HVAC Duct Crossing Building Expansion Joints 3

[sigma1]

We have a 6'x6' square duct with negative pressure crossing from one building to another with 14 inches (+/-) of differntial movement. The duct is not flexible enough to absorb these movemnts without the use of flexible/expansion joints. Any recommendations would be appreciated. Thank you.

 

[MintJulep]

A possible 28 inches of total movement?! Is that at least constrained to movement in a single plane, or is it 28 left-to-right and 28 up-and-down?

That is better than 1/3 of your duct size. So you will need a long flex joint to prevent a rather sudden change in the duct cross-section and associated variation in system pressure drop.

 

[sigma1]

The total movement is 14" (7" each building)and it is in both horizontal directions x & z. No vertical.

 

[ChrisConley]

You mention that duct is at a negative pressure - how negative? Depending on the pressure you may be able to use relatively standard flex duct to allow for movement.

 

[MintJulep]

Is it really possible for the two buildings to move their full possible excursions in opposite directions at the same time?

 

[designbuild]

MintJulep, I would expect that the condition described by sigma1 could occur on only one occasion and that would be an earthquake.
I was thinking along the same lines at Chris. Unless it is a horribly negative pressure your best bet is probably regular ol' flex duct, oversized just a bit to take into account it's huge pressure drop in comparison to standard metal ductwork.

 

[sigma1]

The duct is outddors at the roof level. The joint cannot be too long or it has to be strong enough to support 35 psf snow and/or 21 psf wind in addition to its own weight and negative pressure (P = -9" about -0.35psi). We are thinking of a double joint with a spool between them (similar to a pipe universal joint)but the weight of the spool can be a problem.

Thank you all.