I am trying to correct poorly installed ductwork. Everything is duct-board and flex duct. None of the flex duct is pulled tight and the 90s are pulled around truss webs so those have kinks inside the curves.
I think it would be an improvement to use metal 90s where the duct path makes a turn and pull all the flex duct tight and straight between the metal 90s and take-off plenums. I would tie the 90s back so that they stay where they are at and not be pulled as I pull the flex duct tight.
I have read other suggest this; but, in reality I can not find metal 90s that connect flex duct on both ends. The code states that if using flex to metal connection the metal will have a bead at that connection. Every 90 I have looked at is made to connect flex on one end and metal duct on the other, again, one end of the 90 will have a bead and the other end will not. I need 90s that have a bead on both ends; but, the marketing industry for these materials is making it very hard to find.
Has anyone have experience with this type of installation that will point me in the right direction?
** Update **
Nice response from everybody here! So, from the top down in the response on this page I am trying to give more depth to my thought process on this post. @freeman, this is the same result I came to. I ended up fabricating a collar with one bead, but the bead I placed within 1/2" toward one side so that I may end-up with more meat on the pipe to pipe connection hopes that it has more bite and maybe stays more stabile because the longer length of friction metal in contact between those two pipes and length.
What happened was!, "A hard lesson", hah. The 90 did not have much meat to grab to-start-with so those efforts spent on that endeavor were lost but the best part would have been having the bead so close to the edge with enough meat to grip the flex and allows you to sleep at night. It was way easy! The big box wanted $20.00 for the collar; I thought I was gonna make out budget wise but "time and gas is money", I learned more than it was worth, "you know what I mean?"
About @RMDman's response, there is less volume supplied by the current duct than the true demand on that duct. I am looking to make optimise the current Duct just cut it shorter by pulling it as straight as I can. That area of the house just does not have enough throw to allow the mixing to comfort. Before I did anything, the velocity was less than 5.5 fps, now the velocity is at 10.6 fps. That was from just pulling all the flex as tight as I could.
On the 10" branch take-off there feeds one 4", two 5", one seven", it can not provide the amount of air for the environment. It will be cheaper to add a crucial 90 where it will give significant payback than to replace the system with a 12" trunk-to-branch and possible damper at a cost that includes changing both 10" duct board connections to 12". The parts list and retrofit are more expensive than if I can just trim down this one line that feeds two small rooms, a laundry, a standard bath. So that is way this question is so important to me.
What I want to tell you is that I have taken an eight inch and 10 inch duct out of the box and wrapped it around a bucket and then a trash can to look and see if where there is a problem with the air movement and each time I can visualize that there is more turbulence on the inside of any bent flex than what I have to give. This system is all ready undersized, I do not want to change part of the system out and then adapt the rest, you know? I just want to make it work.