Tube fouling in vertical copper finned heat exchanger

[fjm3188]

I have three vertical copper finned boilers each with an output rating of 2,650,000. Flow through the exchanger is 250 usgpm with a delta 'P' of 2.25 psi. Each vessel is individually pumped and circulator shuts off during 'non firing' mode. Each exchanger has 84 tubes (42 down, 42 up) and design criteria is based on 6.25 BHP per sq ft.

Problems have delveloped with tubes clogging and burning out in various locations around the heat exchanger. There is no rhyme or reason to the location of the fouled tubes, they are at random throughout the heat exchanger. A strainer is installed and when pulled was clean. Although the customer's chemical treatment is not perfect, the exchanger fouled within 6 months. Water hardness is normal for a large city and is not an issue with other equipment.

All three vessels have failed this way while the older atmospheric copper finned boilers have been there for twenty years.

Could there be 'lazy tubes' in the designers flow pattern ???

 

[vpl]

tube diameter?
what do you mean by "burning out?"
Are you sure that the failures are random?

Patricia Lougheed

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[control145]

Boiler tubes are 7/8" ID 38" long.

Out of the 42 tubes availble on the return side of the heat exchanger one of the tubes will foul and plug up. The radiant burner continues to operate as normal and will eventually dry out and warp the plugged tube.

The problem is if it were one tube int he same spot we could look for issues there. Unfortunately it's a different tube in a different area every time.

Are the failures random?? Good question as we are in the preliminary stages of investigating the fault.

 

[control145]

I have attached a rough design of the header for clarity.

 

[vpl]

I'm just going to throw out some items of things that I would look for in hunting down the cause of a tube plug. You may have already thought or done all of these.

Thanks for the information on the tube size. Those are pretty big tubes, so it would eliminate the first type of problem I have seen (too small of tubes plug easily). With 7/8" tubes, I wouldn't expect to have complete tube plugging.

You stated that an strainer was installed, and when pulled, was clean. Is the strainer normally installed, or did the customer install it after one or more heat exchangers had this problem? Is it still installed? Also, what is their tube side fluid? What I'm suggesting is whether there is something in the process fluid that is plugging the tubes.

Is it always a return tube? Then I'd ask if this is a straight-tube heat exhanger where the water goes through, mixes in plenum and then enters the return tubes and exits the heat exchanger? If that's the case, is there the possibility of a coating in the end bell area that is disintegrating and then getting caught in random return tubes? This can happen even in new heat exchangers (and actually, if it's going to occur, is more of a problem with a new heat exchanger than one that has been operating for a while.)

Have you pulled a plugged tube and inspected the "plug" material? Is it plugged in just one part of the tube or spread pretty much all the way through the tube? Can you describe (or provide pictures) of the material that plugging it? Things to look for are the color of the material, the consistency, whether there are any discrete particles (such as clam shells)

You stated that the circulator shuts down during "non-firing" mode and that this is a vertical heat exhanger. Are all the failures in the same part of the various tubes (i.e., all on the bottom or the top or the middle)? Is it possible that your tubes are draining when the circulator shuts off so that when the boiler starts up they're being blasted with hot shell side fluid while empty? (This wouldn't cause plugging, but might cause a tube to fail and leak)

I've kind of been assuming the tube fluid was water, but is it? (You mention water hardness, but that could be on the shell side) Is it something that could harden when in "non-firing" mode that is then breaking into chunks when the boiler starts up?

Finally, since these are fairly new heat exchangers, are you sure they were a quality product? If they were not well-constructed you could be getting construction debris.

Hope this is helpful in at least giving you ideas of where to go look.

Patricia Lougheed

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[fjm3188]

The strainer was installed at the time of boiler installation. Unfortunately it is not always the return tubes, it's both supply and return.

During non firing mode, the boiler vessel is still under pressure, (15psig) but the flow stops. The tubes are never empty.

The fluid is water and the applications are generally light industrial eg chicken processing plants, condos, apartment buildings etc. Water hardness is standard for a big city (Toronto) and not considered hard. Exact grains reading for this city is not known. I will see if I can find out.

All applications are retro-fit nto new construction.

As far as quality, the manufacturer uses high grade copper and the tube fins are extruded. This is a well built machine.

When I go back to the office, I will photograph the sediment we got out of one of the heat exchangers and attach it to another post. Most of the sediment appears to be red in colour.

 

[SnTMan]

fjm3188, would it be possible to keep flow during non-firing mode?

Regards,

Mike

 

[fjm3188]

Mike,

We've tried that with no success. Boilers are located at various locations throughout the city and this fouling only happens on the largest heat exchanger. 3,000 MBH.

System is a closed loop heating circuit. Fresh water from the city would only be introduced if the system had a leak.

Two other boilers firing 4,000 MBH with smaller heat exchangers (4.0 sq ft\ bhp) do not appear to be having this issue.

 

[SnTMan]

fjm3188, I don't have particular expertise in this area, but I am wondering if a difference in flow velocity between the different boilers might contribute. In shell & tube exchangers too low can aggravate fouling.

Regards,

Mike