Air supply and return in a space

[ramnam]

i am designed a room where we need to keep an NC value of 20. (TV studio). Due to the lighting loads the cfm is astronomically large as compared to the room size.

I am supplying hi near the ceiling along two walls and returning low near the floor at the opposite wall. I have to pick a supply register that meets the NC criterion. My question: If I open-end the supply duct hi at the wall without any registers, will the cool air reach the return register at the other end of the room? or will i need to pick a register with the correct throw to the return wall at, say, 100 fpm in order for the cold air to reach the return?

One engineer in our office seems to think that throw is the only factor in non-laminar airflow design that would guarantee that air reaches the correct distance; and that having a low return will not create an effect that will "pull" the air towards it.

 

[KenRad]

ramnam,

My concern with using an open ended supply is insufficient throw -- the cold supply jet can drop into the occupied zone before getting properly mixed with room air, creating a cold draft. Low noise and proper throw will be the two competing criteria here.

I'm a little confused about your question as to whether or not the cool air will reach the return register. Ultimately, the return will pass about the same volume as the supply (neglecting leakage across room boundaries) no matter where it's located. But ideally, you want to return from a warm, more stagnant area of the room.

The return air grille will not affect throw. Its influence on air motion will be hardly noticeable except in its immediate vicinity. I think this is what the other Engineer was saying about pulling the air towards it.

---KenRad

 

[lilliput1]

I have used linear slot diffusers spaced so blows do not conflict. Use lined ductwork & size ductwork to reduce velocity in stages from 1000 FPM max to 300 FPM max in the final 10' before the diffuser. Use series of equal branching to accomplish this & to minimize need for balancing at volume damper. Locate volume damper as far as possible from the diffuser but provide access to them. If the 1000 FPM can not be attained because of space, provide acoustic lagging on the outside of the lined duct but in now way should the velocity exceed 2000 FPM. You should consult an acoustic consultant to be safe.
If ducts have to run through steel joist (truss) coordinate with structural location of "duct chase through joist" for main & large branch size & have him require truss member that normally be vertical at these points to instead be sloped to allow more room for ductwork. In the job I did the truss were not parallel but radial & the ductwork had to curved to follow the location of the "duct chases" which were of uniform distance from the end of each truss. The duct had to be square (not round) lined duct to allow it to be curved accordingly.

 

[VSSriram]

Looking from a slightly different perspective - The acoustic consultants talk about a minimum distance between the main duct and the outlet and this varies depending on the NC level required inside. This may mean taking longer routes of ducting, eventhough you could accomplish the same with shorter routes. I had some data available with me. Have to check. Maybe will try and put the same here in a couple of days, if I can lay my hands on it.

The best thing would be to get an acoustic consultant to give his/her recommendations. They would even suggest the type of vibration isolators for the equipments (chiller, condensing unit, AHU, etc) as well as any silencers requirement for the AHU, if required.

You had mentioned about the lighting load being astronomically high. Try to select light fittings, which reject most of its heat to the ceiling and hence the return air path, thereby reducing the dehumidified air quantity required.

 

[ramnam]

KenRad, thanks for the info on the location of return grille and throw. i think i have design catching up to do this weekend.

Lilliput1, thanks for the input. I am using a mix of linears in some places and registers in another. the room is really small and the architects have managed to contrain its width, height and length, and the room is sandwiched between spaces that ductwork cannot run within. I used ASHARAE guidelines for velocities in ductwork to achieve NC ratings desired. My velocities are within 450 fpm and i have equal duct splits to avoid volume dampers. (all lined duct too).

VSSriram, thanks for the input. Unfortunately, the architects also don't want a ceiling in this space. so all the lights are within the space and i have to account for all the heat.