Temperature at a Distance

[MiketheEngineer]

Structural here with a Thermo question - Thermo class was 35 years ago.

Simply - if I have a heat source - what will the temperature be at a given distance away.

Example: Pipeline running at 1,000F and there is an object (metal or wood) say 2' away. What will the temperature of that object be?? ( time is no concern - the object will be there until it reaches maximum heat )

If I remember - a number of factors come into play: ambient temperature, wind, humidity, density altitude, etc. We currently use cheap infrared thermometers AFTER the fact. Need to have a rough estimate before the fact.

Just looking for a chart (I have) or a simple formula that will give me a rough idea based at 70F, no wind, typical humidity, etc.

Thanks for any help.

 

[IRstuff]

There is no closed form solution for this type of problem, since the temperature is the result of the balancing of the heat flows into and out of the object.

You have to equate the incoming radiated heat with the outgoing radiated and convected heat, while accounting for the view factors.

There will be a temperature that balances the heat flows.

TTFN

FAQ731-376

 

[SImc1]

http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

Seems to me your pipeline will be radiating heat to your wood/metal object.

 

[MortenA]

Worst case is radiation only - and no effect of cooling the object "back side".

For this its a simple energy balance - one side is the surface area of you pipe - other side is the surface area of you object - compared to the "total surface area" at the distance if you consider a outer perimeter at this distance (is that clear )

The at SS in=out so the radiated heat from the object to the pipe= radiated heat from the pipe to the wood

Now use Plank to calculate the temperature of you object - this will depend on the colour of the object.

This link in Wikipedia explains the basic. Theres a link to a black body radiation calculator - thats not exactly what you need. Google around and you may even find what you need

http://en.wikipedia.org/wiki/Thermal_radiation

This may even do the trick:

http://www.efunda.com/formulae/heat_transfer/radiation/calc_2bodies_enclosure.cfm

Best regards

Morten

 

[MiketheEngineer]

MortenA -

THANKS - just what I was looking for... something I can give our field crews who may/may not have a HS education.

Honestly - I do remember that this was something that was not easy to follow - much less get a "real" answer. Too many variables.

I think the trial and error method I suggested a few years ago using cheap infrared thermometers is actaully the way to go. The variables and changing conditions are just too numerous.

The real problem is that they (the 800 lb gorilla customer) want to put wood plank in the vicinity of these vessels and they can't understand why the wood is catching on fire.. Simple answer - it is too damn hot - duh. They make gasoline - you might think they would understand things about heat and fire??!! And wood continually exposed to over 150F starts to lose its structural strength.

Thanks again.

 

[zekeman]

The intensity of radiative heat transferred from a hot pipe to a small surface parallel to the pipe (worst case) is given by:

I*integral W*dx * cos(@)/Sqrt(h^2+x^2)---limits +-infinity

I = radiation intensity=1/PI*sigma*T^4=1/3.14*8000BTU/hr/ft^2 for T=1000 deg F
W=dia pipe, inches
@= angle from elementary pipe surface to normal at receiving surface

The integral is

2*W/h
So the radiation to the small area is
2*W/24/3.14*8000*area=212*W

To get the temperature at the wood surface you equate this to the radiation and convection of that to the surroundins which is conservatively
3*(Twood-70)

The final equation:

3*(Twood-70)=212*W

Twood=212*W/3+70

for W= 2 inch pipe, for example

Twood=212*W/3+70= 212 F

 

[zekeman]

Mike,

My thermo class was was a bit longer than 35 years ago.

Save yourself a lot of trouble.

I wouldn't use the references cited above; at best they are confusing.I tried them to verify my solution but it wasn't working at the time.They depend on you getting view factors which is the heart of the problem.

You could possibly the view factors elsewhere to verify my results.

As a side note,if you are concerned about combustion, wood ignites at over 400 degrees F and , unless your pipe close to a foot in diameter, the wood won't burn.

 

[MiketheEngineer]

Zekeman -

Thanks. That is helpful.

Unfortunately - they often put wood plank within 6''-12'' of a 1,000F vessel. And are amazed when it does catch fire. You just proved it!!

Since this wood plank is often re-used, I am also worried about exposing it to over 150F for any period of time. This plank may stay in this environment for up to 6 months.

I've told them - use steel plank. But at around 450F, the galvanizing starts to let go.

How they work that close - I am not sure, esp in Texas in the summer.

Again, thanks for your help.

 

[zekeman]

"Unfortunately - they often put wood plank within 6''-12'' of a 1,000F vessel. And are amazed when it does catch fire. You just proved it!!"

I thought you said it was pipe and the distance was 2 feet

Makes a huge difference. From the equations I wrote, the temperature rise above ambient is inversely as the distance and directly as the pipe or vessel size, so it would be pretty easy to get combustible temperatures, and it gets worse as wood gets closer, since convectio and radiation to the ambient are getting smaller and were not accounted for.


What is the actual dimensions of the pipe or vessel

 

[MiketheEngineer]

Zekeman -

It is all over the board. This was just one typical example.

Please you have been more than helpful.. I have all the ammo I need to convince the customer that this is definitely not a good idea!!

Thanks again

 

[chicopee]

There has been many house fires caused by wood stove stacks too close to wood framing supporting many of the above responses

 

[gwolf2]

You could use a thin metal heat shield to dramatically reduce the radiation onto the wood.