Copper Chiller - Convection Enhancement

[nickster51]

Dear fellow brewers,
First off, I'm a mechanical engineering student. I've been working on calculations for a homemade counter-flow chiller, or basically an improvement over my 25" immersion chiller I'm sick of waiting on (30-45min cooling for 5-6 gallons). I just finished reading the rib-style immersion chiller thread, so this is partly in response to that.

Which portion of the cooling process limits our chillers? Temperature differential for sure, but isn't a portion of it convection limitations? Assuming copper conducts faster than our chillers are able to convect ( wort -> copper -> water), wouldn't it make the most sense to A. maximize differential (already done with the ribbed design) and B. try to increase convection coefficients.

With B in mind and Having recently reviewed the forced convection chapter in Heat and Mass Transfer: A practical approach (Yunus A. Cengel, p. 477) It seems that the best way to do this is to increase the roughness of the copper tubing. "Heat transfer in turbulent flow in a tube has been increased by as much as 400 percent by roughening the surface". Now my question is, how do we roughen both the internal and external surface of the copper pipe? Would doing so increase the heat transfer limit imposed by the convection characteristic (with the same temp differentials)? Maybe I'm wrong in assuming what limits the transfer.

Let me know if I'm not making any sense. I know there might be strength/sanitary issues, but I think these could be worked around.

 

[HarkinBanks]

Use a convoluted coil of copper, or make yourself a pre-chiller with ice, or both. That will cut your chilling time down.

 

[alan_marks]

Here's a thought as an answer to the question about rough v smooth surfaces; if you have a textured surface in the same configuration as the original smooth surface, havn't you increased the actual surface area in contact with the coolant? Think of a tube with a corragated surface as opposed to a smooth surface. By roughening up the surfaces, you increase the actual area in contact with the coolant, like the ribs on an electical heat sink.

Alan

 

[Hatesfury]

Just taking high grit sand paper to the outside of your copper will increase your total surface area, and increase your heat transfer coefficents. Rouging the inside would as well, but it would also provide hospitable places for nasties to grow, and since anything beyond a sanatizer rinse is hard to accomplish with coiled copper tubing, it would be a very bad idea.

Also, remember that different states of matter transfer heat better. Gas > liquid > solids. If you simply add a screen (just like your kitchen sink) to your inlet, you'll forcibly aerate your wort as it's going into the chiller, also increasing transfer rates.

So for $4 at home depot you could inrease your transfer rates by a fair amount.

 

[nickster51]

I guess I made this unclear, I'm really hoping to save some cash (I forgot to add poor - college student), so I'm hoping to increase the performance of my immersion chiller (through increasing the transfer coefficients) or design an affordable chiller. This could be a new immersion chiller with the ribbed construction or my own counterflow chiller.

alan_marks, I think that is part of the aspect (increase surface area), but I think the roughening of the surface also induces more turbulent flow for a greater portion of the flow regime. Turbulent fluids transfer better.

Hatesfury, is the issue of hot said aeration laid to rest these days? Also, I agree roughening the internal side of a counterflow chiller's inner pipe would create infection risks, but roughening the inside of an immersion chiller shouldn't have this issue.

Also, you mention different phases transferring better. Why does introducing air in the gas phase to wort in the liquid phase increase the heat transfer? Does air-to-copper have a better convection constant than wort-to-copper?

I think I might just end up creating a roughing apparatus and possibly ruining my immersion chiller. I'll compare before and after I do this. (as well as input temps). See if there are any realizable differences.

Let me know any further thoughts, or if you see errors in my thought process. Thanks!

 

[Indian_villager]

Assuming you have good flow in the copper tubing you internal film coefficients should be great. It is the outer film coefficient that is killing you. If you force convection by some form of stirring or mixing you could significantly boost your external film coefficient, cutting your time significantly.