Stainless Steel Vs Aluminium 1

[GrahamB]

I have previously "designed" & built a simple gas to water heat exchanger. That is hot gases passing up a central tube that is surrounded by water and hence the water is heated.

The heat exchanger I have is made from 2 mm thick aluminium but I want to create a "Mark II" version, this time from 1.6 mm stainless steel.

I know that aluminium conducts heat better than steel but how would the heat transfer of 1.6 mm stainless steel compare with 2 mm aluminium assuming same gas temperature/ same water flow rate/same surface area?

Any advice would be appreciated.

Thanks in advance,

Graham

 

[magicme]

hello
to calculate the heat transfer per unit pipe length, you need to calculate the overall heat transfer coefficient, which includes all terms like 1/h + r/k * ln(ro/ri) from one fluid thru the tube to the other fluid.
any text would have the various formulas for the internal and external convection H's and the conduction term thru the (round?) tube. the conduction term is the variable in your problem, as it accounts for t and k of the tube.

see for example the text by Incropera and Dewitt, Section 3.3.1. or Kreith "principles of heat transfer", section 2.2.

daveleo

 

[GrahamB]

Thanks Daveleo, my problem is fairly basic..... I have measured the performance of the heat exchanger made from
2 mm aluminum but want to recreate it in 1.6 mm stainless steel. How much will the performance degrade?

It is a very simple design, an inner round tube with a gas burner inside it and this tube is surrounded by a larger tube thus creating a water "jacket". Terribly ineffficient really but is just for occasional use during camping trips. I don't need an exact answer but I can compensate for the lesser heat transfer if I can get an estimate of the lower heat transfer. Unfortunately I don't have access to any text books hence I'm asking for help on the forum.

Thank you,
Graham

 

[magicme]

hello again

for an unfinned tubular heat exchanger, neglecting fouling terms, the overall heat transfer coefficient, U, is buried in this equation....

1/(U * A) = 1/(Hi *Ai) + ln(Do/Di) / (2*pi*k*L) + 1/(Ho * Ao)

where
subscript i relates to inner diameter or area.
subscript o relates to outer diameter or area.
L is total tube length
k is material conductivity
pi = 3.1415......
H is convection coefficient from inner or outer surfaces
D is tube diameter
A is *surface* area = pi * D * L

then calculate the heat transferred by

Q = U * A * LMDT

where LMDT = log mean temperature difference.

for a cross flow heat exchanger....

LMDT = (DelT1 - Delt2) / (ln(DelT1 - Delt2))

where

Delt1 = Thot(in) - Tcool(out)
Delt2 - Thot(out) - Tcool(in)


Beyond that, it gets messy, but you shoule be able to hack through some comparison based on whatever test data you have plus these equations.

good luck

daveleo

 

[magicme]

PS.....

if the "burner" you mention means that the hot side fluid is at some high constant temperature, in the equation I stated, set Thot(out) = Thot(in) = burner temperature.
I assume here that you know Tcool(out) and Tcool(in) from test data.

also for ballpark cal's.....

k(aluminum) = 170 Watts/m/degK
k(stainless steel) = 15 Watts/m/degK


daveleo

 

[Montemayor]

GrahamB:

I've designed and built heat exchangers with carbon steel, SS, and copper tubes. I've never done aluminum - mainly because it's damn tough to get a 100% reliable weld, it's a weak metal, expensive, and soft (weak mechanically).

I found the gas film to be the limiting heat transfer factor, not the tube metal's thermal conductivity. I believe you are wasting some good time on fretting about the metal's thermal conductivity when you design and time would be better spent on using fins on the gas (flue) side both to accentuate the weak gas film and also to induce helpful convection currents. Another weak point about aluminum is that it will react with caustic to create Hydrogen gas. You don't state the temperature of your hot gases, but Aluminum will distort and twist due to thermal expansion and the stresses built up - it is an inherently weak metal and a thorough mechanical stress analysis should be done. Stainless is tougher and has more mechanical integrity - plus it's easier to weld and can withstand higher temperatures.

I believe aluminum tubes are a weak link and lend no noticeable advantage on heat transfer. Internal fins on the gas side will yield more positive effect. In fact, some years ago there were boilers (both fired and waste heat) that were designed with fins on the internal gas side in order to take advantage of what I have stated and found to be true in the field. I have only used Aluminum tubes in Cryogenic exhangers (where the metal gets tougher at the lower temperatures). You have not mentioned how you plan to make the tubesheet joint for the Aluminum tubes - rolled or welded. If rolled, you will find that the metal thins out excessively and can't retain a sound, reliable joint. If welded, you will find (as I said before) that a good, reliable weld will be hard to achieve without experienced Aluminum welders.

What you've described is a "hobby" exchanger and I would opt for a SS tube with a "spiral" cut outs slipped inside the tube to lend fin-effect and a sweeping convection inside the tube(s). These cut outs are cut out of thin SS metal sheeting (approx. 1/32"). This is sometimes used in domestic natural gas hot water heaters - which is really what you've described.

Art Montemayor
Spring, TX

 

[jay165]

The simple answer, Graham is that the tube metal conduction is not the limiting factor, but the fluid film coeffecients.

Go ahead and build your tube out of SS, you won't see a difference in you're heaters' performance.