Low-cost heat exchanger 2

[ddot]

Hi all, I'm looking at material selection for an ammonia and salt water heat exchanger application. The working temperatures are relatively ambient with a low approach (temperature difference is less than 10 degrees C), and the pressures don't go above 13 bar (188 psi). I was thinking, therefore, that plastics might be an alternative except for their low thermal conductivity (anyone know of some newer plastics manufacturers that are ramping up production of improved heat exchangers?). I was also looking at aluminum, but I don’t know how it performs relative to steels. Steels are probably not as ideal as titanium because of their general higher susceptibility to corrosion effects, however, I'm not familiar with all steels relative to titanium and a cost per performance analysis might be in order. Copper alloys or aluminum-copper alloys seem to be out because of coppers reactivity with ammonia. However, I don’t want to rule out any ideas, these are just my initial thoughts. Again, cost relative to titanium is the biggest issue at this point since titanium is otherwise somewhat ideal. Any help would be greatly appreciated!

 

[thermcool]

See below links:

http://wastechengineering.com/products/heat_exchangers.html

http://www.oxfordplasticsinc.com/geothermalheating.htm

http://www.us.piping.georgefischer.com/index.cfm?uuid=C8EDA72AEE1A4B3DA8F8AA810A435EFA&o_lang_id=2

http://www.thomasnet.com/products/exchangers-heat-teflon174-26645903-1.html

 

[ddot]

Thanks very much Thermcool, for you suggestions. Did you go to a particular manufacturer for your particular exchangers? I contacted wastech myself and haven't yet heard back from georgefischer, but the wastech guys said they didn't have what I was looking for. I'll make sure to consider teflon as well.

Any more details about the conditions that you're working under would be great (temperatures, pressures, etc). Otherwise, thanks very much!

ddot

 

[moltenmetal]

With miles of polyethylene tubing, you'll probably be losing a fair bit of pressurized ammonia by permeation through the tubing into the seawater. As I said in the other thread where you posted this question, the low thermal conductivity of polyethylene will probably require an enormous heat transfer area in comparison to that required for a metallic plate exchanger, which of course means a larger area available for permeation losses. You may (doubtfully) save on the first cost of the exchanger, but you'll be paying for it in lost ammonia, risk of ammonia release in future etc.

When it comes to transferring heat to/from a toxic medium like ammonia, metals are the way to go. Be thankful that you have, in this case, a relatively large selection of metals to choose from! Titanium isn't the only metallic option at your disposal if seawater and anhydrous ammonia at modest temperatures are the only corrodents.

 

[ddot]

moltenmetal, thanks for your critical points! I think you've safely ruled out PE tubing. If there is another plastic available with a higher thermal conductivity, then I would definitely consider it. It sounds like you are correct in pointing out the permeation problem with PE. Do you happen to know anything about the permeation of ammonia through other types of polymers? I don't want to give up on polymers yet because if they are an alternative to titanium, they might be the only alternative of significantly lower cost that can provide similar corrosion resistance. I've been trying to find some, and I may have interesting leads if they check out. We'll see. Otherwise it looks like titanium is the winner out of the available metals.

 

[panduru]

Give me metal (that doesnt cost a bomb,) anyday. Nothing can beat metal when it comes to heat transfer apps.

 

[djack77494]

Glass (not glass lined) may be an option. I know you can buy glass S&T exchangers.

 

[moltenmetal]

Yes, it would be really nice to have corrosion immunity rather than accepting mere corrosion resistance, so I see why you're after polymers. In an exchanger with modest temperature seawater on one side and modest temperature anhydrous ammonia on the other, I would guess that titanium would give you excellent service. But couldn't you also get away with 2205 duplex or perhaps a 6-moly stainless steel? Cheaper than titanium I would imagine, in return for some added but manageable risk of corrosion damage over the long term.

Forget about glass coil in shell, SiC tubed, glass-lined, graphite etc. This is not a severe enough corrosive service to warrant these expensive options. Unfortunately, I would also forget about solid polymer tubed or plate-type exchangers if anhydrous ammonia under significant pressure is involved. There's no fancy polymer I'm aware of which will eliminate the hazards associated with that approach completely, and if you go too fancy on the polymer you've lost your cost advantage anyway.

An option worth considering might be a gasketed plate-type exchanger with plates of a lower grade of stainless steel with a thin polymeric or metallic coating only on the seawater side. While the coating will impair heat transfer somewhat, it may provide sufficient resistance to prevent pitting of the stainless steel. Depending on the coating, this may be cheaper than a titanium plate exchanger or a shell/tube exchanger with titanium tubeside materials (i.e. seawater in the tubes- providing cheaper construction in return for some difficulty in maintenance). How effectively a coated plate type unit would work would depend on the quality, type and thickness of the coating and on the temperatures the metal would encounter. Just as ammonia may diffuse through a polymeric plate or tube, chloride may diffuse through an intact polymeric coating and may lead to SCC etc., and this tendency would increase with the expected coating and metal temperatures. Another issue to consider with this approach is the significant area of, you guessed it, polymeric gasketing that would represent another potential leakage or permeation risk for ammonia.

