Correct material for heating coil

For you pourpose carbon steel is the cheapest option until you didn´t discovered a better one, in my opinion! Yes
a painted carbon steel (crazy idea?), I think...

luis

Common immersion heaters are available with either Incoloy or copper bundles. Either should do fine.

You will want to make the tubes as thin as possible, and allow them to flex a bit.
If a bit of rust doesn't bother your process than CS may be the best bet.
I don't know of any coating that will hold up well in this service.
When you start looking at higher alloys you are in a bit of a bind.
If you can keep the Cl levels modest then 2205 may work, but since you don't know that you are going to be washing off I would hesitate.
Check your data, 40psig steam is only 267F.
A safer option would be Ti gr2. And in thin wall tubes it isn't outrageously expensive.

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P.E. Metallurgy, consulting work welcomed

Ti will cost less than a Ni based alloy, and Cu alloys can be very problematic if you get any S in the solution. Conceivably a CuNi might work, but Ti will likely cost less.

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P.E. Metallurgy, consulting work welcomed

The biodegradable degreaser implies microbial corrosion (MIC). This would be damaging to steel and stainless steel. Sulfides from decomposing bacteria would be harmful to Cu alloys. Monel is a workhorse.

Sorry- missed the fact you're using steam. While you might get away with copper, cupronickel might be a better choice because you have internal pressure.

MIC isn't an issue with copper as far as I'm aware.

SRBs can destroy Cu alloys, whey they die they decompose and release H2S.
Monel might be a good option, but compare the cost to Ti (Monel is more expensive, but easier to work with).

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P.E. Metallurgy, consulting work welcomed

I agree with the wisdom and experience posted above..

Develop a max design pressure and temperature and get a quote for tubes mad from Monel and Ti

How long do you want this device to last ?

Is three, five or ten years a reasonable life for your purposes ?

MJCronin
Sr. Process Engineer

Thank you all of you for your replies!

The device is expected to have ten years of useful life but considering the CAPEX this parameter could be modified (downward).

One supplier recommends us to use a phenolic epoxy coating on a carbon steel pipe, but I am not sure about it, the film need to be thick (16-20 mils) and they do not have the conductive coefficient to estimate the lost in heat transfer efficiency. On the other hand, I am not sure if that coating resists sun exposure.

I'm going to look for quotes for Monel and Ti tubes, then compare these three alternatives. Four considering bare carbon steel that was the initial point.

Please complete this thread and share with us your final decision and the reasons you have for selecting it !!!

MJCronin
Sr. Process Engineer

Diego, you say pipes, meaning that you plan on threading this together?
You really should look for an experienced fabricator that weld this for you using thin gage tubes.
For 40psi at 275F sch5 pipe is likely too heavy for the job.
And in expensive materials every pound matters.

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P.E. Metallurgy, consulting work welcomed

On the epoxy coating, if you're using a steam trap the efficiency of the exchanger only the surface area requirement. Most epoxies are also surface tolerant which means they can be exposed to sunlight. Chalking on the surface is to be expected.

One cost effective solution to corrosion protection, that is often overlooked, is porcelain enamel on steel. No organic material is very resistant to sunlight exposure, particularly in thin coatings.

Hi all!

I want to share the final decision.

Before, it is neccesary to tell that the client informed that the pool will not have continuos operation.

We look for experinced suppliers of 2205, CuNi or Ti heating coils but no one offers that materials in my location in the required dimensions, in each case they offers just austenitic stainless steel. As foreign suppliers would be required, we discard these options due the cost.

Considering that coating suppliers did not offer a paint that will withstand substrate temperature under immersion conditions, Finally it was selected bare CS, recommending the client to always keep the coil submerged, if it is necessary to drain the pool to replace the degreaser, dry the coil well and submerge it again as soon as possible. For longer periods of no operation to apply an anticorrosive paint. Due to comparative lower cost of CS, the client accepts the eventual replace of the tubes.

Thank you to all of you for your replies, they were a valuable input to handle the problem and to learn a bit more.

Do consider treating the water with a film forming corrosion inhibitor. So long as the water level is maintained concentration of the inhibitor shouldn't be a problem. Sodium nitrite or carboxylic acid based inhibitors are common. Start with clean, bare tubes.

What is the degreaser composition? Again I think to paint the cs coil it is not a good option.

luis

Thank you for the advice TugboatEng.

0707, the main component is fatty acid esters (i just have that information). The coil will be not painted for the operation, the anti corrosive film will be only necessary for "storage" while the pool is not in operation, before to operate again it will be removed.

I have heard of phenolic epoxy coating. IMO this is not a good option if you are talking about putting some kind of grease while the coil is not in operation I can agree with you.

luis

I believe you had already reviewed all candidate materials. Once you have selected a bare CS instead of ASS, DSS or Ti Alloys which are the most common materials in swimming pools, you may have to consider the followings continuously for the 10 years design life.

- Corrosion Allowance: to be considered per the corrosion rate expected by chemical analysis and velocity of the service. If the steam is condensed in the heat exchanger, the corrosion will be noticeably appeared.
- To avoid the dissimilar metal contact, such as stainless steel, Ni alloy, copper alloy, etc.
- The tube bundles should be easily replaceable.
- All water (internal and external) should be completely drained during suspension (idle) of the pool service. After drained, the relative humidity near the heat exchanger should be less than 65% (60% preferable); or VpCI/Desiccant should be treated; or nitrogen blanketing is to be used.
- After drained for suspension, if there are some scales on the coil surfaces, they should be removed by water jet or chemicals accordingly.

References: MTI 34 Mothballing/ NACE Corrosion Journal, p515-522, June 2004/ NACE Paper 98244, 10147, 14-4335, 15-5450, 20-15004, Others

Thomas Eun