Air Cooled Exchanger Failure

Dirty fins are a common cause of low efficiency in fin fan coolers. Assuming that the blade pitch is as per design and they are turning at the right speed (no belt slippage or wrong sized pulleys), then I would get someone to do a very good cleaning job. This can be done with only water or with one of the proprietary "foams", depending on the makeup of the dirt. Care must be taken that an over enthusiastic contractor does not flatten your fins with a pressure-jetter. I have seen gains of >30% in efficiency after washing and where I am at present we get around 3% loss of efficiency in a year. This increases rapidly as fouling gets worse. Tried to insert a picture but with little success, (too higher grade task for me) but it does not need much dirt to loose the kind of efficiency you are, and it is the fins that you can't see that seem to have the greatest fouling effect. Other areas to look at are the blade tip clearances and the condition of the inlet bells.

Do you know the materials of construction? How many fins per inch?

I have seen cases where 500+ F oil was pumped into the tubes of a cold, plate fin exchanger and those fins got really loose. Try to move your fins on the tube. If you get any movement, that would be the problem, especially at high fin densities. If you can't detect any movement, it still might be the problem since the fins tend to interlock.

10 Fins per inch with a SS 304 tube and AL 1100 fins. The facility cleaned the exchanger earlier this summer but I wasn't able to be there to witness it so I don't know how thorough they were. There has been little positive effect from the cleaning though.

"an upstream exchanger has no cooling flow going to it so the inlet temperature of the process going to the air-cooled exchanger can rise to 440 °F for up to 10 hours at a time"

I am not a real genius but I would think that this would be the first thing that I would fix.

Two things come to mind. One is the viscosity of your fluid might be high enough and the velocity of the fluid low enough that you get laminar flow. This can be especially irritating of the fluid has a high rate of change of viscosity with temperature, these can lead to 'channelling' of the hot fluid in the core area with viscous cooled fluid against the walls.

Cure, lower viscosity fluid, higher intensity pumping.

Another problem can be the accumulation of cooling induced precipitation products inside the pipe. 'boiler scale' type of thing.

cure, clean the pipe inarrds somehow.

Yeah, I'm pretty sure that you got loose fins. What happens is that the tube expands and pushes the fins along with it. I'm not sure at what temperature aluminum anneals, but at your elevated temperature, I'm thinking that they have a permenant set. Then, when you try to operate at the design conditions, your tubes are not quite expanding into the fins anymore. So it does not have the performance that it did when it was new.

It's time for a new coil. If you can't fix the upstream issue, then make sure the new heat exchanger is designed to take the high temperatures.

Talked with the boss and he likes the idea, we are going to move forward on that assumption. Thanks to everyone for your feedback and opinions.

I agree that the fins are most probably causing the problem. 'L' footed Aluminum on stainless steel tubes is not a good match to retain the bond. I also want to add that if you have not done so, an can, inspect the inside of the tubes, flouling inside the dubes can cause a reduction in heat transfer as well.

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