 

[JRMacGregor]

I don't know about the ammonia side of the equation, but on the seawater side I can say that a number of PHE suppliers delivering for SW/FW cooling service in the shipbuilding market have been offering Hastelloy C22 and similar instead of the normally used titanium (due to long lead times on titanium).

 

[sirsa1]

Have you solve your need?
there is another option for material on this, as far you are in low temperatures you can use duplex stainless s31803 tubing and ss T316 tubesheet, the plate and frame heat exchangers are the most efficient in size and are but they are not the most practical for sea water use because the marine life becames a problem on this type of ammonia condensers on boats

 

[ddot]

Thank you all for your input. I have not found a viable replacement for titanium. In the end, I began to evaluate plastics in terms of their structural properties as well as their permeation issues. I essentially assumed nearly any plastic would do the job on the seawater side. It just seems that plastics don't cut it (unless someone invents a plastic with stiffness that can rival metals...).

Anyway, unless I'm mistaken, I hear from many people in industry that S316 does not have the same level of corrosion resistance necessary for marine applications. What is the problem with plate exchangers that is not encountered by tubesheets?

 

[EdStainless]

Will any of the P&F guys work with superferritic stainless? Both SEA-CURE and AL-29 4C have been used extensivly in seawater. They have good heat transfer, excellent corrosion resistance, high strength, and they cost a heck of a lot less than Ti.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[rmw]

EdStainless,

I mentioned one of the superferritics as a solution for a heat transfer problem based on better HTC and presence of chlorides and their reaction wasn't so much that it wouldn't work than it was "not invented here".

So, if the SF's can be pressed into the PHE plate form, it should be an excellent PHE material with respect to both corrosion and HTC.

rmw

 

[ddot]

As far as their website advertises, SEA-CURE has been used as heat exchange tubing, but not for P&F designs. There is a significant amount of cold-working that will go on, so ductility may be an issue. However, I'm sure it could be overcome with enough money thrown at it. Steels and aluminum alloys seem to be the near term replacements for titanium given the large development costs of getting new exchangers designed and tested.

ddot

 

[rmw]

I don't know that EdStainless was advocating Sea-Cure and/or AL-29 4C specifically as much as he was extolling the virtues of superferritics in general. Sea-Cure is the trade name of a tubular product I think, not an alloy and part of its properties that give it good corrosion resistance is determined by certain steps (I don't know if I am allowed to say which one) of the tube fabrication process which I have a hard time visualizing being done to strip or a stamped plate.

Wade in here EdStainless and bale me out.

rmw

 

[25362]

Sea-Cure is considered equal to S44660 thus a visit to

http://www.copperweld.com/pdfs/S44660_Brochure.pdf

http://www.seacuresolutions.com/pdf/PerformanceofSuperferriticinHighChloride.pdf

thread330-150259

may be of help.

 

[EdStainless]

Sea-Cure is the tradename for S44660 made by Plymouth tube. The tradename applies to the alloy, not the product form. The guys at Plymouth are the only people out there with tube experience in these grades.

That said, both of these alloys are regularly fromed into deaply convoluted shapes. They are used in the HX of high efficency furnaces.
You do have to be very careful when you form these grades, but it has been done.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[rmw]

EdStainless,

I was told by DJ while on a plant tour at Plymouth that there was a specific and particular part of the tube mfg process that gave S44660 material the corrosion resistant characteristic and that any variations in that particular process or part of the process would sacrifice the great corrosion resistance that SeaCure has.

I just don't know if I would be giving up a trade secret to say what that was, but I can say that it is something that I would consider difficult to do (control) in a plate forming process as opposed to a tube forming process.

I may need to ask you the question via e mail.

rmw

 

[EdStainless]

Wellllllll, sort of.
If you buy S44660 sheet and you want to use it as sheet, with the best corrosion resistance possible, you are going to need to do some work with the supplier. Often the strip supplied to tube mills is under annealed and over pickled. The steel mill knows that the tuber will be processing it further so it doesn't matter.
If I were buying strip for this application I would require ductility tests in L and T, NDTT, and corrosion tests on the as shipped surface (G48D).
The forming of these alloys will be slower than with 3XX grades. You need to make sure that you don't have any sharp corners or edges. You need to use low strain rates.
One advantage is that there is no need for anneal or stress relief after forming. The material will not CSCC.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm