AHU and humidifier space constraints?

[remp]

Im building up an Air handling Unit on site with a humidifier and due to space constraints etc... I have the following question. The AHU fan handles approx 15m3/s (31000cfm)

Q. The humidifier is going to be located down stream of the fan outlet. The fan is a DWDI centrifugal and the humidifier will be placed in a plenum the same size as the AHU cross sectional area and about 2 metres long (6 ft)....straight off the fan outlet. After the humidifier section there will be a bank of HEPA filters. The question is, will the velocity and turbulance in the plunum due to relatively high dischage velocity of the fan be too high for the steam humidifier...will there be carry over of condensate onto the HEPA filters???

 

[ChasBean1]

Billyg, I'd say you're adding risk with this configuration. I imagine the fan blows out the discharge at a pretty high speed - even without the humidifier there I wonder what kind of velocity distribution you'll have across the filter bank. You might consider some sort of air blender right at the fan outlets.

If you absolutely need the humidifier there, I think the Dri-Steem Ultrasorb has something like a 9-inch maximum vapor trail. You could also limit humidification by lowering the high limit humidistat (in the supply duct, downstream) to less than 80% or so - if you can still achieve your objective at that value...

 

[imok2]

Billieg
Please disregard my first post I misunderstood. The fact of the matter is that a proper humidifier should handle velocities anywhere from 300 to 3000 FPM. 10 duct dia is still a good idea for proper mixing

 

[quark]

I think it is better if you create turbulence while humidifying air. I feel that it is high velocity stream line flow that can carry water droplets with it and not turbulent air. Moreover, you won't have any problems with HEPA because they are hydrophobic.

Velocity in a duct purely depends upon the duct size. For a recirculatory system we always go with equal sized supply and return main ducts. In this case, if the velocity is a concern, you may have problems both at supply and return of the fan. The main reason for water carryover is not the velocity but the pressure (I mean static pressure at discharge) that balances the weight of the liquid particle and keeps it in the air stream. (This is the same principle used in pneumatic conveying)

Regards,


Eng-Tips.com : Solving your problems before you get them.

 

[quark]

As I don't want to sound weird I owe a little explanation to my earlier post. What I mean is that if the velocity in supply and return is same then the reason for carry over in supply side is due to the comparitively high static pressure. However you can rule out carry over, as suggested by imok2, if you maintain 500 fpm across the humidification section.

I hope imok2

Regards,


Eng-Tips.com : Solving your problems before you get them.