CurtPalme.com Home Theater Forum

[ncc1701d]

I am finished with my hushbox, left now is how to get the heat out.

Is it enough with 3meters 10cm diameter pipe from the back and a 107m3/h fan?

[jask]

here this should help:
http://www.comairrotron.com/airflow_calc.shtml

[garyfritz]

10cm pipe is 4" -- yes, that's probably OK if it's smooth inside and not too many bends.
107 m3/h is 63 CFM. That would be fine *IF* you were actually pulling that much through the hushbox.

Realize that the fan is rated for an unloaded (free air) situation. When you put load on it -- 3m of 10cm pipe, pulling it through the projector, etc -- you reduce the amount of air the fan will actually draw. How much depends on how much resistance there is, and the specific fan's particular load characteristics.

If you actually pulled 50 CFM = 85 m3/h, that would replace all the air in a 1m x 1.3m x 1.3m box (1.7 m3) 85/1.7 = 50 times per hour, or roughly every 72 seconds. I think most people prefer to pull at least 2x that much air through, replacing the air every 30 sec or so.

Your situation will be unique, depending on how big your box is, how much heat your projector puts out, the ambient air temps when you're running your projector, etc. You might want to try it, and measure the temps. If the internal temps stay constant, you're good. If they keep climbing, you will need a bigger fan.

Now, consider noise. a 10cm pipe has a cross-sectional area of pi*5^2=78.5 cm2 = 0.00785 m3. 85m3/h = 1.42m3/min. In that pipe, 1.42 m3 is 1.42m3 / .00785 m2 = 181m long (!). So the air has to travel 181m per minute = 10.86 kph in the pipe to exhaust 85 m3/hr. If the pipe is smooth, and if 85 m3/hr is sufficient to keep things cool, 10.86 kph airflow will probably be fairly quiet. If you need more airflow, or if you want to make sure there is no wind noise in the pipe, you might want to consider a larger exhaust pipe -- 12 or 15cm.

And you should definitely check my math.


EDIT: Jask -- excellent resource!!! Guess I got my math right!

[ncc1701d]

Not really.. it just tells me that 106m3/h is 63 cfm.

What i need to know: in my barco there are 3 80cfm fans blowing, should my
vent be able to do 240cfm then? (80x3) or is 60-100cfm enough, and is it possible to
get that flow in a 100mm tube?

I tried the type of tube used for kitchenfans (the bendable aluminum type) but the noise from the
tube was horrible even though the fan itself was pretty quiet. The sound came from turbulence in the
tube.

[garyfritz]

That's what I was addressing with my wind-speed calculations, which jask's site also calculates. The bendable aluminum tubing is BAD BAD BAD!!! (Trust me on this -- I have some built into the ceiling/walls of my HT!! ) You want SMOOTH inside surfaces, and as few bends as possible. That will reduce the turbulence noise AND the resistance on the fan. Then larger-diameter pipe will reduce the air speed, which will further reduce noise and resistance. Using jask's site, increasing the pipe diameter from 10cm to 15cm decreases the air speed from 3 m/s to 1.34 m/s.

[ncc1701d]

Thanks

I guess i have to do some experimenting then.
I might get by with ~2m pipe also if it is noisy.

The hottest it has been in the room is about 30deg C in the summer (the fans run at max all the time then).
But then it just sucked the hot air around in the room, thats why i built a hushbox now and need airventing into another
room.

[ncc1701d]

Hehe yes i did notice that noisy as hell.

the tubes i got now is plastic plumbing tubes, completely smooth on the inside.
And there will be one 90degree + one 45degree turn, thats all.

[jask]

the bottom line is..if you can use a larger tube,do it! this lowers velocity and potential cavitation noise.