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I am installing a 5 ton package unit. The supply air duct size is 12X19. Evap fan 2000 CFM. I plan on using a 24' horizontal duct with 10" round takeoffs. I would like to use 7 of these. The recommended velocity is 1000 FPM in trunck duct and velocity in branches not to exceed 800 FPM. My question is, am I in the ballpark here? Thanks for any input!
In just a wink (I didn't particularly check it out) I found this online duct size calculator...
http://www.aes.uconn.edu/tools/airductm.htm
Thanks very much...I'll check it out right now!
Your 12" x 19" main duct will give you a velocity of 1263FPM which is in excess of the recommendations. To obtain the required maximum velocity your duct needs to be 12" x 24", or slightly larger to reduce resistance.
The branch duct size of 10" will give you a velocity of 377FPM within each branch if you balance the system correctly.
You do not say what the maximum external static resistance is for the fan so I can't say if the amount of ductwork you are planning is within spec.
Hope this helps
Frank
I find this question rather worrying....
An installer of a ducted system who is unable to calculate the simple basic facts of his proposed system.
Or am I wrong and he is fitting his own equipment and praying that it will do the job correctly?
Call me fussy, but.....:confused:
Your absolutely right Brian...I find it worrying myself. Please understand this is a factory which is under constant changes. Room sizes change, ceiling heights lower, newer equipment can run hotter or cooler depending on the technology. Equipment can be added in a heartbeat or the entire way we do things can call for equipment that we haven't even seen yet. With all this, I have to compensate for a 100 ton R-12 system that was shut down this year by me cause I refuse to pump R-12 through the building. This is what goes on in my world! It's something that takes a little more then a calculator to solve. I wish "simple basic facts" could be applied here, but they can't. Thanks for your replies guys.
I wish you all the luck in the world buddy, it sounds like you nedd it :)
If it is a factory, then I wouldn't worry about velocity limitations related to noise considerations... not on returns, supplies, nor registers. Be sure to check those requirements on your duct-a-lator, they may not apply.
On the other hand, I would worry about delivery velocities required from the registers... and, of course, design total static pressure considerations. Depending upon your ceiling height, you may have to deliver the air at higher velocities.
I think you might have been been precipitate in condemning an R12 central system. But I don't know your situation.:)
It just doesn't sound like you have a simple package unit solution here, that stands a chance of adapting to what you are facing.
Have been battling this situation for the last 25 years or so. Two phase motors on the old R-12 system is one of the deficiencies of the system not to mention open drives, one recip and one radial. I am really relieved that I have made the choice. Package units and splits have been added over the last 3-4 years and another this year is almost finished. I seem to hold temp okay except when the plant goes on 24 - 7...It never gets a chance to cool down. It's gonna be an interesting summer for me! :cool:
LOL! I understand. Good for you!