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Author Topic: Fan Question  (Read 2491 times)
KA1OWC
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Posts: 94




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« on: September 27, 2017, 12:19:58 PM »

Building a go-box and am including a small vent fan...Should it blow cool air into the box or hot air out? Thanks
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Steve, KA1OWC
Retired Lieutenant Colonel, US Army
Army Nurse Corps
K0UA
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« Reply #1 on: September 27, 2017, 12:24:19 PM »

Building a go-box and am including a small vent fan...Should it blow cool air into the box or hot air out? Thanks

One of each, is even a better idea if you can do it.  Get some real powered air flow going.  24 volt muffin fans run on 12 volts are low current and don't make a lot of noise, yet can still move quite a bit of air if you get the right ones.
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AC5UP
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« Reply #2 on: September 27, 2017, 12:45:27 PM »

It's generally best to pull the hot air out.  If practical, position the fan in the hottest part of the enclosure so the heat travels the shortest path to buh-bye...
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WY7CHY
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« Reply #3 on: September 27, 2017, 01:54:33 PM »

Building a go-box and am including a small vent fan...Should it blow cool air into the box or hot air out? Thanks

If it's a go box, I'm assuming you won't have an amp. Why do you think you need a vent fan? Do you think the equipment is going to get that hot? Radios don't normally have fans. Only amps. But, if you're anticipating some very hot environments and want a fan anyway; then as mentioned, you want the fan to SUCK OUT the hot air. You also need to have INTAKE VENTS on the opposite side of the container. The fan will produce a negative low pressure by sucking out the hot air; but you need a way for fresh cooler air to replace it. But you don't need another fan blowing in. Just put the vent on the opposite side of where the fan blowing out is. It will create it's own draft. You don't need a 2nd fan; unless you make it so the incoming air doesn't pass the equipment.
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Born Wild - Raised Proud: 73
Cheyenne, Wyoming
KS2G
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« Reply #4 on: September 27, 2017, 02:45:48 PM »

Radios don't normally have fans. Only amps.

What?!!!

Every HF radio (5 Kenwoods) and every 2-meter and 2-meter/440MHz mobile radio (I've lost count how many but both Yaesu and Icom) I've owned since licensed in 1977 have had cooling fans.
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N8EKT
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Posts: 586




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« Reply #5 on: September 27, 2017, 07:40:51 PM »

You never want to fight natural convection so you put a intake vent on the bottom left side of the box and put a exhaust fan on the top right side for example


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W8JX
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« Reply #6 on: September 28, 2017, 04:00:28 AM »

One of each, is even a better idea if you can do it.  Get some real powered air flow going. 

I do agree with this. Good way to get positive ventilation. They can be small quiet fans too.
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Ham since 1969....  Old School 20wpm REAL Extra Class..
W5FYI
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Posts: 1053




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« Reply #7 on: September 28, 2017, 06:43:48 AM »

Here's another take on the conundrum: if the fan draws hot air out, then cool air will enter the box wherever it can, taking with it dust and smoke that can accumulate on the components within. If, on the other hand, the fan blows filtered air into the box, most of the dust and crud will be caught in the filter, which can be cleaned or replaced when it gets dirty. Just a thought....

Stew, W5FYI
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N2MG
Administrator

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« Reply #8 on: September 28, 2017, 07:29:55 AM »

Regardless of there being an amp, or equipment with their own fans, ventilating an enclosing box is a good idea. If the box had one, you could just "lift its lid" but assuming it's to remain closed up, any equipment inside will heat the box up and without natural convection, will likley get (perhaps a lot) hotter than "ambient".

So, if you go the fan route, put a fan on one side and some vents on the other.

A (removable/washable) filter on the air-input side is a good idea.

Mike N2MG
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W8JX
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« Reply #9 on: September 28, 2017, 07:33:42 AM »

Here's another take on the conundrum: if the fan draws hot air out, then cool air will enter the box wherever it can, taking with it dust and smoke that can accumulate on the components within. If, on the other hand, the fan blows filtered air into the box, most of the dust and crud will be caught in the filter, which can be cleaned or replaced when it gets dirty. Just a thought....

Stew, W5FYI

Well what looks good in theory (filtered air) does not always pan out in practice. If you had a chassis mounted device that ran 24/7 in there could be some merit to filtered air intakes (they do not have to be on fan to work) but for a carry box it is over building for needs. KISS.
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Ham since 1969....  Old School 20wpm REAL Extra Class..
K8AXW
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Posts: 6306




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« Reply #10 on: September 28, 2017, 10:31:24 AM »

JX......AMEN!  Several years ago I caught myself over-engineering stuff I was building.  What triggered my seeing the light was that I was wrestling with a problem with a sold-state circuit when the thought came to me; "what the hell's wrong with you.....a simple relay would take care of all this!"

I then found everything much more fun when I learned ......KISS!
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W9IQ
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Posts: 1706




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« Reply #11 on: September 28, 2017, 11:49:05 AM »

There are subtle differences with push vs pull fan venting designs. But given this application, I doubt that they will make any significant difference.

It may be more instructive to determine the size (CFM) of the cooling fan that is needed in the application.

Start with calculating the power consumption of the electronics in the go-box. This includes all loads in the go box divided by the power supply efficiency (use 80% if you don't know this number). Don't forget to allow for the transmit to receive duty cycle. If the transmit duty cycle is anticipated to be high (e.g. data modes), assume 100% transmit to be safe.

Next determine the maximum temperature in degrees C that you would like the inside of the box to reach. This should be at least 10°C less than the device with the lowest rated maximum operating temperature in the go box (check your manuals/spec sheets). Subtract the maximum ambient operating temperature of the go box and call this ΔT.

Plug these values into the formula:

CFM = 1.767 * Watts / ΔT

As an example, a box with 200 watts of power consumption operated in a 30°C ambient temperature with a maximum allowable internal temperature of 40°C:

CFM = 1.767 * 200 / (40-30°C) = 35 CFM

Note that this is the minimum CFM since once a fan is installed in an enclosure, the effective CFM is lower than the rated CFM. If this were a formal engineering exercise, the back pressure (H2O) of the box and the ducting design would determine the adjustment factor. In this less formal environment, a 1.1 to 1.25 multiplier should suffice. So a fan in the 40 to 45 CFM should prove sufficient for this example.

- Glenn W9IQ

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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
WA3SKN
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Posts: 6490




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« Reply #12 on: October 02, 2017, 09:24:23 AM »

Are you planning a "squirrel cage" or "axial" designed fan?  There are merits to each.  And if axial, consider 2 fans wired in series... it will lower RPMs and quiet things down.
You can have one push and one pull.

-Mike.
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N4MQ
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Posts: 138




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« Reply #13 on: October 02, 2017, 02:49:05 PM »

I prefer to blow in cold air as it is denser and the pressure at the fan inlet is at full atmospheric pressure.  Sucking out hot air has both issues, also I can direct the inlet stream air to my desired location(s).  Woody Enjoy
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W8JX
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Posts: 12080




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« Reply #14 on: October 02, 2017, 07:53:41 PM »

I prefer to blow in cold air as it is denser and the pressure at the fan inlet is at full atmospheric pressure.  Sucking out hot air has both issues, also I can direct the inlet stream air to my desired location(s).  Woody Enjoy

It is always more efficient to suck hot air out of case than to blow it around in case as it can heat up other areas in process. Muffin fans tend to loose efficiency quicker with back press vs same an\mount of suction pressure differential. Squirrel cages fans do better with back pressure.
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Ham since 1969....  Old School 20wpm REAL Extra Class..
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