Flexible duct 2

[coffmarm]

I've unsuccessfully searched the internet several hours for a supplier of a heavy-duty, 42" diameter, spiral-wire-reinforced, flexible duct for conveying clean ambient air at 5" WC. Does anyone know where I might find such an animal?

 

[CountOlaf]

That does sound rather large. Probably 20" is the max normally available. I've spec'd/seen some 42" diam. fabric ducts like DuctSox, but that's probably not what you had in mind.

Could you not employ some smaller parallel paths to accomplish this task?

 

[coffmarm]

No, it's an existing duct arrangement used in four different places so it would be a big deal to modify them all - even if I could get the funding approved. The existing 42" flex ducts are almost 30 years old and I can't find any record of who the original suppier was. I found that

http://www.ducting.com

sells flex duct up to 60" diameter, but their's can't take the 5" WC pressure.

 

[ChrisConley]

ABC Manufacturer's of Canada makes custom fabric and PVC lined duct for the mining industry.

http://www.abccanada.ca/

I know they will consider specialty runs.

 

[EmeraldCoastHVACR]

Can't help you on this, but I attached an engineering study by ASHRAE professors on the performance of flex vs metal duct.

Very informative

 

[coffmarm]

Thanks Chris, ABC says they can make it - I'm waiting for a quote now!

Nice study EmeraldCoastHVACR, that one goes in my references folder.

 

[willard3]

Contractors and I always have discussions about length of flex duct and the manner in which it is installed. I detail its use carefully both on drawings and specifications and contractors pooh-pooh the reasons for the details but do it after they have to remove and replace a couple of pieces.

I have seen the bad results of sloppy flex installations in too many balance reports.

I knew about the high pressure loss, but the paper indicates that my calculations are off by about 10%. All the more reason to limit the use of flex duct.

Excellent paper.

 

[ChrisConley]

I'm glad they could help, they're a good bunch down there. I worked on the expansion to their plant a few years back.

 

[EmeraldCoastHVACR]

To summarize that paper:

The only time flex came close to galv steel was fully extended and there was considerable loses when at 94% efficiency, which is extremely rare (i've never seen it) in the field.

Expect to see flex duct losses approaching 50% compared to galv metal; in addition, expect to see losses using ductboard with liners more than 33% compared to metal(ASHRAE design manual).

From a techs point of view, my experience shows that ductboard & flex systems lower seer ratings 2 to 3 points with all the deteriorating effect that low-airflow systems give(too numerous to mention here).