steam/water heat exchangers volocity wear

[captsmith]

we are making our own heat exchangers to save money and teach inmates a skill, my question is reguarding volicity wear on our pipes. we use type k copper and have a problem with it being destroyed at the incomming water point, the rest of the exchanger wears well. Is there a beter tube material and were do you get it? Is there a "paint on" abrasion resistant coating around? Would applying silver solder to the pipes at the water entry hurt or help? The original exchangers made by Paterson Kelly have the same problem with wear at the water entry point, I'm not sure what material they use, but it does not hold up any better.

 

[EdStainless]

As you alloy the pure copper it get harder and the surface film that provides corrosion resistance gets stronger also. In order of increasing flow erosion resistance, Cu, Admiralty brass, Al bronze, 90/10 Cu/Ni.
I assume that you are staying with Cu alloys for ease of working with them.
Let me know if you want to talk stainless steel.

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Plymouth Tube

 

[captsmith]

we are using the regular type k copper because thats all we can find, I'm not an engineer so the abriations are over my head, but I would love to use stainless as its shown the best wear properties for our hard water and flow. We are a state owned and run prison, so our funds are always short and our access to peers outside in the real world is very limited. We have a wilson torq-air-matic tube roller that I don't know if it can roll stainless. I know that stainless will not transfer heat quite as good, but I can live with less efficiency if the tube bundles will last more than 6 months. Our only problem with the copper is some kind of abrasion is comming from the high flow and its like somebody held a sand blaster on the outside of the pipe. All the copper pipes are getting "sanded" this way. The only real solution I know is to replace the pipes with the correct size, so volicity will be reduced.

 

[davefitz]

wear rates at the inlet to copper tubed heat exchangers has always been a problem. When condensers were tubed with copper, a max tube water velocity of 7 fps was often stated in the specification. This could be increased to 10 fps if you were to use a bellmouth at the entrance or a reducer soldered onto the inlet, with a rate of increase in diameter of about 2 lengths axial for each 1 length increase in diameter. For example, a 1" tube might have added a 1.25" to 1" reducer soldered to the inlet, and permit a 10 fps water velocity based on the 1" tube diameter.

Water quality also affects the wear rate.

The problenm occurs only at the inlet because as water entgers a tube inlet, the streamlines converge into a flow area minima called a "vena contracta" which has an effective flow area of only 64% of the measured open tube area. The velocity at the centerline of teh tube inlet will be about 160% of teh average velocity, and so will the velocity of water in the recirculating eddies near the inlet. The vena contracta can be eliminated by use of an inlet bellmouth.

 

[EdStainless]

What size tubes are you using? Maybe I can get you some to try.
The trade off with stainless is that you can use thinner walls because it will resist erosion and it is stronger. This will offset most of the performance difference.

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Plymouth Tube

 

[SnTMan]

captsmith, you obviously have access to the net, I would advise, first, go out there and learn some HX lingo, it will help immensely.

Second, is this a shell & tube type exchanger and is your erosion ocurring on the tube or shell side?

Maybe some impingement protection is needed.

Regards,

Mike

 

[rmw]

Davefitz,

I was going to suggest tube liners. Is that what you are referring to by the term bellmouth?

rmw

 

[captsmith]

we are using 3/8 I.D. 1/2 O.D., that's what the original bundles were and that what our bender die's are made for so using a smaller stainless pipe would only be a viable option if we could stay at 1/2 O.D. We have both Paterson-Kelly and Ajax heaters and both of the shells are piped with the same size supply pipe as the inlet on the shell, I know reducing the supply pipe would lessen the wear due to your bellmouth principal, but we can not get enough flow to the point of use. I don't think tube liners would help, because the wear is on the outside of the tubes. Thanks for all the help so far. I have thought about coating the tubes at the inlet with silver solder for wear, but am afraid that the temper in the pipe would change to much at that point, and don't know how the silver solder would wear.

 

[SnTMan]

captsmith, from your last description of the your problems, this sounds to me like erosion of the tubes due to fluid impingement, particularly if the wear is at the shell inlet end only. Stainless tubes may help with this, or, if you have room in the shell above the tube field, a plate say 1-2" larger than the nozzle ID mounted on the bundle below the nozzle can perhaps protect the tubes. An alternative arrangement could be to mount "impingement" rods above the tubes, probably two rows on a staggered pattern.

A third approach sometimes used is a distributor belt, a short, larger shell containing the inlet nozzle, mounted outside the main shell which has cutouts to admit the flow to the bundle. This increases the inlet area to the shell and reduces the velocity of the fluid. Sorry, I don't have any pics or so forth handy to post at this time.

Can you provide some further details of your designs, maybe tube layouts, shell and nozzle diameters, flow rates, etc? It might then be possible to make some calculations, etc. to compare to industry standards.

Do you have any data from the manufacturers you list, to compare your operating conditions to?

My guess (and a guess only) is kind of that your flowrates and therefore fluid velocities are above what the original equipment, which as I understand it, you are "copying", was designed for, leading to the erosion wear you mention.

Regards,

Mike