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radiator design question

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LAN_403

IamSmooth

Mon Jan 21 2013, 04:06PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

I wanted to know what the most efficient method for using material for a radiator. I have a tube with a stream of gas that I am cooling. I've improved on the heat radiation by adding fins. If I have a piece of 1" x 2" aluminum for a fin, which is the best method for removing heat: using one large fin, or cutting it in half and using two 1"x1" fins? Is the heat transfer about the same?

The formula I've found is dQ/dt = k A (Te-Tc), where A is the area for heat transfer. Just using this equation one would think both methods are the same. However, there is a finite interface area where the fin makes contact with the tube.

Any thoughts?

Dr. Slack

Mon Jan 21 2013, 05:44PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Insufficient information. Would you post a diagram of the configuration you intend please.

IamSmooth

Mon Jan 21 2013, 06:48PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

I'm at work now without the means to post a picture.

Imagine a 1x1 square sheet of aluminum (0.016" thick) with a hole punched in the center. The fluid runs through a tube through this hole, radiating the heat. There are two of these square pieces attached to the tube. Now imagine another tube carrying the fluid, but the fin is 1x2.

Which is more efficient, or are they the same.

BigBad

Mon Jan 21 2013, 07:45PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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Fins shouldn't be longer than a few times their thickness, the extra length doesn't do much; so having a fin longer than ~0.064" long is a waste if the contact area is 0.016" wide.

Ash Small

Mon Jan 21 2013, 09:12PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Two one inch fins is more efficient that one two inch fin, because two fins will have twice the contact area with the tube.

The parts of the two inch fin that are more than half an inch from the tube will radiate less heat than the two one inch fins will.

There comes a point where adding more fins becomes less efficient, but this point is only when airflow is restricted.

When I was looking into this, and designing heatsinks ten years ago, I learned that the most important factor is turbulence in the airflow over the fins, so that cool air continually comes into contact with the surface of the fins.

I therefore design my heatsinks to create as much turbulence as possible.

I also found that most of the formulas quoted on manufacturer's website are fairly meaningless. You just have to use common sense.

One point worth mentioning is that, if you don't have a forced air supply, you want to mount the fins vertically, so that the air rises as it warms, ensuring cooler air is constantly fed to the fins.

teravolt

Tue Jan 22 2013, 03:50AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

if you dont mind what type of gas are you condensing

Dr. Slack

Tue Jan 22 2013, 08:31AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

I'm puzzled by your use of terminology.

The fluid runs through a tube through this hole, radiating the heat.

Do you really mean radiating, so improved heat transfer if you paint the fins black? Or do you mean convection, so improved heat transfer if you blow cold air across them?

Anyhows, it's a complicated topic. I spent a few weeks researching at work for the best geometry heatsink to mill into the side of a hot instrument we were making, and some of the results were completely counter-intuitive. All sorts of things are important, turbulence is good for instance but there are more important things. Did you know that in certain commonly-met flow regimes, the thermal resistance of a finned heat sink is independent of the velocity of the cooling fluid?

Study this paper, to find out why and when, and a lot more besides. Try not to go cross-eyed at all the equations.

If your cooling tube only has to operate down to 0C, then you could do worse than to simply water cool it, without messing with fins (but having to mess with water).

BTW, I suspect that, in practice, there will be f*all difference between your two different configurations of two sq.ins of alli fin. Until they're closely spaced, more total area of fin is the best way to go.

BigBad

Tue Jan 22 2013, 04:15PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600

You could try reading this free 766 page book:

I didn't download it, but I think that's the one I usually refer to; it's bloody good. There's more in there than you could ever want to know.

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