Wednesday, June 20, 2018

Inlet Plenum Claims

I was asked recently by an engineer whether there is any truth to the claims that a 12” inlet plenum attached to a diffuser would reduce inlet generated sound by ~10NC.

The answer is no, not really. Keep reading to understand why…

There was an ASHRAE research project conducted at UNLV on inlet effects. The short of the story is that diffuser NC values are obtained under ideal test conditions (specified by ASHRAE 70) where there are several diameters of straight duct used. In reality, when you account for the effect of a sharp 90° flex duct bend and realize that rooms don’t really absorb 10dB, the reported value may be underestimated by about 5NC.

At Krueger, we used to sell 90° inlet plenum boxes, but we were shipping mostly air and local contractors could always under bid us. We reported (in 1982) that it would reduce sound by 3 NC.

Where we should be looking is at the ducting…

A straight, hard duct connection to a plenum box will transmit system noise to the diffuser. Even though straight, it may result in being louder than a flex connection, as flex duct has a high “insertion loss” with a low “breakout”, meaning that it makes for an excellent system sound attenuator.

As far as what we've seen in the field, there's often a 90° bend at the connection to the diffuser (outlet). Doing this can seriously affect the discharge pattern. As it turns out, more air will come out of one side, which then results in a longer throw in that direction. In some situations, this may cause occupant discomfort. (PS: This is bad.)

In my experience, air piled to one side by a hard 90° connection will remain piled to one side unless the plenum is HUGE. So to go back to the initial question we were asked, 12” won’t do it, and neither will placing a perforated plate in the path. It will still stay piled to one side. In fact, we found in testing that after 10 feet of 14” spiral duct that follows a 90° connection that the air was still not very uniform at the discharge. So anything that promises to work, just expect that it will add both noise and pressure drop.

In my opinion, I think the approach in unnecessary and likely ineffective. I think it’s better to support the flex duct so it doesn’t lay on the adjacent ceiling tile. Doing this will likely cost a lot less.

Authored by: Dan Int-Hout, Chief Engineer Krueger