Chilling/cooling of anodising bath Q&A's

Aluminum How-To
"Chromating - Anodizing - Hardcoating"
by Robert Probert

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A. Yes, this has been done with lead coils. However, I think you are making progress in the wrong direction. Here in the U.S. many shops are doing exactly the opposite: converting from direct cooling to indirect! Most seem to think it's inappropriate to be pumping refrigerants around a shop and into vulnerable coils in tanks like that.

A. Although direct cooling is more efficient, the chance of acid being introduced into the chiller is a very real and serious problem. In addition, with indirect cooling more precise control is possible with modulating valves. Also, indirect cooling provides a cooling buffer to minimize chiller on and off.

A. A common way of handling the problem, but it is not cheap, is to use an external heat exchanger. The "brine" flows through one circuit and the H2SO4 through the other. Since there is high velocity and large surface area, it is possible to maintain a small temperature difference (less that 10 degrees F) between the "brine" and the H2SO4. (I put "brine" in quotation marks because we actually use ethylene glycol in the water, not salt brine).

A. We have a solution to your need in form of chilling your H2SO4 solution directly through M-22 refrigerant gas, in an all stainless steel heat exchanger outside your finishing tank.This way you can reduce your temperature losses and maintain your acid very close to the refrigerant gas temperature. Also you will avoid refrigerant gas leaking into your finishing tank.

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A. Hi John. I have been involved in correcting a number of undersized cooling plants over the years, and I have never seen any indication that less than 100 percent of the anodizing power is converted to heat.

If you are running at 1000 Amps and 80 volts, you will be generating 80 KW of heat. If you are anodizing at this current and voltage 60 minutes out of every hour, you are generating 80 KW-hours of heat or 273,000 BTUs per hour. 12,000 BTUs per hour is a ton of refrigeration, so you're looking at 22.75 tons. Please check my conversion factors because this was a quick on-the-fly thumbnail sketch.

But usually you don't run a rectifier flat-out, and you need to ramp up your voltage, so it can't always be 80 V either. After looking into those duty factors, you'll probably come up with 15 to 20 tons of chilling required.

A. Hello, Arun. Ethylene glycol is automobile antifreeze, and the thermal properties of brine/water and glycol/water are probably reasonably similar. But commercial antifreeze probably includes descaling and corrosion-fighting additives that will somewhat reduce the tendency of scales to form which interfere with the "U factor". If you want to take that tip and run with it, please do. Or if you want to retain a consultant to do an engineering and economic study of the two heat transfer systems for you, please see our Directory of Consultants. But this is a public forum for camaraderie and sharing of quick tips, so we need to warn you that there is little point waiting for someone to undertake a detailed engineering study for you for free :-)

Good luck with it.

A. I have a suggestion and I think that you wanted to eliminate these problems
1. Improve the efficiency
2. Reduce the heat gained during the circulation of indirect cooling.

You can make a safe approach:
cascade the cooling coils with freon inside a SS case filled with brine(...) immerse this in the tank (cool pack method) that will prevent your coils with freon from being damaged and also avoid to a greater extent the mixing of H2SO4 in the freon or vise versa.

But this will increase your maintenance work in terms of regular check by NDT for any possible hole or thinning of the outer SS shell which contains the brine.

A. Hi Peter.

Regardless of whether or not you are reading stray currents, make sure that you have dielectric unions, or lengths of radiator hose, or some positive electrical insulation in between the anodizing tank and the heat exchanger.

Regards,

A. We are manufacturing chilling plants for Hard anodizing, the sulfuric acid solution is coming directly into the Evaporator. We are using 316L tubes, Tube plates & Shell.
We have had no leakage problems in the evaporators.

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A. Hi Janaka. Unfortunately, that is impossible to answer definitively. Although many systems (including most anodizing chillers) recirculate the chilled water, some chilled water systems might be open loop: for example if you go to a water fountain in your shop, it may chill the potable tap water before serving it to you.

Regards,

Thermowell

A. Hello again Janaka. In certain situations it can be impractical to insert a thermometer or other temperature sensor directly into the liquid. In that case the piping is specially built with a sort of a nest into the pipe so the temperature sensor can be in the middle of the flow but protected from it by metal; this also allows quick change-outs without draining anything.

If at all possible you should try to get a look at this chiller system because it is problematical to design a control system if the things you are trying to control are too ethereal :-)

Luck & Regards,