SEAWATER - AMMONIA Exchanger tubes selection 1

[Mac001]

Can anyone help me?
the exchanger is SEawater tube side and ammonia shell-side

I discarded CU/ni and AISI 316 ...

Someone suggested titanium, i think it should be ok , but i'm looking for something less expensive.

What do you think OF AISI 446/447 and duplex 2507 ?
what are they cheaper ( considering the nickel prices)?
What backdraws does any choice present? ( welding ?)

Do you have any other suggestion?
thak you in advance
Best regards

 

[EdStainless]

S44660, superferritic tubes.
Use 2507 or AL-6XN for tube sheets.
You can autogenously seal weld to the AL-6XN. For seal welds in the 2507, or strength welds in either, I would suggest C22/59 type filler metal.

Yes there are limits. They cannot be used above 600F (ever, even briefly), and you will not want to use them much below 0F.
They are strong and stiff (use thin walls) and have the best thermal conductivity of any high alloy stainless.
Keep your flow velocities on the seawater side high, at least 12 ft/sec. This will help keep the tubes clean and improve heat transfer. Don't worry about erosion.

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[unclesyd]

b]EdStainless[/b],

I don't know what the OP's process is but anhydrous NH3 condensers routinely autorefrigerate to -40F.

Is this too low for the alloys you propose?

 

[EdStainless]

I'll have to check.
What temp are we actually dealing with?

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Rust never sleeps
Neither should your protection

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[Mac001]

Thank you for your answers

this is an normal ammonia cicle not a low temperature one.

The condensing temperature is 35°C with Seawater entering at 30°C . So there will should be no problem with low temperature.

In any case in order to avoid any surprise, I will rate the exchanger for a range of temperature from -10°C to 110°C on the ammonia size.

 

[EdStainless]

That would not be a problem for a superferritic that has some Ni in it. The Ni free alloys have slightly higher DBT.

Just one reminder, keep the tube walls as thin as possible. This will help heat transfer, cost, low temp issues and make you look smart. I have worked on many jobs where ht old units were 0.065" wall in a Cu alloy and it was replaced with 0.028" wall superferritic.

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Rust never sleeps
Neither should your protection

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[Mac001]

Opps I forgot ...

In order to improove the exchange ratio ,the tube will be finned , how does the duplex behave compared with AISI 316 or titanium .

I think that i will need at least 1.65 mm which is 0.065''

 

[EdStainless]

I'll look for some experience on finning. I presume that you are looking at low profile roll fins, and the 0.065" is the starting wall?

Are you sure that you checked the wall thickness using hte correct allowed stresses? With either S44660 or 2507 we are dealing with alloys that are much stronger than 316L or Ti.

You can fin a superferritic, a superduplex, or a superaustenitic stainless. It all has been done. They are much more difficult to fin than 316 or Ti because they are all much stronger. I'll get some specific info for you.

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Rust never sleeps
Neither should your protection

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[unclesyd]

You might want to take a look at Fine-Fin tubes.

http://www.highperformancetube.com/default.htm

 

[EdStainless]

There have been a number of ammonia/seawater applications of S44660. Some of these have been finned, those started with 0.049" wall tubing. There is a firm in the US that has done this fin work.
There are also a number of similar applications that used fined Cu alloy tubing that use slick S44660 tubing. Unless you are way beyond the heat transfer that you could hope for with smooth tubing you might consider it. Is this condensing service or evap?

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

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

 

[Mac001]

You are right, I will try to dimension the condenser for smooth plain tubes.