Heat Transfer: Copper vs SS Chiller

[GutsyBrewing]

I've seen the question of copper vs. SS wort chiller debated at length. The main argument I hear is that copper conducts heat 25x better than SS so it must cool faster. At the same time I have read many a post of people saying the SS chills just fine. So I began to wonder...what's the deal?

While copper is a better conductor, there is more at play in the heat transfer. There are actually three modes of heat transfer happening.

1. Convection from cooling water to inside wall of metal tube
2. Conduction through the tube
3. Convection from outside wall of tube to the wort

It turns out that the limiting mode is #3, convection from the tube to wort. This means that all that "extra" heat transfer that copper provides is not doing anything. Imagine you have a six lane highway that at one point only has one lane open. It doesn't matter how fast you COULD go with all lanes open, it only matters how fast the single lane is going.

I modeled the system and found that if you have no re-circulation in the wort (i.e. natural convection), the resistance to heat transfer in mode #2 is much lower than mode #3 (500 times for copper, 20 times for SS). In other words, that six-lane highway is actually more like a 500 lane highway that gets reduced to a single lane. With re-circulation, the factors drop to more like 200 for copper and 8 for SS. So having some kind of motion in the wort (whirlpool or re-circulation) is a huge benefit in the overall heat transfer.

The take away here is:

"Copper and SS chillers have nearly identical heat transfer capabilities because the heat transfer is NOT limited by conduction through the tube"

PS: I can show more details in a follow-up post for all those nerds out there

 

[k1ngl1ves]

I have read this 8 times, and I don't see any evidence that the two metals have "nearly identical heat transfer capabilities." Just based on the physical properties of each metal, your results are incorrect. If testing on both metals is carried out exactly the same way, copper will outperform ss every time in regards to heat transfer(until you get to temps where copper will change it's state.. but that's not what we are here for now). It's really not a big deal what metal we choose to use(lol that rhymes!) in homebrewing. A couple minutes either way is no big deal. Though one IS better than the other at heat exchange.


As for your analogy, you're more or less referring to surface area and how quickly the heat is being stolen by the metal. Copper's single lane is still moving faster than ss's. Now if ss's tubing was larger than copper's, then ss at some point would transfer heat faster, because it's surface area in contact would be greater. We're talking a great difference in tubing diameter though, and that still has nothing to do with the actual physical properties of the metals. The only thing that is clear in your post is different techniques produce different results. Thanks Mr. Obvious!

So, I guess my question is...

LOLWhat?!!!




TLDR; Doesn't matter for small scale brewing.




Please... enlighten us

 

[jbaysurfer]

Two things (one which was covered already) (edited for clarity)
A)The lane passing more cars per minute will still be the faster moving freeway.
B) The mitigating factor referenced is not mitigating in the vast majority of homebrewer setups. Very very few homebrewers use an IC without agitating the wort in some way (recirculating, stirring, moving the chiller up and down or around). Thus you can't say #2 doesn't matter because #3 evens the playing field. #3 doesn't even the playing field unless the brewer does nothing to agitate the wort, in which case they're wasting a lot of water, and ignoring the entire goal of cooling quickly.

 

[rlmiller10]

what gutsy is saying is that the wort to metal interface is the limiting factor. He is also say that for both metals the thermal transfer across the tubing wall is sufficiently faster than the wort to metal interface that the type of metal does not matter.

Think of it this way, for heat to move there must be a temperature difference, a delta. In this system there are three boundaries that BTU must cross. One is the boundary from the wort to the tubing surface. Two is the boundary from the outer tubing surface to the inner tubing surface. Three is the boundary across the inner tubing surface to the cooling water. Factors affecting the rate of heat transfer at the boundary is temperature delta, the resistance of the substance to conducting heat, the thermal conductivity, and a factor in a fluid system is laminar flow and laminar layers.

