Re-circulation Fans/Plenum Question

[JumpNWork]

I've attached a screenshot of what I've detailing out for a project I'm working on. In short its just a plenum to allow the fans along the back to recirculate and keep airflow over product that's being cured in a buffer inside.

Fans are 4" axial fans @ 100cfm with .325" H20 max Pressure.

2 questions:
1) Will I see any huge effects on flow because all the fans are drawing vertically through the plenum? As in, will the top fans be denied any air? (I'd do SW/COSMO flow analysis if my company had the full version...)

2)Because its internal, I shouldn't need to flow balance. I just need to ensure that the fans can overcome the friction loss over the height of the plenum. The top fans are 10ft from the bottom plenum opening. For a 6 round duct, @ 100cfm thats .1" H20 per 100ft or .01" per 10ft. Less than the max pressure of the fan. Are these right assumptions? Can I assume each fan is only responsible for its own friction loss?


It doesn't need to be precise, so that's why I'm a little liberal with the estimates. I'm just trying to ensure I'm not going to burn up the fan motors. I'd appreciate any tips or suggestions on the plenum; however the recirc system must remain internal as the inside is humidity,temperature and co2 level controlled.

 

[MTES93]

I tried to figure out what you were doing in your diagram but couldn't. Are the fans connected to 6 inch ducts on their inlet and blowing into the common plenum? Or, are they drawing from a common plenum and discharging out a 6 inch duct..

From the diagram it appears they are drawing from a common plenum in a vertical arrangement (as you mention). Not sure wehre they go from there If this is the case and the plenum is amply sized for the full air flow (all fans running) it should be fine. I imagine you could get these little fans turning slowly backwards if they were off and a slight negative pressure existed in the plenum.....

 

[JumpNWork]

Yea the circle cutouts you see are fan locations and they are pulling from the plenum and blowing into the machine(away from you-into the page). What you can't see is that on the inside there a stack of products that are being stored and allowed to cure after a coating process. The fans are there to help facilitate that curing process. The cut out on the bottom is of course the supply opening of the plenum. I used the 6" round duct for friction loss as i had the chart quickly on hand and I had already sized the plenum as rectangular for equivalent diameter as a 6" round. Sorry for the confusion.


And the plenum will see a slight negative pressure anyway since the fans will be pushing air out, which is why i looking at the static pressure and friction loss.


The best I've come up with since then is that the fans might be considering a parallel fan array pulling from the plenum into the machine. But my courses in HVAC didn't delve into fan arrays or deep into flow patterns, so I'd need to find a book and dig deeper.

 

[MintJulep]

The flow at the bottom of your duct isn't the flow of one fan, it's the flow of all the fans.

So the velocity in the plenum is high at the bottom, and lower at the top.

Since the total pressure in the plenum needs to be more or less constant, that means that as velocity, and velocity pressure, decrease static pressure increases.

So I'd expect the top fans to have higher flow than the bottom fans.

Also, you have crappy inlet conditions, so your fans won't be operating anywhere near catalog performance.

 

[JumpNWork]

Seems I've been over thinking this though. I wasn't looking at the fans as a branch and that was sending me in circles. Gonna bump up the plenum size just a bit to compensate for the rectangular duct losses and to keep the velocity and pressure down at the bottom.

Yea the inlet conditions are crap, but don't have much to work with. The frame of the machine is set, and they want to use in-house sheet metal they can make/bend themselves. Making the assemblers try and sheet metal field fab the ducting would be a nightmare, I'd end up doing it myself.

In fact, the fans being used are stock parts for us. So I'm being forced to design this backwards and as an add-on. But fortunately the goal is just to promote air circulation. So as long as air is being moved and the fan life isn't being drastically reduced its good in the eyes of my boss. Kind of scary how lax the design requirements can be sometimes around here.