I have the economy chiller from midwest, works ok in a 5 gallon batch about 45 minutes or so from boiling... i just picked up a keggle and plan to do a 10 gallon batch in a few days and will attempt putting the chiller in ice water and gravity feeding the wort through the chiller into a carboy.. has anyone tried this?
I plan on doing pretty much the same thing but with a few important changes.
I'm building a eherms rig and plan to reuse the herms coil as a chiller. Heat the water bath, circulate wort through it to heat the mash. Cool the bath and circulate wort through it to chill before pitching. It's simple and elegant and I see no reason why it would not work.
That said, I would use a pump and continually recirculate the wort through the coil until pitching temp is reached. This means you need a pump. Since you need a pump you probably also should use bigger copper tube so as to not restrict the flow to much - I'm going with 1/2" by 20. The length, for me, is really irrelevant since I will be recirculating through the coil.
I would also begin with tap water in the water bath which is quite warm (about 80 F) in FL in the summer and recirculate until the water bath and wort reach equilibrium or close to it. Then drain and add ice and recirculate some more to get it down to pitching temp. This would minimize water waste and also the quantity of ice you'd have to buy/make. What matters is the temp difference between the wort and the water bath. The difference between 80F and 212F is sufficient enough to do the bulk of the cooling (of course this is only true if you are recirculating).
Your setup, gravity feeding through the coil, requires a longer coil to insure adequate chilling in a single pass. But longer means more difficulty cleaning and slower (or no) flow. How long is right? I honestly don't know. It also requires vigorous stirring of the water bath to maximize single pass cooling. I second the recommendation of drill + paint stirrer (paddle). I would also just use ice. You gain nothing with the rock salt except having salt water everywhere which is corrosive and can't be dumped on the lawn or used in the laundry and would be particularly bad if it somehow made its way into the wort. The salt simply lowers the freezing temp of the water it doesn't give the ice any additional "cooling capacity."
The initial runnings would be the coolest and probably below pitching temp but subsequent runnings would be warmer as the ice melts, water bath temp rises, and temperature difference shrinks. Your hope would be that the temp of the runnings would average out to somewhere around pitching temp.
I think in principle it is a great idea and that's why I'm designing my rig this way. But as you have the setup designed based on spare parts you have on hand I would be hesitant. It may work. It may not. Worse case scenario you pitch this batch warm or let it sit and cool overnight before pitching. Then modify the system (probably with a pump) before trying it again.
As to the copper cleaning issue it is a non issue. Copper has been used in breweries for ages. Patina on the copper is fine and it will be removed by wort and consumed by the yeast. You don't want verdigrass which is different from patina (blue green) but I have never ever seen that on my coils or copper manifold. You can clean with a vinegar soak and I do from time to time, particularly if the manifold hasn't been used in a while. Vinegar is super cheap anyway. I would sanitize by running boiling water through the coil before chilling (or by pumping boiling wort through it).
Look at CFC and plate chillers. This would be easily as sanitary as a CFC chiller and far cleaner than the plate chillers everyone is so enamored with. I shudder to think about what could be trapped in all those little passageways and yet everyone uses them without any apparent issues.