Plenum Fan

[ChaChaMan]

Never used this type of blower before, I was going to use a DWDI blower. I am very concerned about system effects. Does this 1200 cfm blower configuration make sense to all those with experience with this type of blower ? (See attached pic)

 

[KiwiMace]

I think you'll get some undesired system effect from the bend into the fan, but not that this is not done time and again.

The plenum fan is typically more compact and appears to provide reasonable efficiency in the instances I have seen.

 

[PatPro]

It appears as a residential application and this configuration has been used thousands of times. You won't get away from the system effects unless you have straight duct runs downstream of the fan and residential manufacturers are not too interested in that. You might want to consider using a VFD on the fan to minimize energy since most of these systems can operate at less than full flow rates and still achieve design set points. Energy savings might not be applicable since you already have electric heat.

Patrick D. Propst, P.E.

http://www.pro-engr.com

 

[ChasBean1]

(Admittedly without having done the research): I think the plenum, aka plug fan may be more efficient than your standard centrifugal. It’s basically a backwards inclined centrifugal without the casing (or shroud). Some of the losses associated with shrouded units related to friction and leakage (air between blades and shroud leaks by and the next blade must again apply work to it) might become mitigated with no shroud.

 

[ChrisConley]

The plenum fan is also more installation tolerant, and system effect will be reduced.

VFD or ECM motor is highly recommended. Just the sound reduction is worth it in many cases, the energy is a extra bonus.

 

[ChaChaMan]

Thx for your feedback everyone ! I will install a VFD too.

As you can see, the installation is tight and I want to minimize system effects.

Like many of you mentioned, I am seeking noise reduction which I am hoping the backward curved blades and direct-drive motor offer.

Once it's done...I'll give you all feedback.

Thx again all !!!

 

[Waramanga]

Plug fans are great. They are slightly more efficient with a shroud of some sort (EBM engineer told me that it is like 1% improvement in peak efficiency) but you loose some of the flexibilty. Get the manufacturer to approve the layout you are proposing.

 

[BlueSandman]

Have you considered the effects of the A-coil in this arrangment on your electrical components? I don't know your climatic design conditions, but if you operate close to dew-point off that coil or toss droplets (condensation drain plugs, refrigerant runs low, etc.) then the motor as well as any other electrical components may become really wet. And when the unit turns off on reaching cooling SP then the motor sits in a damp environment.

 

[ChaChaMan]

BlueSandman, Thx for your reply. Actually I share your concern and yes there will be dehumidification. This is how I address the problem :

I cannot put the coil after the fan because there is no room. I am hoping that by ensuring that the air flows through the coil at a speed of < 450 fpm, I will ensure that I don't entrain water droplets downstream during cooling.

Might you have arranged the components differently ? Then how so ? Keep in mind I cannot redo the ductwork around the air handler sinced it is fixed through walls. Therefor I am constrained by the existing ductwork.


Please see an alternative arrangement I elected not to use. This arrangement made use of a DWDI blower.

Regards

 

[BlueSandman]

Tough spot to be in. Clearly any ideas for optimal arrangments have been considered and weighed against the cost/time/space/rework limitations you have. Ideally the cold coil would have been after the heater, but you have to play the cards you are dealt.

Given the mosture I suspect all that I would have removed as much of the electrical connections to being outside the housing as possible (extend all leads, wiring and controls outside the plenum). If the humidity could be great in the fan/motor area then perhaps I might have used a motor rated for external exposure so that it would not be affected by the water.

I agree with slowing the air down to minimize droplet throw. If that is still an issue there are mist eliminators that are very low dP that could work to help with that (Basically they are stainless still brillo pads, about 1" thick which attach hard to the discharge face of the coil. Any condensation that forms attaches to these pads and drains down.)

 

[ChaChaMan]

I came up with this arrangement : (Please see attachment)
Plenum Fan + N-Coil +Discharge Plenum.

Following everyone's suggestions,

1.I placed the N-Coil after the Fan Motor so as to remove the risk of a damp Fan-Plenum environment.
2.I use a Discharge Plenum so as to do away with transitions such as elbows, pant fittings, offsets, etc.

Concern : I seem to be pressurizing 2 Plenum chambers, the Fan Plenum and the Discharge Plenum. Is this a problem ?

Any other comments on the present configuration ?

Thx in advance !