Estimating how far away a cable rated for 90 C has to be from a stack

[SlavaU336]

Hello folks,

How would you go about estimating how far away a cable rated for 90 C has to be from a stack?
This cable is a 4-20 mA signal from a temperature transmitter installed on the stack itself.

I thought I would estimate the thermal radiation from the stack using AERflare spreadhseet (Publicly available).

Then add on top of that (worst case scenario):
- Radiation from sun, convection with worse wind blowing hot air from the stack to the cable, neglect any conduction and somehow come up with a worse case temperature.

What do you guys think? I'm just curious how you would approach this!?

 

[IRstuff]

how hot is the stack?
what is its emissivity, since that's the most likely thermal load?
Can you wrap the cable with metal or reflective film?

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[trashcanman]

I would treat it the same as a fireplace chimney. Bldg. codes dictate how high about the roof and how far away (horizontally) the muzzle of the stack must be from a flammable surface and add your choice of a safety factor.

 

[SlavaU336]

@trashcanman : That's a good idea. I'll look up the code, Thanks!

@IRstuff : I have stack temp and emissivity. But then I'm stuck at what equation to use.
If I need to use a radiative view factor I'm using the "From small hemisphere to a large sphere).
But the result I get for the cable temperature doesnt make sense. I must be missing something.

I'm isolating T1 based on the equation below. Since I know the heat flux q(2=>1)(W/m^2)from the stack, I just assumed Q(2=>1)/A1 = q(2=>1)
Q(2=>1) = A1 x F(1=>2) x sigma x (T2^4 - T1^4) x Epsilon2

With:

Q(2=>1) = Heat transfer rate (W) from surface 1 to surface 2
q(2=>1) = heat flux (W/m^2)
A1 = Surface 1 (m2)
F(1=>2) = View factor from surface 1 to surface 2 (Using 0.5*(1-SQRT(1-1/h^2)+1/2*h^2)) with h = H/R with H = 1m and R=0.305m = 2.71
sigma: Boltzmann constant = 5.67e-8
T2 = Temperature of surface 2 = Assumed to be around 480 (K)
T1 = Temp of surface 1

epsilon2 = emissivity of surface 2 = 0.3 (Not sure - its a steel stack)

Surface 1 = Cable
Surface 2 = Stack

Writing equations is painful here, any tips for that?
I'm attaching my spreadsheet for the braves lol. Easier than typing all that stuff.

 

[IRstuff]

Did you not read anything about the equation you chose? You essentially placed the cable almost next to the stack. Your spreadsheet link doesn't work

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[IRstuff]

see p18 of

http://imartinez.etsiae.upm.es/~isidoro/tc3/Radiation View factors.pdf

for strip view factor of cylinder

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[pierreick]

Hi,
Could you use an infrared camera or pyrometer to check at the wall along the stack? This will give you more confidence than calculation?
My 2 cents
Pierre

 

[SlavaU336]

Thanks guys, good idea @pierreick
I'll check this @IRstuff, thanks!

 

[TugboatEng]

All of this focus on the cable but what about the 4-20mA signal amplifier? Those also have temperature limits. I would consider using a thermocouple and running a high temperature extension cable rated for your worst case scenario. Convert to 4-20mA in a location that isn't as harsh.