Effect of radiation on thermocouple

[googool]

Hello
How can i find the effect of radiation on temperature measurement in high temperature stream with thrmocouple for example if i want to measure the high temperature of gas stream in a furnace with thermocouple without shield how much error arise due to the radiation
thx

 

[IRstuff]

What temperature is the furnace?

TTFN

 

[googool]

It is 1100°c

 

[hacksaw]

At those temperatures, radiation is just about all you measure.

 

[IRstuff]

What accuracy?

TTFN

 

[25362]

A combustion furnace is quite a complex environment from the heat transfer viewpoint. The combustion mass contains radiating solids and gases, as well as diathermic (infrared radiation-transparent) gases.

As for the temperature reading error, it all depends on the radiating enclosure; for example, whether the measuring thermowell "sees" a radiating flame or just cooler walls and tubes.

If we call temperatures

T1: the true gas temperature
T2: the temperature reading
T3: the enclosure temperature
T1-T2 = [Δ]T: the error

I think we could say that in the radiant section, where radiating flames are "seen" by the thermowell, T3>T2 [≥] or [≤]T1, while in the convection section, T1>T2>T3.

As hacksaw says the convection heat transfer coefficient, h[sub]c[/sub], would be small in comparison with the radiant one, h[sub]r[/sub], at ~ 1100[sup]o[/sup]C. Thus h[sub]T[/sub] = h[sub]r[/sub] + h[sub]c[/sub] [≈] h[sub]r[/sub].

Have a look at Holman's Heat Transfer example 8-11 for an illustration. The exercise refers to air and a bulb thermometer, but it could also be applied to a thermocouple in a ceramic thermowell. The example refers better to the case of gases in the convection section.

 

[googool]

Thanks for your comments is this true that if i apply 2 thermocouple one with shield and the other without shield (with same characteristic )the difference between result is corresponding of radiation on furnace environment
thx

 

[hacksaw]

It all depends on the details of the thermal environment offered by your process.

There plenty of general responses to your question, but I think you are looking for those that can be substantiated. That requires more detail than is generally suited for this forum.

 

[IRstuff]

While the gas temperature is of importance to whatever process you're monitoring, there are a couple of additional issues:

>> The workpieces also see the thermal radiation, so the question is whether your measurement of the gas temperature alone is meaningful in the context of the process.

>> A decent thermal baffle is non-trivial. You need to allow access to the gas, but not the radiation. This generally requires a multilayer shield, with no direct line of sight of the furnace walls.

TTFN