The Once and Future Theater - Page 38 - AVS Forum
Forum Jump: 
 8Likes
Reply
 
Thread Tools
post #1111 of 1120 Old 07-06-2014, 01:21 PM - Thread Starter
AVS Special Member
 
HopefulFred's Avatar
 
Join Date: Mar 2007
Location: Atlanta, GA
Posts: 2,619
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 58 Post(s)
Liked: 172
Grills on order: (2x) 4"x24" bar grill and (1x) 8"x18" bar grill (all three brown, plus one balancing damper for installing inline in the return duct, just in case. I figured I need a coupler anyway, since the 25' duct I started with wasn't quite long enough, so why buy a coupler when the damper is only a couple bucks more. The damper will be easily accessible under the stairs, a few feet from the air handler. Price is actually very competitive with what's available on the shelf at Lowe's, plus this is actually the right stuff...
HopefulFred is online now  
Sponsored Links
Advertisement
 
post #1112 of 1120 Old 07-06-2014, 01:35 PM
AVS Special Member
 
doublewing11's Avatar
 
Join Date: Feb 2010
Location: Timber Country!
Posts: 3,206
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 24 Post(s)
Liked: 173
Quote:
Originally Posted by HopefulFred View Post
Grills on order: (2x) 4"x24" bar grill and (1x) 8"x18" bar grill (all three brown, plus one balancing damper for installing inline in the return duct, just in case. I figured I need a coupler anyway, since the 25' duct I started with wasn't quite long enough, so why buy a coupler when the damper is only a couple bucks more. The damper will be easily accessible under the stairs, a few feet from the air handler. Price is actually very competitive with what's available on the shelf at Lowe's, plus this is actually the right stuff...
Great to hear......one step closer!

BTW, I screwed up on my info JFYI. That 85 square inches for returns should be for each grille.......there are four return grilles, not two.
doublewing11 is offline  
post #1113 of 1120 Old 07-06-2014, 01:40 PM - Thread Starter
AVS Special Member
 
HopefulFred's Avatar
 
Join Date: Mar 2007
Location: Atlanta, GA
Posts: 2,619
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 58 Post(s)
Liked: 172
8x18 is about the largest I could fit, in any case. I'm not really doing any real math here, just putting it together in a gettin'-it-done kind of way.
HopefulFred is online now  
post #1114 of 1120 Old 07-06-2014, 01:56 PM
AVS Special Member
 
doublewing11's Avatar
 
Join Date: Feb 2010
Location: Timber Country!
Posts: 3,206
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 24 Post(s)
Liked: 173
Quote:
Originally Posted by HopefulFred View Post
8x18 is about the largest I could fit, in any case. I'm not really doing any real math here, just putting it together in a gettin'-it-done kind of way.
Well, if room volume was directly related to cross section of supporting ductwork, you're well with in required air exchange.....but it's not. I'll bet you are fine........
doublewing11 is offline  
post #1115 of 1120 Old 07-06-2014, 05:33 PM
AnalysisParalysis Analyst
 
TMcG's Avatar
 
Join Date: Mar 2010
Location: North Carolina
Posts: 2,379
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 22 Post(s)
Liked: 203
Quote:
Originally Posted by HopefulFred View Post
With 2 supplies and 1 return, the return should be approximately equal to the total area of the supplies, right? Or is there some adjustment to be made since I'm combining the two into one? (I wouldn't think so, just checking.) So I'm expecting to need somewhere around 200 square inches for return.

Shopping at HVACQuick.com, I'm looking at (2) Dayus DABL - Bar Linear Grill for an opening of 24" by 4" or do I need to go larger to get that much actual grill area? (pn in my basket is DABL-00-A-24x4-Brown-STD)
Area of the diffuser has nothing to do with the CFM. Very simply, you calculate the CFM of supply you will need for the room based on the calculated heat load. Your return CFM should approximate the total supply number otherwise you can get into a positive / negative pressure situation in the room where the door will either be very hard to open or very hard to close depending on the room's pressure and because the room is so tightly sealed. The best approach to get equivalent CFM is to install an in-line baffle on the return side at the end nearest the Air Handling Unit. All the diffuser size "area" tells you is what the face pressure will be and nothing more. 4x24 is the minimum for a 6" flex at max CFM, but you could easily go bigger and lower the face pressure even further....although sometimes this can be at the expense of slightly less efficient air mixing if the CFM is spread out over too great of an area (i.e. too big of a diffuser).

