Are Air-Cooled Heat Exchanger Tubes Always Finned? 5

[Christine74]

In my experience, all of the large air-cooled tubular heat exchangers I've seen have used aluminum finned tubes (tension-wrapped, embedded, extruded, etc.) to enhance heat transfer.

These finned tubes can be very expensive in comparison to bare tubes. I'm not sure what the effectiveness is of these finned tubes, but even if the effectiveness was quite high I don't see why it wouldn't be cheaper and more compact just to used many more bare tubes instead.

Any ideas?

Thanks,

-Christine

 

[MikeHalloran]

Some airplanes built in the thirties used non-finned tubes for cooling their engines. I've never seen one up close, but plastic models and photographs are distinctive; the tubes comprise a substantial band, looking like a horizontal corrugated stripe, just hanging out in the airstream, running pretty much the entire length of the fuselage. I wouldn't describe the assembly as 'compact'. Given the speeds attained by the airplanes, I wouldn't describe it as 'low drag' either.






Mike Halloran
Ft. Lauderdale, FL, USA

 

[MikeHalloran]

Not long ago, I bought a couple hundred stainless finned tubes, custom made to my specs, and they cost barely more than the same length of straight tube. I.e., at least one outfit is tooled up to bang out straight finned tube in small lots in a variety of materials and fin pitches.

Adding u-bends or headers and brackets to make a heat exchanger assembly adds considerably to the cost, but still there's a point to be made: Full custom can cost less than 'stock', if you know what you're buying, because you skip a couple levels of distribution.



Mike Halloran
Pembroke Pines, FL, USA

 

[IRstuff]

C74,

It all boils ;-) down to a question of what is being traded off. All engineering designs are compromises. In the case of your particular example, the performance driver is obviously exposed surface area. However, one can imagine that pressure drop might be an important secondary consideration. Another would be weight.

If every subsystem decided to use 3 unit lengths of bare tubing instead 1 unit length of finned tubing, you'd get 3 times the pressure drop. This might require a bigger pump and/or higher flow to compensate, resulting in more expensive maintenance for the pump as well as potentially more leaks from the longer piping run at higher pressure or flow.

Additionally, the bare tubing will incur additional weight, compared to the equivalent surface area of finned tubing. There are many systems where the added weight of the tubing and bigger pump are non-starters, compared to paying some extra for the finned tubing.

TTFN

 

[speco]

Christine,

Finned tubes are usually used because of their economics. However, there are some cases where bare tubes are used in air-cooled exchangers.

Since the finned surface is typically 20 to 25 time the bare tube surface, the overall design is usually much more compact and much less expensive that it would be with all bare tubes. This takes advantage of the fact the the inside heat transfer coefficient is relatively high.

On the other hand, in cases where the inside coefficient is low, there is very little advantage to using fins. Examples of this might be coolers with viscous oils in laminar flow, or low pressure gases on the tube side. In these cases, adding fins would simply add resistance for the fan to overcome, and do little to make the cooler smaller.

Regards,

speco (

http://www.stoneprocess.com)

 

[rmw]

Fins can also be detrimental to overall performance of a Hx notwithstanding their contribution to heat transfer if the finned side of the tube is in an atmosphere where the fins would tend to plug up due to contaminates in the air/gas passing over the fins.

An example of this is steam coil air heaters used in paper mill chemical recovery boiler combustion air ducting. (at least in the older days) Chemical recovery boilers have (had) a lot of salt cake in the atmosphere around them, and in some cases adjacent boilers were using solid fuels including waste wood, and coal, which produce dusting of the fuel as it is handled from storage into the boiler, and/or from ash products of combustion of those solid fuels from leaky flue gas clean up equipment.

In short, the environment was nasty, and fin tubed STAH tubes tended to plug badly, so bare tubes were often selected because a bare tube at a poor HTC was better than a finned tube with a great HTC plugged solid with 'stuff.'

And, yes, it resulted in a larger heater at higher capitol costs, but it ran good and accomplished the goal intended, namely the heating of the combustion air.

rmw

 

[25362]

There is a variety of fins beside circumferential, spiral and longitudinal. There are corrugated, slitted, segmented and even semicircular fins.

The latter were developed especially for evaporator tubes in WHB in conformity with the limited friction drop allowable, when there is a risk of dryout, and where compactness is a requirement.
BTW the missing half is the downstream one, in the wake region of the flowing gases.