Saturday, July 14, 2012

Slots are adjustable? Who knew!

I made a call on a design engineer firm today to do a “Basics of Air Distribution” Lunch and Learn. It turns out that I learned something (which I usually do). It seems a number of young engineers weren’t aware that linear slots were adjustable. They assumed they were placed at the window and designed to blow down. No wonder so many perimeter environments are uncomfortable and drafty.
Here’s the deal. Almost all linear slot diffusers are adjustable from horizontal to vertical. There are a few with fixed deflection. (Often, of course, these are located where they should have been adjustable.) The deflectors in most slots can be switched from deflecting left to deflecting right (and of course, down). When shipped, the factory has no idea how they should be adjusted, and often there is a wide degree of variance in the factory settings, which are also subject to shipping, installer manipulation, and general handling issues.
In practice, almost all slots should be adjusted to blow air horizontally along the ceiling. For a perimeter, the ideal location, according to research published by several manufacturers back in the late 70’s when we were actually researching this issue, is a couple feet away from the window, set to discharge some air towards the window, and some into the room. This results in a good compromise between heating and cooling performance, and has a chance of complying with the requirement in ASHRAE 62.1 that the 150 fpm throw make it to within 4.5 feet from the floor. (Failure to comply with this requirement requires a 25% increase in ventilation air to compensate for the inevitable short circuiting that will result). There is also a requirement that the discharge air not be more than 15-deg F above room temperature, or the same penalty applies. The data (and the ASHRAE Handbook) also explains that delivering air more than 15-deg F above room temperature will likely exceed the ASHRAE Standard 55’s maximum room vertical stratification limits, and void compliance with the Standard.
Failure to adjust slots in interior spaces results in cold air being directed down on occupants. This is always unacceptable. I can’t begin to tell you the number of times I have suffered in an ASHRAE technical meeting in a fancy hotel where there was a slot overhead blowing cold air directly on me. I have learned to wear a sport jacket in all ASHRAE meetings. I am tempted to take gloves and a scarf.
Adjusting a linear slot requires that the design engineer state clearly in the design documents that the slots must be adjusted, per instructions, prior to balancing. When adjusted from vertical to horizontal, the pressure drop almost always increases significantly, and must be accounted for in the system balance. While some may argue that a balancer won’t do this even if required to, it is assured that if the engineer does not state this requirement, it won’t happen.
The ball is in your court Mr. Design Engineer.
Authored by: Dan Int-Hout, Chief Engineer Krueger