Hopefully this makes sense as I just got finished with a 10 hour car ride, complete with screaming baby *sigh*....
Mfusick likes this.
TMcG is online now  
post #1116 of 1120 Old 07-06-2014, 05:40 PM - Thread Starter
AVS Special Member
 
HopefulFred's Avatar
 
Join Date: Mar 2007
Location: Atlanta, GA
Posts: 2,619
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 58 Post(s)
Liked: 172
Thanks, TMcG - that does make sense. I was just trying to back into a determination for face velocity (<250 fpm) at the intake grill, working from the assumption that the pressure would be equalized. In any case, I feel pretty comfortable that the grills I have ordered will be appropriate, and I have the balancing damper to go along with them - I think I'll be good!
HopefulFred is online now  
post #1117 of 1120 Old 07-07-2014, 06:15 AM
AVS Addicted Member
 
Mfusick's Avatar
 
Join Date: Aug 2002
Location: Western MA
Posts: 20,937
Mentioned: 2 Post(s)
Tagged: 0 Thread(s)
Quoted: 51 Post(s)
Liked: 589
I feel your pain on screaming baby
Where you headed ?

-


"Too much is almost enough. Anything in life worth doing is worth overdoing. Moderation is for cowards."
Mfusick is offline  
post #1118 of 1120 Old 07-07-2014, 08:07 AM
AVS Special Member
 
J_P_A's Avatar
 
Join Date: Dec 2008
Location: L.A. - Lower Alabama
Posts: 3,802
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 62 Post(s)
Liked: 184
As usual, I'm late to the party. Best I can tell you should be good on the grills as well. You've got .7 s.f. of grill for each 6" supply. At 75 cfm, that gives you a face velocity less than 110 fpm.

With balanced flow, the return is drawing 150 cfm from the room. An 8"x18" grill gives you 1 s.f. of grill, so that puts you at 150 fpm on the return.

Both look good to me (I'm sure you did these calculations yourself, anyway). I think you've got a little extra margin considering you will likely have less than rated flow form those supplies.
Mfusick likes this.

Dude, are you made of leprechauns? Cause that was awesome!

The Plains Theater Has Begun
J_P_A is online now  
post #1119 of 1120 Old 07-07-2014, 08:33 AM - Thread Starter
AVS Special Member
 
HopefulFred's Avatar
 
Join Date: Mar 2007
Location: Atlanta, GA
Posts: 2,619
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 58 Post(s)
Liked: 172
I did not do that math - so thanks. If i do math, it's usually in public so someone can check my work.
HopefulFred is online now  
post #1120 of 1120 Old 07-07-2014, 02:30 PM
AnalysisParalysis Analyst
 
TMcG's Avatar
 
Join Date: Mar 2010
Location: North Carolina
Posts: 2,379
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 22 Post(s)
Liked: 203
Quote:
Originally Posted by J_P_A View Post
As usual, I'm late to the party. Best I can tell you should be good on the grills as well. You've got .7 s.f. of grill for each 6" supply. At 75 cfm, that gives you a face velocity less than 110 fpm.

With balanced flow, the return is drawing 150 cfm from the room. An 8"x18" grill gives you 1 s.f. of grill, so that puts you at 150 fpm on the return.

Both look good to me (I'm sure you did these calculations yourself, anyway). I think you've got a little extra margin considering you will likely have less than rated flow form those supplies.
Quote:
Originally Posted by HopefulFred View Post
I did not do that math - so thanks. If i do math, it's usually in public so someone can check my work.
CFM = velocity x surface (in square feet)

Therefore

Velocity = CFM / Surface (in square feet)

A 6" flex duct has a max velocity of 75 CFM and about 0.2 square of surface area (28.27 square inches). So velocity (face pressure) is calculated at 75 / 0.2 = 350 fpm.

If you used a 4" x 12" bar diffuser with 75 CFM, that equates to 0.33 square feet and calculates to 225 fpm which is under your 250 fpm target. However, it gives you very little overhead to deal with any air turbulence.

The 4" x 24" diffuser is a nice size that gives you plenty of overhead at around 107 fpm without being obnoxiously huge.

When I was working with David's (Savoy Theater) HVAC company, his dedicated system for this single room had enough output that we needed a very long bar diffuser to keep the system perfectly quiet. It worked like a charm and he said that he can barely hear anything, even standing on a ladder with his ear toward the diffuser. His recent GTG pushed the limit on the system with as many as 20 people in the room at once (including standers), but the dedicated system was able to keep up with the high demand without being heard.

EDIT: I forgot to mention that the face velocity calculation is just that....a calculation....and that the only way to get the real numbers is with an anemometer as no supplies ever run at perfect maximum and no AHU or duct layout is ever the same. The cost difference between what you ordered and the absolute minimum is negligible compared to the benefit of having the diffuser slightly oversized to be under the 250 fpm max target.

Last edited by TMcG; 07-07-2014 at 02:37 PM.
TMcG is online now  
Reply Dedicated Theater Design & Construction

Tags
Pioneer Electronics , Sherbourn , Dayton Audio Um18 22 18 Ultimax Dvc Subwoofer 2 Ohms Per Coil

User Tag List

Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page


Forum Jump: 

Posting Rules  
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off