Need a physics expert (thermal)

**loopyloo** · 13-04-12, 02:01 PM

Can you help here

A friend and I have built a small coal burning forge to melt aluminium.
The melting pot is made of thick steel.
In an attempt to transfer as much heat as possible to the contents of the pot, my friend has suggested welding fins on the outside of the pot to make the pot hotter.

My understanding is that the fins are for dissipating heat due to the larger surface area and I thought heat transfer would be more efficient to the contents of the pot with the fins in the inside.

I realise thermal dynamics is very complicated but if I can get a reasonably simple answer would be good.

What's the story ?

**lsemmens** · 13-04-12, 02:07 PM

Logic would dictate that your rationale was correct. The issue with welding fins on would also create hot spots where the mass off the pot were greater because of the fin. Think of it like the heat dissipation thing. Get a block of ice and place your finger in the middle. The whole block does not melt evenly because of the heat source (finger) but it does melt faster around that heat source.

I'd say no fins at all.

**nbound** · 13-04-12, 02:19 PM

Thats only because ice is a poor thermal conductor. It wont be so bad with steel, and considering you are heating aluminium which is a good thermal conductor (is used in heatsinks for this reason), the hotspots wont be as big a deal.

Whether you are better or worse with the fins i dont know.

Note: Im not a physicist :P

**Gerbertron** · 13-04-12, 02:27 PM

Im no expert but i'd guess if the fins were right in the heat source then it would pick up more heat, and heat up the pot faster. Interesting, is it possible for you to try:
fins on the out side,
fins on the inside.
and fins on both sides?

**mtv** · 13-04-12, 03:38 PM

I don't think adding fins inside or out will actually make the pot hotter.

The temperature will be determined by the furnace, not the pot itself.

Adding fins on any part of the pot will increase the surface area, which may actually take longer to heat the pot to maximum temperature.

I'd leave it without fins, but it would be an interesting experiment to make comparisons.

**Mods** · 13-04-12, 05:33 PM

I worked in a foundry when I was about 17 (a long time ago), and there were 3 furnaces, but they all had ceramic crucibles. This was an aluminium foundry that made basic corners/frieze/balustrades. They used to put silicone (in rock form - silica ?) into the mix (I think about 7% ?) so the aluminium wasn't so brittle.

Cheers.

**loopyloo** · 13-04-12, 05:40 PM

I don't actually intent to put fins on the inside of the pot.
That would only impede working with the aluminium, and of course, would be a hindrance to transferring heat until the aluminium is in liquid form,
But I would like to be able to explain to my mate reasons why fins are no good on the outside.

Imagine in the following drawing that 1, 2 and 3 are the base of a container with water inside. Which one would heat the water quicker ?

Fins.jpg

**trash** · 13-04-12, 06:34 PM

No, fins will not make the pot hotter, they will make it cooler.
While it is logical to assume a larger surface area will collect heat faster, the reverse is true and it will cool down faster.
You're much better to go with a standard crucible and spend more time and energy getting it up to temperature and because it is not loosing heat faster, less effort and fuel is required to keep it at the elevated temperature.

In this case, KISS. Keep it simple stupid - holds true

What I can suggest is that you test the logic on a small simple scale.
Take two pieces of aluminium roughly the same size and mass.
Leave one as a solid block and the other with fins.
Put them both into a pot of boiling water to heat them both up to exactly 100 degrees C.
Leave them for a few minutes so they have a chance to fully equalise and then take both out with some tongs and place them on some styrofoam. Monitor their temperature with a laser thermometer.

Chart their temperatures as they both cool down.

When they are both cool you can heat up a piece of metal plate on a BBQ or stove etc.
Turn it off and give it a minute to equalise. Then place both of your metal analogs on the metal place together and measure their temperatures with the laser thermometer.
Again, chart how much they warm up.

If dT1/dt is not equal to dT2/dt then you have something of interest.

If however they heat and and cool down the same, then it is a waste of time.

You could also test the surface area by having two flat pieces of metal, both the same mass, but one with a bigger surface area.
I can also think of a few other variations for testing with conductive chokes etc.

**loopyloo** · 18-04-12, 09:51 AM

Am now in the process of building another pot which is larger in diameter and less in height.

The base area has increase from 14sq" to 20.5 sq". I can't make the base any larger than this or I won't have enough room to feed more coal in.

Larger base should absorb more heat from the source and the lower sides less chance to dissipate heat since the sides will also act as a cooling fin from some point up the sides. Although the sides have increased in surface area due to the larger diameter, at least the top of the sides are now going to be closer to the heat source.

That's my theory anyway.

Loopy

**gordon_s1942** · 18-04-12, 04:49 PM

If you ever watch any doco's on melting any metals, they use a thick pot both for strength and to keep the heat INside the pot.
All those I have seen the height is usualy twice the width at least not the wider.
Part of that would be to get a decent grip on the crucible so it wont tip as your lifting it.
Most as Mods says were ceramic lined as they want to minimise heat escape as well as making the pot reusable.
Most apply heat either from under or in the case of Aluminium, from paddles inserted into the pot itsefl that use very high electrical currents to melt the material.
Be warned not to allow even a drop of water/moisture near your pot as it can explode with diasterous results and injuries from molten metal can be fatal.
I think you will need an old Blacksmiths forge with some sort of means to force air thru the coals and that should be COKE or even charcoal, not coal as I dont think it can be made hot enough on its own withou air being forced through it.
What you need is a load of good Newcastle coal, best I have ever seen for heat and low ash content but it burns too hot for many users in the Power Stations but when converted to Coke, ideal for Blast Furnaces.

**trash** · 18-04-12, 10:17 PM

I've burnt hunter coal in my heater. It burns hot, but if you want really hot, I would try shredded cardboard. When you can't get the heat up any more, then use the coking coal to take it further.