Al vs. SS. Which Heats Faster?

Systems, Gear, & DIY
1

AL vs. SS myths, time for an experiment.

AL can be used for a boil kettle.  No it is ok if seasoned.  Busted.
AL causes Alzheimers.  This has been Busted by scientific research.
AL will heat the wort faster due to the higher conductivity of the AL over SS.  Well how much faster?

The overall heat transfer coefficient is what is important for a system.  The system I use is a propane burner, the pot then the wort, so there are 3 heat conductors in series.  People on the internet always say that AL will be faster due to the much higher conductivity.  If you look on the web you will find the following for AL, and SS.

AL =  250 (W/m-degree C)
SS = 16  (W/m-degree C)

So the AL has greater than an order of magnitude of  advantage in heat transfer per unit thickness

So to see how much time I was wasting on a brew day, I was able to do a quick test, since we recently were given a  AL pot that as very close to a ss one we had.  These are both about 10 quarts, and are very close in diameter and height, you have to get them side by side to see that the AL one is about ¼ inch taller.  I thought there would be a small advantage to the AL, for what it is worth, but not enough to worry about.

Both pots were filled with 2 gallons of water.  The Aluminum pot was put on first, and the temp was measured with a digital thermometer.  Started  the time at 110F, since my brother called and interrupted,  and went to 200F, just because I was heating some of the sparge water.  The AL was taken off and the SS was put on, and the propane valve was not adjusted (30 pounds in a 40 pound tank, so it was running steady).  Both were uncovered.  So what was the time?

AL = 15 min. 55 seconds
SS = 15 min. 20 seconds

Whoa!  What the heck?

One clue is that the digital thermometer was jumping 3 to 4 degrees +/- with the AL pot vs. about 1 degree with the SS pot.  The AL pot was even stirred several time while trying to take a steady reading, which should have helped its convection heat transfer.  My guess it that the AL pot had higher convection currents out the sides  These pots are as high as they are in diameter, so when you do the math, there is 4 times the surface are on the sides than on the bottom.  The AL can transfer heat to the wort better in the bottom, but it can lose it faster through the sides.

This is far from a conclusive test, but when someone on a random website says my AL kettle will be faster, I will not lose any sleep.  I have some bigger SS and AL pots to try.

AL is faster - Busted?  More heating experiments are in order.  If you have any data to share, please do.  Or just flame me if you want.  ;D

2

Might be that. Conduction is a two way street.

3

What are the thicknesses of the bottom of the two different kettles?  Do you know?

I like your hypothesis that the Al kettle will lose heat faster through the sides than the SS kettle, I hadn’t thought of that before.

4

…and, does the SS kettle have a 3 ply bottom? Aluminum layer sandwiched between 2 SS layers?

5

Both pots were single layer.  The SS was not so thick.  The AL was not all that thick either.

There are some clad SS pots in the kitchen, maybe I should take one to the garage for the next round.

6

I think that would be interesting to see if a tri-clad (SS-Al-SS) bottomed SS kettle is just as fast, or even faster (due to less heat loss out the sides).

7

I work in industrial heat processing.  I asked our Phd R&D manager who works in a lab testing out and simulating heat transfer scenarios and he said his gut told him SS pot of similar thickness would heat up a set amount of water to a boil quicker due to greater wall losses in the aluminum pot.

8

Thanks for asking.  In my engineering specialty (N&V), you often have to account for all of the energy flow, so that is how I came to my guess.

9

That’s true, but the constants you listed aren’t the heat transfer coefficients (h), they’re the thermal conductivities (k). The thermal conductivity of any metal is massive relative to the fluids on either side (air and water in this case), so either kettle would be very nearly the same temperature on the top and the bottom. Moreover, the layer of metal is very thin (note that the units for k are meters). For an 18-gauge (1.27 mm) layer of metal with air on one side and water on the other, the heat transfer coefficients would be:

hAl = 43.3908 W/m2-K
hSS = 43.3909 W/m2-K

with the extra digits added for emphasis. I’d expect no detectable difference in their performance.

10

My heat transfer course was a long long time ago.  Nixon was President.

Yes, I listed the conductivity, just the lazy look up.

So if 1/Heq=1/H(air) + 1/H(metal)+1/H(liquid), I do remember that the liquid and especially the air(well the fire) side will control the heat transfer.

The expected result was that the SS would be about the same as the AL, maybe slighlty longer.  Faster was not expected for the SS.  But once again, the energy flow has to be accounted for in the system.

