One more vent question

[tiggybubba]

I had my venting guy over today to go over the final design. I think I'm ok but Angelique got me thinking.

I have a 790cfm fan. I am running 10" rigid ducting. But there is about 22' of it. Will my fan become ineffective with that much ducting. There will be 2 90 deg turns, one out of the vent box and one exiting the roof.

Any advise? It may be possible to move the exit vent but I wanted it away from as many windows as possible.

[NMLinda]

You'll have some degradation, although it's good that you ran 10" dia rigid ducting. I recommend the following linki:

http://mikeaurelius.wordpress.com/ventilation-primer/

Part 2 walks you through the calculations so that you can find out how much static pressure the long run of pipe and the two right angles will create for you. Hopefully, the fan you bought has curves or a table to tell you how much the cfm degrades with statid pressure. Once you know that, then you can decide if your fan is big enough for your vent hood.

Linda

[tiggybubba]

Ok I know that the fan has enough cfm for the area of the barley box. If based on 100 cfm per sq ft, I should only need a 466cfm fan. If based on 125cfm/sq ft then 625. My fan is 790.

But if I use Mikes formula of cfm/area of ducting to get my velocity in fpm my fan would have to be 1635? and that is not taking into consideration the duct run. Only the size of the duct. I thought bigger was better when it came to the ducting size. We shouldnt use reducers because it brought up the static pressure?

This has given me a headache and left me confused. Please, anyone? Help?

[AVC-Ed]

Mike ran the numbers for you given what you wrote,

Your total system static (back) pressure is 0.07 inches. You don't have to worry about back pressure, the 10" ducting is a very good thing!

As far as velocity, according to the spreadsheet, you have good velocity at 1448 fpm. Again, nothing to worry about. You are golden.

[tiggybubba]

even with 20' of ducting?

[RSimmons]

The links in the article don't work, so I couldn't repeat the full calculation. The SP that Mike came up with seems a little low for 20 feet of duct with 2 X 90 degree bends, even for large duct.

Robert

[AVC-Ed]

Mike re-ran the numbers and came up with the same answer.

Remember, a 10" duct has 78.5 square inches of area, a bit over a half foot.

With an 8" duct, the total SP was .34 inches, at 7" it was .99 inches and at 6" it was 3.68 inches.

Robert, what links weren't working for you?

[tiggybubba]

here are the numbers I came up with....
velocity pressure .60
bend loss factor 1.5
static pressure 1.668

in part two of the ventilation article it said to aim for a velocity of 2500-3000. I calculate mine at 1449. 790/.545

[RSimmons]

The links to Table 1 (Bend Loss) and other things in Mike's article that you referenced above.

Robert

[AVC-Ed]

I just checked all the links, Robert, and they are good.

[AVC-Ed]

Originally Posted by tiggybubba

here are the numbers I came up with....
velocity pressure .60
bend loss factor 1.5
static pressure 1.668

in part two of the ventilation article it said to aim for a velocity of 2500-3000. I calculate mine at 1449. 790/.545

Mike says that a higher velocity is desirable, but not totally necessary, especially with the long run that you have. Because of your long run, jumping up to 1600-1800 CFM really puts you into the range of pushing "too much air".

The only drawback to having a lower velocity in the duct is the possibility of dropout of dust and debris. He suggests that at least once a year, you open the duct and clean it out.

[AVC-Ed]

Originally Posted by tiggybubba

here are the numbers I came up with....
velocity pressure .60
bend loss factor 1.5
static pressure 1.668

in part two of the ventilation article it said to aim for a velocity of 2500-3000. I calculate mine at 1449. 790/.545

The problem is in your math.

Velocity Pressure is (Velocity / 4004) squared.

1449/4004 = .3619

.3619 squared = 0.131

[RSimmons]

Originally Posted by AVC-Ed

I just checked all the links, Robert, and they are good.

Loss Factor: A multiplier, usually fractional, that is the amount of friction induced by ducts. This is number is a constant for specific duct types and is usually presented in a look up chart form. The chart we will be using in all these calculations can be found in the posting “Ventilation 101: Static Pressure” (insert link here), referred to below as Table 1.

Sorry Ed/Mike but the link (insert link here) isn't functional.

R

[AVC-Ed]

Robert -- scroll down to the very bottom of the text, you will see a highlighted link called "Table 1". I'll ask Mike to also add the link in the upper part of the text. Sorry for the confusion.

Here's a link to it as well. http://mikeaurelius.files.wordpress..../12/table1.pdf

[RSimmons]

I got it. Thanks Ed.

Robert

[AVC-Ed]

No problem!

[tiggybubba]

Vent is in with ducting. Went with 10' of 10" rigid ducting. Did the smoke test and it works like a hot damn!!!! Now just have to get the electrical to the fan (did the test with a long extension cord) and with a flick of a switch, voila!