts at the midpoint between diffusers where the jets collide, and primary air then enters the occupied zone. (This is a bad thing). To meet the stringent sound limitations in classrooms, four diffusers are probably required, but the throws will often collide.

One proven classroom solution is to use a 3-way diffuser near the windows (with the non-open portion facing away from the window) and a 4-way behind it. The throw from the rear 4-way diffuser will simply combine with the 3-way, towards the window. The 3-way, and 4-way diffusers should be spaced so that they don’t collide in the parallel-to-the- window direction. Remember, it is usually no issue if one overthrows a wall, as the comfort zone starts a couple feet from any wall. The result will be that the diffusers won’t be located evenly, but likely somewhat more towards the side walls of the room.

The K-Select “room design” program is an interesting tool, and works well selecting diffusers for large spaces with no walls. In smaller spaces, ensure that the sum of half the diffuser spacing plus the ceiling height less 6ft is always less than the 50fpm throw values at maximum airflow.

Authored by: Dan Int-Hout, Chief Engineer Krueger