Replies

1. 
   Taylor | Dec 10, 2005 07:54pm | #1

   BS. Shape does matter, oval more friction than round, square more friction than oval. Look here for a way to convert.

2. 
   profengr | Dec 10, 2005 08:17pm | #2

   round is sheet metal is best.  stay away from fiberglas ductboard, difficult to seal against moisture, therefore possible mold issues.  higher velocities in ducts = more noise, typical range is 1,000 to 1,500 fpm for residential.

1. 
   brownbagg | Dec 10, 2005 08:20pm | #3

   I got round sheetmetal because I was leary of fibers, but boy is it noisy.. 2+3=7

3. 
   highfigh | Dec 10, 2005 09:45pm | #4

   Round ducts allow laminar air flow better than any other shape. Keep any directional changes as gentle as possible, too. This means, no tight bends. The air will flow better is this is done and rectangular ducts with tight corners have a lot of turbulance, which causes a lot of noise.

   "I cut this piece four times and it's still too short."
4. 
   experienced | Dec 11, 2005 05:35am | #5

   Your forum heading mentions return ducts also.

   It is recommended that return ducts be 25% larger than supply ducts.

1. 
   Matt | Dec 11, 2005 03:07pm | #6

   So, if you have 1 return that services four 1 sq ft supplies, does that mean that the return should be 5 sq ft?

1. 
   experienced | Dec 11, 2005 04:20pm | #7

   Yes, sort of.  For the best design, you just can't go from the sq footage but from the volume capacities of the ducts for your application. For example, for the velocities that conditioned air in houses is delivered, about 400 feet per minute, 4" round comfortably (with out noise) handles 40 cfm, 5" = 70cfm, 6"=100, 7"=140, 8"= 200.

   These are volumes I've had in my head for years but on Friday saw some volumes that were higher for the duct sizes. This was in the literature from an HRV manufacturer. For example, they had 8"= 250 cfm. This may be OK for an HRV as it operates on low speed 95%+ of the time with 40-60% of the high speed volumes. For the 5% of the time (if that much) it operates at high speed, any duct noise (small at this cfm and velocity) will be very low and maybe even drowned by the high speed fan operation noise.

1. 
   Matt | Dec 11, 2005 04:37pm | #8

   Right - I was just trying to simplify things for conceptual purposes. 

   >> For example, for the velocities that conditioned air in houses is delivered, about 400 feet per minute, 4" round comfortably (with out noise) handles 40 cfm, 5" = 70cfm, 6"=100, 7"=140, 8"= 200.  <<  If you don't mind me asking what would be some #s for 10", 12" and 14"?

1. 
   experienced | Dec 11, 2005 09:54pm | #9

   9"- 300 cfm

   10- 400

   12- 500

   14- 700

   Edited 12/11/2005 2:04 pm ET by experienced

2. 
   profengr | Dec 11, 2005 10:49pm | #10

   at the 8" thru 12" sizes, you are in the 600 to 700 fpm neighborhood.  universally accepted equation is Q=VA, where Q is the voulmetric air flow, A is the area of the duct in SF and V is the velocity of the air in the duct.  high velocity air noise is only a concern in occupied spaces where branches are most likely to be installed.  mains can be sized for higher velocities while in attics, basements, etc.  sheet metal costs are going up and money can be saved with judicious application of duct sizes.

3. 
   experienced | Dec 11, 2005 11:43pm | #11

   Agreed! The calulated velocities for the 9"-14" diameter ducts are between 637 and 727 fpm. But ducts this size will trunks and usually never be the last of a supply run in a home.

   Do you oversize the return ducting?

4. 
   profengr | Dec 19, 2005 04:03am | #15

   My experience is in institutional and commercial and have always used friction loss as the guiding factor.  Supply trunks up to terminal units are sized at 0.3" of static loss per 100' of duct and at 0.08" of static presure loss per 100' of duct for anything downstream of the terminal unit.  Return duct is normally sized at 0.08" of static loss per 100' of duct.  Will use higher numbers for return duct if the length of runs between the grilles and suction side of fan are shorter.

5. 
   Tim | Dec 19, 2005 05:07pm | #16

   "Return duct is normally sized at 0.08" of static loss per 100' of duct. "

   In medium and high pressure systems, 0.3" is marginal to poor design. Returns are not normally sized at 0.08"/100ft in low pressure systems, at least not by good designers.

   Edited 12/19/2005 9:11 am by Tim

6. 
   profengr | Dec 24, 2005 06:34am | #17

   I would be interested to hear your design criteria for low and medium pressure air distribution systems.  In my 20+ years, low pressure has been 0.08"/100' for low pressure returns, low pressure exhaust and low pressure supply (downstream of terminal units) distribution.  Upstream of terminal units is medum pressure @ 0.3"/100' for supply distribution. Some discretion can be exercised based on space limitations, noise tolerance ans energy use.

7. 
   Tim | Dec 27, 2005 09:29pm | #18

   Using the same design criteria for all (supply, return and exhaust) duct system is, in my professional opinion, inappropriate. Applying such a blanket across all designs is like "square footing" your load calculations, also inappropriate.

   My design criteria for low pressure system duct design is as follows:

   0.05" - 0.08 (per100') for returns depending on the details of the layout, length of runs, restrictions, duct lining, enviroment and noise sensitivity for the space. What you state as THE criteria, I consider as the upper limit. Certainly all design situations are not the same and criteria must be revised based on the specifics of the job. In residential or other system based or marginal /low static equipment, even 0.08" is too high.

   For low pressure supply side duct design, I also apply a range of friction design criteria from 0.05" to 0.1" near the unit, and the same criteria as returns farther away from the units. Adjusted as necessary for the job.

   For exhaust systems, designing based on friction rather than velocoty is also poor and/or sloppy practice. Depending on the exhaust stream, different velocity requirements apply. Grease ducts, for instance have to be designed at 1500 fpm or greater (usually up to 2500 fpm, depending on the AHJ) which is a far cry from 0.08". If you were refering to "fart fan" exhaust, 0.1" is fine. For other exhaust systems, friction based design is inappropriate for duct sizing.

   For medium pressure supply distribution systems, I use 0.4 to 0.7" IF I can use double walled w/perf liner spiral, else 0.3 to 0.5".

   ALL return ducts, BTW, are low pressure.

   My design criteria are conservative and the reasons are very simple and unfortunately, realistic. Have you ever had a system be installed with every detail, turning vane, splitter, transition, radiused turn, shoe tap, takeoff, etc., exactly as it was specified? Ever had incomplete structural and/or achitectural details (or those that were changed unbeknownst to the HVAC designer) available and then have to reroute or "squeeze" the ducts to fit. Ever has some DA highschool educated, union "tinner" decide he knows better than you and install whaterver they happen to have nearby? In a pefect world, less conservative design criteria would be acceptable, but I haven't had that fortune in my carreer, and I doubt you have either.

8. 
   Matt | Dec 11, 2005 11:58pm | #12

   Thanks much.  I'm gonna add those numbers to my misc - everything - numbers cheat sheets.  I would never be sizing ducts, but when the HVAC guy does the layoug I kind of like to check over it and see if what he is doing makes any sense at all.  Once a trunk line return (for example) is in a duct chase and all sheetrocked up, I'd hate to hear "it's always hot upstairs" (cooling season) from a customer.

9. 
   experienced | Dec 12, 2005 03:49am | #13

   Do a bit of web searching. There's a lot out there.

   http://www.Snipsmag.com       http://www.warmair.com

10. 
    Matt | Dec 12, 2005 03:59am | #14

    If you don't mind me saying so why don't you take a look at this guy's thread: 66965.1