Consistent Air Velocity 1

[R744]

HVAC Engineers,

Need your advice on the design of a makeshift supply air diffuser that would provide consistent air velocity across the entire opening.

Due to the constraints of this project, I need to come up with a way to insure consistent air velocity across a diffuser that will be at a considerable length (+8'). This problem is exacerbated with the supply air having to come in at one end of this diffuser and it cannot be centered in the length. I need to maintain 150 FPM across the entire length of this diffuser.

I am having a hard time coming up with the correct formula for this to ensure the air opening area is properly sized so that the air does not decrease or increase in velocity at one end of the diffuser over the other. I need to make sure the diffuser is properly pressurized over its entirety to maintain the velocity.

This would be a rectangular diffuser as far as the plenum goes, but I am open to suggestions on whether rounded would serve me better.

Thank you.

 

[chicopee]

Would air duct vanes be helpful?

 

[Wilbur55]

What you want is constant static pressure along the inlet side of your diffuser. Maximize the cross-sectional area of a plenum behind your diffuser to reduce pressure drop due to flow rate along the length of the plenum behind the diffuser. Consider dumping air into the plenum through another slot (or perforated duct) at the back of the plenum to disperse the air behind the main diffuser along the length of the plenum.

 

[willard3]

Investigate "static regain" which varies duct size to recover static pressure.

 

[Drazen]

custom-made plenum with varying width, calculated by static regain method.

 

[SAK9]

If this is a critical application with zero tolerance no amount of calculation will help you.Build a prototype and perfect it with trial and error.As a guide line,you need to drop the velocity to the minimum possible value well before it enters the diffuser boot.A perforated plate at the outlet level is preferred.

 

[berkshire]

I would second Wilbur55's method, drop the inlet velocity to almost zero, then use the static pressure regain to disperse the air through a perforated plate set back behind the diffuser.
B.E.




You are judged not by what you know, but by what you can do.

 

[R744]

[ignore][/ignore]Thank you all for the suggestions. It has been a little while since I have been able to get back on here. I haven't had time to research the static regain method but here is my shot at the perforated plate idea. If you take a look at the attached image of a rough 2D layout, would this be what you had in mind? It seems too simple to work.

The diagonal line would be the perforated plate and the arrows designate the airflow through the diffuser.

 

[berkshire]

R744,
You are on the right track, give your self a little more room at the right hand end. Maybe an inch or so behind the screen.
then make absolutely sure that the free area of the perforated plate, is less, than the surface area of the incoming duct.
You want the air to pile up in there and re-distribute itself through the perforations.
B.E.

You are judged not by what you know, but by what you can do.

 

[R744]

Berkshire,

Are you saying to angle the perforated plate towards the diffuser outlet on the right hand side to within 11-10 inches of the outlet?

Also, would Solidworks fluid simulation be of any help here? I have access to it but not sure if it would be of any help.. has anyone had experience with it for a similar situation?

 

[Wilbur55]

I doubt that static regain would produce a straight line along the entire 8-foot length of the plenum. Along with more room on the right side, I would suggest more room on the left. There is no depth between the plate and the diffuser for air to flow toward the left side of the slot.

 

[ChasBean1]

Don't reinvent the wheel. I don't have experience with either of these and don't vouch for their function, but:

Linear slots:

http://www.acutherm.com/products/linear-vav-diffusers/tl-diffuser.php

Square diffusers:

http://www.titus-hvac.com/file/8313/vavdiffusers.pdf

 

[uofc2003]

I agree with ChasBean1. Have you gone to any equipment/grille suppliers for their input. They may provide a product that already meets this requirement. Depending on the space you have you may be better off using shorter sections combined to create the one 8'+ length.

 

[R744]

The problem with the acutherm option is that like most linear diffusers, the air inlet is centered on the diffuser outlets and I would need the inlet to be on one end in this case.

It might seem like an insignificant requirement but I need to stick with it.

I'll try to run a fluid simulation on it and see how it goes.

Thanks for the input.

 

[cry22]

consider laminar flow diffusers