Duct Sizing, how to?

[wchowe]

Hi All,
I need some help designing a duct for a laser. If you can picture a box that is 3ft square x 10ft long, thats the rough size of the laser. I get maybe 140 degree air at the top inside surface of the cabinet. I built some ducting that runs from the top of the cabinet down to the bottom where the heat exchanger is. At the bottom I have a heat exchanger coil with fans mounted on it. The fans pull air down from the top of the cabinet through the coil and out into the bottom of the cabinet. Convection takes care of the rest. I am using 3 small fans that produce 300fpm given no static pressure. Once connected to the duct however, my fans are only producing about 125 fpm. The duct crossection is 48" x 2" and runs about 30"long. I am really limited in the 2" dimension because of design limitations. I would like to get the flow up. How do you go about calculating static pressure through the duct? How can I improve the duct design. There is only one 90 degree turn (int the coil). If I were to go the entire length of the cabinet (10ft) x the 2" width, would that solve the problem?
Thanks for the input
Bill

 

[willard3]

The hydraulic radius of a 48" by 2" duct is horrendous and static loss will be the same.

I will assume 125 fpm x (48" x 2")/144 duct size = 83 cfm
83 cfm x 3 fans = 250 cfm

From a Trane wheel, static will be 0.05 in w.c./100 lf duct

If you double velocity you double the volume, static will be 0.15 in w.c./ 100 lf

Elbow will be in the range of 0.3 x velocity pressure or 0.02" w.c.

 

[wchowe]

One thing I am wondering about, is crossection. Being that the section is 48x2=96in2, is that the same as any (say 9.8x9.8) 96in2 section? or does the one narrow dimension add to my problems?
The static is 0.05in w.c./100lf so the shorter I make the duct run the better? Correct? How much better does it make the fan performance? If I am 1/3 of 100, is it saying that the static is 0.05/3=0.017 ?
Thanks
This is interesting, but I never do any of this work.
Bill

 

[willard3]

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One thing I am wondering about, is crossection. Being that the section is 48x2=96in2, is that the same as any (say 9.8x9.8) 96in2 section? or does the one narrow dimension add to my problems?
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Look up Hydraulic Radius and you will figure it out. Equal area is NOT equal friction

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The static is 0.05in w.c./100lf so the shorter I make the duct run the better? Correct?
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Yes

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How much better does it make the fan performance?
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I would have to see the fan curve to know

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If I am 1/3 of 100, is it saying that the static is 0.05/3=0.017 ?
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No, static pressure calculations are quadratic and not linear

 

[wchowe]

I found a calcultor at

http://www.freecalc.com/ductfram.htm.

I input 48x2 duct section, 900 cfm, and 2.5 ft duct length. I get 0.768in h20. My fans are rated for 200 cfm @0.04 in2 h20. 0.768 is not on the graph, so I assume that the fans will stall? My air friction through the heat exchanger is 0.08in h20. Do I have to add the 0.786in+0.08in=0.866 in h20 and find a fan that will be rated for air flow at that combined pressure?
I know this is basics, but I haven't done anything like this since school.
Bill

 

[willard3]

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My fans are rated for 200 cfm @0.04 in2 h20. 0.768 is not on the graph, so I assume that the fans will stall?
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Yes, fan will stall

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My air friction through the heat exchanger is 0.08in h20. Do I have to add the 0.786in+0.08in=0.866 in h20 and find a fan that will be rated for air flow at that combined pressure?
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Yes, pressure is additive

A propeller fan (I assume it is a propeller fan) does not do very much pressure. You may have to change fan types, ie, forward curved centrifugal, backward inclined centrifugal and etc to reach rated flow at 1" wc

 

[wchowe]

Alright, so I either make the duct bigger or the fan stronger!
Can you explain, "pressure drops in a system to me?" I can't find anything that sinks in for me. I keep seeing it ,as the opening gets smaller the force required to push through it gets bigger. Like a cylinder head, as the projected dia gets smaller, the same pressure provides less force. But I believe its, areas of less pressure due to air evacuation?
Thanks
Bill

 

[willard3]

Read the whole page here for pressure drop:

http://hyperphysics.phy-astr.gsu.edu/hbase/pber2.html

 

[wchowe]

Nice read. I think I am straight now.

I recalculated my ducting. At

http://www.freecalc.com.

Its 0.045in H20 + the coil at 0.008 in H20= 0.053in H20. My fans put out 200 cfm at 0.05 in H20. My test rig using a vane airflow meter registers 125 fpm. Based on your calculation earlier= 83cfm? I wonder why I am not getting the 200 cfm? I guess I have more static pressure in the ducting? My duct is 42x2 as I said, it travels 2.5 ft, then I have the coil on the side of it. So effectively I have a sharp 90 degree turn. If I smooth the transition maybe I'll get back to 200 cfm? The coil opening is not a restriction itself at 31.5in x 12in =2.58 ft2.
Any input?
Thanks
Bill

 

[willard3]

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The coil opening is not a restriction itself at 31.5in x 12in =2.58 ft2.
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Yes it does and it's probably in the range of 0.01 to 0.1 depending upon velocity and coil design.

Velocity across the coil, either heating or cooling will be in the range of 300-700 fpm to be turbulent and have good heat transfer.

If it's a cooling coil that condenses water from the air, a wet coil has a higher pressure drop than a dry coil.