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Old 06-25-2014, 03:59 AM   #1
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Air Flow Calculator?

Refitting my van's rear heater, adding a short section of duct to direct about half my available CFM's to the rear area just behind the wheel wells, '03 E250 EB.

Have a choice of 2" 2.5" & 3" OD duct and louvers, space isn't an issue for this part but the goal is proper sizing in order to not raise the static pressure so much it bogs the blower motor down.

Anyone know of a HVAC calculator or have knowledge how best to make the right choice for duct sizing?

Would this same question be better asked in another forum here on SMB?

TIA
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Old 06-25-2014, 10:24 AM   #2
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Re: Air Flow Calculator?

It obviously depends on the flow rate of the blower. It also depends on the fan's pressure capability. Commercial fan manufacturers provide a fan curve that shows this, but I doubt if you can find one from Ford. One way to design it would be to use the same size Ford did for the passenger vans or slightly bigger. Another way would be to base it on the size at the discharge of the blower. For a commercial fan, a good guess would be 2x the area of the blower discharge. So if the blower has a 3" diameter discharge maybe use a 4" duct. (7 sq. in. vs. 12.5 sq. in.). Area = diameter(squared) x pi / 4. Pi=3.14

Another way would be to base the design on duct air velocity. You need to know the fan output to calculate this way. I would recommend maybe 800 to 1000 ft. per minute (fpm) to keep it quiet and minimize pressure drop. So if the fan puts out 100 cu. ft. per min. (cfm) (a total guess on my part) the duct should be 0.1 to 0.2 sq. ft. or 9 to 18 sq. in. (4 inch duct would work for this). Area = Volumetric flow rate (cfm) / velocity (fpm)

Or another way of looking at it is, if 3" is the biggest size available, use it since space isn't an issue. However 3" sounds too small to me for the total output of a Ford rear heater.

The size can be reduced after a tee. So if you start out with a 4" duct, it could be split into 2 - 3" ducts and the velocity would remain about the same.

Straight duct doesn't create a lot of back pressure, it is the turns and tee's that create most of it. A rigid duct has less pressure drop than a flexible duct. A Y fitting is better than a T to split the flow.

Another thing to consider is how to balance the flow between the outlets. If there is no damper, then the ducting to each outlet needs to have roughly the same amount of pressure drop, or all the air will come out of the one with less pressure drop.
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Old 06-25-2014, 12:00 PM   #3
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Re: Air Flow Calculator?

Okay let me plug some numbers into this then............

Aftermarket Pro Air brand heater only unit, total CFM is 315, outlets are currently 2 @ 3 7/8" x 1 3/4" I would need to come out of those discharge openings and turn one 90*, blasting straight outward. pretty much parallel to the van floor. The second would be first turned 90*, run about 30" then be turned once again to blast straight out from that opening, again pretty much parallel to the floor.

I can get straight solid oval shaped duct in 2, 2.5 & 3" "diameters", yse normal spiral wound flexible duct as the connections for that straight run.

Conceivably I can install a Wye fitting with adjustable damper but its largest size is 2 1/2". The outlets can be one of a few as available in the Coyote Int'l catalog above.

I have the capability to build a plenum that would provide separate "chambers" to better isolate each openings discharge away from the heater blower.

Hopefully this makes enough sense to better pin down what OD ducting I should use.

BTW thanks for the info---really appreciate the expertise!
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Old 06-25-2014, 05:56 PM   #4
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Re: Air Flow Calculator?

Since the heater actually has a separate blower for each outlet, it won't matter as much that the duct pressure drop is equal between the two discharges. Adjustable louvers should be able the balance the flow.

I calculate the area of each blower discharge as 6.9 sq in or 0.048 sq. ft. With half the flow going through each (157 cfm) the velocity would be 3,300 fpm. That just seems really high for quiet operation.

So one idea would be to just run one flex duct from each discharge to a louver. But I think a better plan would be to install the plenum you talked about and then use 4 ea. 3" ducts and louvers.

Don't forget you need a big return air louver as well, if the heater is going to be in a cabinet.
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Old 06-25-2014, 06:14 PM   #5
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Re: Air Flow Calculator?

See if this helps or hurts you.
It a screen shot of a Trane Ductulator set to a 3" diameter duct.
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File Type: pdf DOC015.pdf (197.0 KB, 7 views)
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Old 06-27-2014, 04:38 AM   #6
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Re: Air Flow Calculator?

Charlie that chart would be very handy to have in-hand huh? Wonder if they're still out there or its all on an "app" these days?

Thanks for posting that BTW!

I've mentally designed a few work arounds to most of this--had to remember this same heater unit was previously installed and worked fine in a raised roof van I still have. Thinking a few ideas from that installation might transfer to the current one as well.

Thanks for the input about my (air flow) output guys!
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Old 06-27-2014, 02:35 PM   #7
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Re: Air Flow Calculator?

Here is an on line chart that may help. http://www.engineeringtoolbox.com/sizin ... d_207.html
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Old 06-29-2014, 09:46 AM   #8
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Re: Air Flow Calculator?

Hey Larrie---thanks for that link. If I can't solve almost anything there I'm dumber than I think!
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