# Insulation of Thermocouple

1. Aug 13, 2013

Dear Seniors and Friends

We have a thermocouple consisting of two pipes . In the inner pipe there is the flue gas and the outer pipe we have a cooling water . Currently we are facing a problem that gases are being condensed below 100 C because of cooling water .We have to apply insulation between the inner pipe and the cooling water so that gas outlet temperature is limited to 100 C.

What approach can be used of insulation thickness so that condensation can be avoided .
Flue gas enters at 1500 C and should leave at above 100 C to avoid condensation.
Cooling water enter at 20 C .

Thabking you in anticipation

2. Aug 14, 2013

### sanka

Am I correct in saying that you have some sort of double-pipe heat exchanger? In any case, there a probably a few things you can do to avoid condensation of the gas, although I am not sure if your test-conditions will permit some of the suggestions.

1. You can reduce the flow rate of the cooling water. This will increase the thermal resistance, maintaining a higher gas temp.
2. If you can increase the temp of the cooling water. This will reduce the heat transfer rate.
3. As you suggest, you can insulate the wall between the 2 fluids. Something like an air layer or inert gas would be good as they have low thermal conductivities.
4. A thicker wall (possibly a composite wall, containing multiple layers of different materials with low thermal conductivities)

Thats about all I can think of at the moment, good luck with it.

3. Aug 15, 2013

Thakyou Sanka to give me a quick response ....First two point that u told me is really helpful for me and 3rd point is impossible for use air or inert gas in my case. I have to apply insulation between gas pipe and coolig water. Can you help me which approach i have used to get insulation thickness.

Thanks

4. Aug 16, 2013

### sanka

To get an estimation for the thickness you require, you can use Fouriers Law of Heat Conduction which is;

$\dot{Q}$=kA$\frac{dT}{dx}$

where dx is the thickness of the medium through which the heat travels (i.e. the insulation thickness). For a given material you will obtain some k value (thermal conductivity), you know the area. You need to assume a temperature difference (dT): in your first post you said the gas cannot go below 100°C and cooling water is 20°C, thus, you dT should be based on this. Remember it is the temp of the hot fluid AT THE WALL - the temp of the cold fluid AT THE WALL.

Also, you will need to know Q to solve the equation, you can get this from an energy rate equation:
Q=$\dot{m}$CpΔT

This leaves one unknown in the equation and by manipulating the conduction equation, you can solve for the thickness, dx.