First take the third out of the equation as we can assume copper or SS makes no difference as the flow is the same and the thermal conductivity of water to copper or SS are close enough to ignore. (note: in reality things like surface finish can make a difference as it can assist or disrupt laminar flow which is a bad thing for heat transfer, so the assumption may not be 100% correct)

So looking at the wort to tubing interface there will be X btu per hour per area. X will be larger if there is circulation so that laminar layers do not hinder conduction. And X gets smaller as the wort temperature approaches the cooling water temperature.

On the outer surface to the inner surface of the tubing boundary there will be Y btu per hour per area. This is driven by the conductivity of the metal and the delta of the temp between the surfaces.

What gutsy is saying is that Y is sufficiently greater than X for both SS and copper that the outer wall temperature of the tubing at a given time in the cooling process is the same for both copper or SS. As long as that outer wall temperature is the same the wort will transfer the same amount of heat regardless of the metal.

I am not saying he is correct as I don't have the numbers and don't care enough to look them up or conduct experiments. But it is a feasible hypothesis.

 

[k1ngl1ves]

As long as that outer wall temperature is the same the wort will transfer the same amount of heat regardless of the metal.

Correct. The difference is that the outside wall of the copper tubing will be at the wort temp for less time than the ss tubing will because of copper's ability to transfer the heat through itself to the water at a faster rate.

On the small scale we are discussing, it's rather pointless to argue. Overall, copper is more efficient. Does it really matter based on how we are using it? No. An extra minute or two will not ruin a brew day.

Stainless does have it's advantages. The greatest one is that it's much more durable. My stainless IC looks brand new, even after a year of service. After 3 years, my copper IC looks like it's been through a war, and recovered from it's wounds at Dr. Frankenstein's hospital. Both still work fine. Both still transfer heat, and cool down my wort at a reasonable rate.


Let's all find something else unimportant to argue about now...

 

[jbaysurfer]

Correct. The difference is that the outside wall of the copper tubing will be at the wort temp for less time than the ss tubing will because of copper's ability to transfer the heat through itself to the water at a faster rate.

On the small scale we are discussing, it's rather pointless to argue.

Let's all find something else unimportant to argue about now...

Your first paragraph I'd add "increasing delta". To the rest I'd say, welcome to the internet. Very very few arguments are really that important, and this isn't an argument, it's an interesting discussion that people can learn from.

 

[GutsyBrewing]

Copper's single lane is still moving faster than ss's.

like rlmiller10 said, it is the geometry of the wort chiller and the movement of the wort that really affects the heat transfer, not so much the material. I will admit that copper will be slightly faster than SS due to having a little colder outer wall temp, i calculated about 30-60s faster. As far as my brew day is concerned, that is the same amount of time.

This is why it takes me so long to make decisions on my equipment buys...

 

[SilverZero]

So why does my 50' aluminum IC seem to take so long to cool 10 gallons? The water coming out the end is only hot for a couple of minutes, and then it feels cool while the wort is still fairly hot.

 

[Chainsaw_Mechanic]

Aluminum is slow to dissipate heat. It has a much higher capacity to store heat, but slows it's transfer considerably compared to SS, and hilariously slow compared to copper.

 

[JaDeDBrewing]

So why does my 50' aluminum IC seem to take so long to cool 10 gallons? The water coming out the end is only hot for a couple of minutes, and then it feels cool while the wort is still fairly hot.

Make sure you are utilizing the full potential of your wort chiller by following the recommendations on our "Immersion Wort Chiller Optimization" page. Also, you might have a stainless IC as there are no commercial manufactures of aluminum ICs that we are currently aware of.

Another thing that comes into consideration when choosing a metal for your IC that most homebrewers are not aware of, is that copper is antimicrobial. The EPA has certified multiple copper alloys as killing 99.9% of all bacteria within 2 hours of contact. Stainless does not have this benefit, in fact, stainless could be used for a bacteria culture tray to grow bacteria.