Antifreeze in carbon steel? 1

[StoneCold]

I have a small heating loop that originally was going to use Dowtherm J but do to a decrease in the operating temperature (150C down to 100C) we could use 50% antifreeze and water mixture. I want to use the antifreeze water because it is much easier to keep contained than the Dowtherm J. The piping is carbon steel. The part I am a little worried about is the vapor space above the liquid in the expansion tank. I am concerned that vapor condensing in the top of the tank could lead to corrosion of the tank and then flaking into the liquid. Maybe I should just use straight antifreeze? I intend to use automobile Antifreeze instead of just propylene glycol to take advantage of the corrosion inhibitors in the Antifreeze.

Any experience on this type of thing?

Thanks
StoneCold

 

[MikeHalloran]

Use a separate expanson tank made of brass. Or a remote tank made of plastic.

Use 50/50; that's what the product is blended for.







Mike Halloran
Pembroke Pines, FL, USA

 

[pennpoint]

StoneCold
You do not state the operating pressures of your system. Why not use copper tubing (brazed or silver soldered).
To you're point of carbon steel reservoir I have used both automotive grade ethylene and polyproplene glycols in HX systems. Polypropelene is enviromentally safer than ethylene glycol. Both have inhibitors. Look for Sierra Brand automotive antifreeze as this is polypropelene.
But! for the few bucks extra build the reservoir out of 304 SST.

 

[Montemayor]

Stone:

I've used both ethylene and propylene glycol with 100% carbon steel vessels and piping and never had any corrosion or metal "flaking". After 45 years in the business, should I thank the good Lord for taking care of me? I never have had any problem with the glycol aqueous solutions in carbon steel. Have I missed out on a good corrosion experience?

Today, I would still go with carbon steel. Stainless is great and I'll take it to the bank without asking any questions. However, I think that is a knee-jerk reaction. If you maintain a pure and stable glycol solution you should have no problems with carbon steel, in my experience. However, if you can justify the stainless steel, then go for it.

 

[sshep]

If your expansion tank is N2 padded, you will not have any corrosion problems in carbon steel even if there is water condensation in the vapor space.

Just a thought, sshep

 

[TBP]

There shouldn't be issues with carbon steel. Corrosion issues can develop over time, if nobody is checking the glycol annually. It can gradually turn acidic. If the water treatment people check it periodically, there shouldn't be any problem.

I'm not aware of any particular problems using ordinary bladder expansion tanks, with compressed air. Plain steel tanks work fine as well, as long as the Airtrol fitting gets installed in the bottom of them, and not just a pipe nipple.

 

[Montemayor]

TBP:

You've expressed the basic points in an excellent and logical manner. I failed to specifically point out the very important basic truths you bring out: any chemical solution used in a closed cycle process will degenerate to a point, sooner or later. Nothing lasts forever. It takes engineering supervision and control to maintain an efficient system. Additionally, engineers should not tremble and run the moment they have a carbon steel material of construction in a chemical process. With diligent care and well-organized maintenance carbon steel will carry its weight and contribute a profitable result to many processes - if the design engineer fully understands the limitations and strengths of this material.

 

[StoneCold]

Thanks Guys
You all had excellent points. Since the system is built out of carbon steel the choices are down to the type of fluid. I have decided to go with the antifreeze water mixture. I will keep an eye on it and let you know if it turns south on me.
Thanks for your advice. That is what I love about this place.

Later
StoneCold

 

[TBP]

Mechanical people usually have very little understanding of chemistry, and chemical people frequently have no knowledge of things mechanical. They're often like ships passing in the night.

Thanks for the star, Montemayor.

 

[JimCasey]

Automotive antifreeze has very effective corrosion inhibiting additives. That's why it's possible to run it in a cast iron engine with a steel water pump impeller through an aluminum radiator connected to the heater core through brass fittings held together with solder. It works well in cars even though you and I both neglect our coolant and don't change it as regularly as we should. Without the corrosion inhibitors this combination of metals should have enough galvanic potential to keep the battery charged :-/

The carbon steel will be fine. One place where stainless has a big disadvantage: It has lousy thermal conductivity. If you consider stainless, you will have to either derate the capacity or increase the size of the HX for the same number of Qdot.

 

[TBP]

There are applications where the toxic nature of ethylene glycol is a big factor. Propylene glycol can have additives that provide the same level of corrosion protection.

I think you'll find that by the time all of the other factors come into play for heat exchangers, the type of metal used is of no real consequence. It's much more of a theoretical issue than a real-world one.