Finned tube natural convection 4

[ione]

Hello guys,

Usually when I have to deal with heat loss Q (only due to natural convection) from a smooth horizontal tube I use the formula in the attachment.

Now my questions is:
Does this formula stand still for an horizontal finned tube? And what value have I to enter in the formula for A (area)?

Regards

 

[IRstuff]

More, or less.

The area now includes some fraction of the total (two-sided) fin area, with decrements for inefficiency and thermal conductivity of the fins themselves.

TTFN

FAQ731-376

 

[SNORGY]

How accurate do you want to be?

If it was me - a generalist, not a heat transfer guy - I would simplify it via the following approximate approach:

Get a couple of air cooled heat exchanger data sheets that report the overall "U" values and "A" values for bare tube and extended surface areas.

Let:

A = total convection heat transfer area of interest
Ab = bare tube area without fins
Af = fin face area (two faces per fin...)
Aoe = outside fin edge area
Aie = inside fin edge area

A = (Ab - Aie) + (Af + Aoe)

Ub*Ab*c*LMTD = hb*Ab*(T2-T1)
U*A*c*LMTD = h*A*(T2-T1)

This would allow one to approximately compare h (extended) against hb (bare).

Be careful with L-footed fins...there is another "area" to treat somewhat differently in that case.

Of course, separate "U" values for different materials (e.g., aluminum fins onto ASTM A179 tubes) would really make things more complex, so you would end up with something in the form of:

Q = Ub*Ab*c*LMTD + Uf*Af*c*LMTD = h*A*(T2-T1)

In the end you might be able to average out to a single U-value.

I can just see the heat transfer guys cringing by now...

Regards,

SNORGY.

 

[ione]

IRStuff,

Is the decrement you mentioned due to the fact the overall surface of the fin is not at the same temperature (I mean there is a temperature gradient from the base towards the tip of the fin)?

Moreover how can I evaluate the fin efficiency? That how I calculate an area A(EFFECTIVE) < A to be entered in my formula for the heat loss?

Thanks

 

[ione]

Snorgy,

Your approach from a practical point of view seems to be good, but I miss a point.
You made reference to air cooled exchanger, and that implies forced convection while what I am looking for is something that is valid for natural convection.

 

[SNORGY]

ione:

Absolutely a good point. In another earlier post I stated that I typically use U = 5 BTU/(hr-ft^2-F) for bare pipe to air natural convection. other folks on here suggest a lower value, coser to "1". So, split the difference, call it "3".

IF the ACHE data sheets (or GPSA e.g. Section 10 Figure 10-10) stated U = 35 (forced convection), for example, maybe I would ratio the heat transfer duty downward by a factor of 7. It's really crude but it might be all you need at the "roughing-in" level.

With that said, GPSA Section 10 is probably worth looking through. Lots of good methodology there.

If you had access to something like HTC-ACX, you could run a simulation at "almost no air flow", and the results you need might be accurate enough.

But, a star for you for stating the obvious - and serious - shortcoming in my post.

Regards,

SNORGY.

 

[SNORGY]

7 x 5 = 35; 11.7 x 3 = 35

Ratio down by 7 or 11, depending on what number you believe.

Regards,

SNORGY.

 

[IRstuff]

Temperature drop along the fin is a consequence of the thermal conductivity of the fin itself, but there are other factors, including the thermal conductivity of other parts of the fin/pipe structure, as well as the convection efficiency at the base of the fin compared to the top of the fin.

As to your second question, SNORGY's approach can be used. Find a finned design that matches yours, and use their lowest heat transfer coefficient, which should either be natural convection, or close to it, and back out the apparent area of the fins used in the calculation.

TTFN

FAQ731-376

 

[ione]

Thanks all you guys, a star for you for help.

Snorgy: It is never a matter of faith (5 rather than 3 for U-values).I have already posted the link below in the same thread you’ve stated U = 5 BTU/(hr-ft^2-F) for bare tubes and natural convection.
I have checked the results from the calculator with the formula attached at the beginning of this thread and they match (so the calculator works pretty good).

http://www.thermal-wizard.com/tmwiz/default.htm

Regards

 

[SNORGY]

ione:

You guys all do a better job of finding these kinds of things than I do.

I've been in this "upstream oil & gas plug'n'chug" mode for too long, I fear. Too much "...close enough...that'll do..."; it's more like training a border collie to herd sheep than it is like calculating things properly.

Then again, maybe I tend to see things a bit too macroscopically at times.

If only more folks in this racket saw (or placed) any value on doing engineering the right way.


Regards,

SNORGY.

 

[ione]

Snorgy:

Your reply seems somehow sarcastic, but maybe I have misinterpreted it. I considered your approach helpful and recognised this with a star. If I offended anybody with my previous post, because of my poor command of English (I am from Italy), accept my apologies. I just wanted to point out it was not necessary to make any hypothesis on the U value for bare pipe and air natural convection, since this value can be calculated.

Regards

 

[IRstuff]

Calculating the heat transfer coefficient of air is only something you would rarely do, since the number is almost invariant,, and what variation there is occluded by other factors.

And, everyone has their own favorite value; my personal preference is 2.5 W/m^2-K, simply because I'm usually interested in a worst-case scenario.

TTFN

FAQ731-376

 

[SNORGY]

ione:

Absolutely no sarcasm intended! Your post was extremely valuable and welcomed and well-received by me.

What I meant by my comment is that there are many people on this website - you included - who actually do things *correctly* and place value on that, whereas in my line of work in this culture there is more of a "cowboy" approach to engineering.

I come here often to surround myself with smarter people, you included.

Regards,

SNORGY.

 

[ione]

Snorgy:

So I definitely misinterpreted your post. I reaffirm your approach was a good one for my requirements. A “cowboy” approach (as you defined it in a funny way) can be more valid than a too picky approach, especially whether it comes from people skilled as you (I have read your profile and realized you are one of the MVP’s).

Regards