CurtPalme.com Home Theater Forum

[snkby]

im wondering if putting 2 fans together piggy-back style would have any effect for greater airflow ?

i can argue it in my mind both ways so...........

thanks

[Ile]

Series operation have same airflow, but almost double pressure.
http://www.nmbtech.com/images/engsec/multiplefan.gif

Parallel operation have viceversa.

Here's whole story.
http://www.nmbtech.com/html/fans/engineering/engineering_101.html

[garyfritz]

Unless I'm seriously mistaken (and it wouldn't be the first time ), you've got it wrong.

Static pressure is a property of the enclosure or ducting that you're blowing air through, not the fan. Long, small, or rough ducts increase static pressure, as do bends and kinks. Pulling air through a hushbox increases static pressure. It can change at different airflows -- hence the "system impedance" curve in that article -- but other than that the fan doesn't change it.

Fans can move a certain airflow through a specified static pressure. So you look at the airflow chart, find the static pressure you're fighting on the Y axis, and move to the right until you hit the curve. Then you drop down to the X axis to find the airflow that fan can deliver through that static pressure. Look at these charts:

Let's say each tick on the axes is one "unit." So if you have 1 "unit" of pressure, you get about 2.2 "units" of airflow with 1 fan, and about 4 "units" with 2 fans in parallel. Now look at the second chart and you'll get only about 2.8 airflow with 2 fans in series. Now look if you have 3 units of resistance: you get almost zero airflow with 1 fan, not much more with 2 fans in parallel, but about 2.5 with 2 fans in series.

So the charts show that the 1-vs-2-parallel-fan lines are close together (similar X values) at high pressures, and farther apart at low pressure. That says parallel fans work best with low-pressure situations. Conversely the series lines are far apart at high pressures, so series fans work best with high-pressure situations. Which is exactly what the article says:

[dropzone7]

Gary, do you have any pictures of your hushbox and or ducting/fan system? You may recall that I am working on a hushbox and have been posting over at AVS. I'm going to be using 8" duct and a super strong fan that was once part of a HEPA filtration system.

[garyfritz]

Actually I don't have a hushbox any more. I got rid of the XG, and the 8500 is quiet enough that a hushbox isn't necessary. It would be nice, sure, but given my budget (time and $$), it'll never happen.

I could post a pic of my old hbox, but I'm too embarrassed. It was nothing to use as an example, trust me.

Good God, 8" duct? You won't see much of ANY resistance from that, provided it's not 50' long with half a dozen bends. That's got 4x more cross-sectional area than a 4" duct. Put a "super strong" fan on that and you're likely to have a wind tunnel! You don't need to exchange the air in the whole HT every minute or two. All it takes is about 100 CFM for any projector -- that would completely exchange all the air in a 2x3x4' hushbox in about 15 seconds. You could probably do fine with half that.

With an 8" duct you hardly need any air movement to get 100 CFM. The duct has a cross-sectional area of pi*4^2 = 50.24in^2, so one cubic foot is 12in^3 / 50.24in^2 = 34.4" long. You need the air moving 34.4*100 = 3440" per minute to get 100 CFM, which is 57.3" per second. That's only 3.23 mph -- barely a breeze.

[ecrabb]

The 100 CFM figure is in the ballpark, but it does depend on the duct size/shape (we're talking static pressure, which dictates type of fan and CFM rating), projector, supply air temp, and it what air temp delta you want the projector to see.

When my engineer friend ran calcs on a G90 for me (I can dream, can't I? ) using my 6" duct with two 90-degree bends, he told me I could use a 140CFM fan with pretty low static pressure capability - like a bath fan. That was for a 5 degree differential... Meaning, that if the supply air (in my case my room) was 75 degrees, then the exhaust air from the hush box (and therefore most of the inside of the hushbox would be 80 degrees.

Since we used the G90 as a worst-case scenario, and arrived at 140CFM, most projectors would probably only be 2/3 to 3/4 of that. So, Gary's 100CFM figure is probably about right - as long as you don't have static pressure and use a fan not designed for static pressure.

There's a sweet spot, too. First and foremost, you don't want the projector to get hot. But, you don't want to drag 250CFM through the hush box, either. In that case ,you'd be pulling twice 2.5 times as much dust, lint, skin, dander, etc. through the box AND your projector... unless you filtered the supply air somehow.

SC

[garyfritz]

Hm. So your friend said a G90 would heat about 24 cu ft of air by 5° in 10 seconds!? (2x3x4' hbox, a bit snug for a G90 but it would fit; make it a bit larger and you've still got less than 24 cu ft due to the displacement of the tubes &etc. 140cfm cycles 24 cu ft in just over 10 seconds.)

I'm no HVAC designer but that seems fast to warm that much air by 5°. But considering how much hot air one of these beasts puts out, maybe he's right.

But did he say to use a 140cfm *fan*, or did he arrive at 140cfm as the target airflow? Because even with your big duct, there's going to be some static pressure (from the hbox if nothing else), and a typical bath fan will deliver well less than its rated zero-back-pressure numbers.

I'd say 140cfm airflow is overdesigned just enough -- meaning it would keep the projector quite cool without sounding like a vacuum cleaner.

[dropzone7]

Just doing a little testing pre-hushbox tonight. I placed a temperature probe 1 inch from the projector just behind the front of the tubes. I measured ambient temperature at 77 degrees before turning on the projector. After 1 hour of operation running video it reached a temperature of 93 degrees and held at that. Again, this is free open air operation. The hushbox is not in place yet.


I found the specs on my fan and it has the following speeds:

Low 100 CFM 10 X 12 room 6 times per hour
Med 200 CFM 16 X 16 room 6 times per hour
High 300 CFM 17 X 22 room 6 times per hour

My room is roughly 13W x 18L x 8H or 1872 cubic feet.

The completed hushbox is a little larger than the XG itself so roughly 3W x 4L x 2H.

We are going to hook this puppy up tomorrow night and hope it does not implode the box.

[ecrabb]

Gary,

Here's the old thread where we were talking about hush boxes, static pressure and CFM:
http://www.curtpalme.com/forum/viewtopic.php?p=39965

It's been awhile, but I went back and looked at my email from my buddy, and he worked up a little table for me. Based on an "equivalent duct length" formula, which took into account my duct length and the two elbows I have, and on a performance curve for a Panasonic duct fan I was looking at (that mapped fan performance and static pressure), he was able to calculate that the 240CFM rated inline axial duct fan would achieve 190CFM in my system.

He was also able to calculate that a G90 @ nearly 1000w of heat dissipation (that's damn near a hair dryer if you think about it!) would result in a 13-degree temp increase - or 88 degrees from 75-deg ambient air with that 190 CFM air flow. Mos scary, was that dropping down to a 110CFM fan (not sure what the actual CFM would be), he said the G90 would see 105 deg F in the box. The G90 is rated to 104 according to specs. I don't know about you, but I wouldn't want my G90 running in a 105-degree box all the time.

SC

[AFryia]

My hushbox runs two 87cfm PC fans @1200rpm, a little more than 1/2 their rated speed (2000rpm). The box direct exhuasts through one 90 degree turn.

This allows my G70 to hold <90 deg F in the box with a delta of 17 degrees.
http://www.curtpalme.com/forum/album_page.php?pic_id=656&mode=previous

[dropzone7]