11

Then there’s the question of which is better after you reach boiling.  If stainless doesn’t conduct heat out of the system as easily as aluminum, it could maintain the boil w/less energy input after you reach equilibrium?

12

Yes, that is probably true.

I will have to look closer at the Peter Ausin system at one of the local brewpubs.  If the boil kettle is all copper, it would help explain why the sides have a brick vener.

13

If direct fired, perhaps you’d see a fire-proof liner (e.g., made of asbestos, or modern equivalent) between the vessel and the brick veneer.
Another possibility is that the brick veneer conceals the steam jacketed vessels.

14

Very interesting problem.  I had not thought about the heat losses through the sides of the kettle, but they are obvious.  Insulating the kettle sides can be difficult if you’re dealing with a direct-fired kettle as most homebrewers do.

Off hand, I’d say that fabricating a metal skirt with a circumference about the size of the outer diameter of the pot lip might be a good energy saving enhancement.  The skirt length would be about the same as the pot height.  I’d say that if you formed the top of the skirt to sit on the lip of the pot without gaps, you would create an somewhat insulated air-space between the pot and the skirt down the pot sides.  That would provide some insulation to heat loss through the sides.

A better option would probably be to insulate the pot sides with another material, but I’m having difficulty thinking of a fire and water resistant material that could be wrapped around a pot.  It would need to be relatively durable.  Maybe that stiff fiberglass sheeting with the metal foil that they use to create HVAC ducts?  Maybe it could just be banded onto the pot somehow?

The same argument for heat loss through the sides can be applied to the lid of your pot.  I partially cover during the boil to reduce heat losses.  That could be further insulated too.

Any other ideas?

15

For a given geometry, the conductivities would be essentially identical. Even if the walls were one inch thick, the difference is 0.003%.

16

Martin - converted kegs have a skirt, that is what I use for my main pot.

a10t2 - You are probably right, again.  Time to try to see if my results are repeatable when I brew in a day or 2.

17

I’m with Sean on this one. Getting the heat transferred into and out of the metal is more critical for fast heating.
An interesting idea would be the adoption of the JetBoil design for a brew kettle: http://www.jetboil.com/products/technology where a series of meal fins provide a larger surface for the fire to metal heat exchange. This makes for more efficient use of the heat from the burning gas.

Another aspect of fast heating is even convection in the kettle. This is not necessarily given when we bring wort to a boil. Because all bottom of the kettle is heated evenly the hot wort wants to rise upwards from all these places. Unfortunately the cold wort that is falling down from the top gets in the way and there are many turbulences and the all so familiar sudden rise of large bubbles.

An improved kettle design would heat the kettle only in the middle or offset to one side. By doing so you promote a more even convection of the wort and better heat transfer between kettle wall and wort.  But the more you move the burner off-center, the less heated bottom area you are providing.

Yet another thing that gets in the way of good kettle wall to wort heat transfer is the formation of steam pockets/bubbles on the kettle wall. Those end up creating small spots of insulation between the wort and the kettle wall. That’s what you hear crackling when you are heating the wort. As soon as you stir the wort this crackling goes away since the hot wort close to the heated metal is replaced with colder wort. This is yet another reason why good convection helps with heating wort faster.

All this being said, I brew with a 5 gal SS pot that has none of the enhancements I talked about. Maybe someday when I have a new burner stand I can try off-center heating  to see if it improves convection.

Stirring the wort while heating is an elegant option as well. Especially you have a motorized stirrer which you could also use during the chilling process with an immersion chiller. Or you may want to keep the pump running while you are heating the wort. But make sure that the heat loss in the hoses is less than the gain you should be getting from better circulation in the kettle.

Kai

18

I seem to recall hearing a seminar at NHC (maybe Randy or Ray) talking about putting a metal plate under one side of your kettle for just this reason.

19

Denny - the hear tamer that Randy and Ray talked about was to eliminate hot spots.  Could of had other reasons. I only went to the second one they did in Oakland, and they said they had talked about it in the first.

Kai - I do run my pump when bringing the wort to a boil.  Don’t know if it really helps or not.

20

I think what Marting is talking about is more like a 2nd wall for the kettle which provides a small gap. This gap provides some insulation. The skirts on keggles are different and I wonder to what extent they actually make heating less efficient. This is because they trap hot exhaust and prevent this hot gas from flowing across the surface evenly.

So far I got the most energy savings from having a wind shield and keeping the lid on the pot while heating the wort.

Kai