Calculate where the dew point ends up ?

[frma71]

Hello, how do I calculate where the dew point in/on a pipe occurs?

As an example with the input parameters:

Temperature of the liquid in the pipe: 2 degrees
Outside temperature: 25 degrees
Humidity: 50%
Insulation thickness: 15mm
Lambda 0.04 W/mK
If it matters, the pipe diameter is: 32mm

/Fredrik

 

[useless]

6 mm into the insulation assuming that the outside of the pipe is 2 degrees and the temperature gradient in the insulation is linear.

 

[frma71]

Thank you, that sounds reassuring. How did you arrive at that figure?

One cannot assume that the outside of the insulation is the same as the outdoor temperature, otherwise, there would never be a problem with condensation, right?

The relevant question might be what temperature the outside of the insulation will have?

6 mm into the insulation if one assumes that the outside of the pipe is 2 degrees and the temperature gradient in the insulation is linear.

 

[useless]

On the outside, the insulation will have the same temperature as the room. Condensation problems only arise if the insulation is diffusion-open so that condensation can occur inside the insulation (except perhaps in the special case of 100% RH in the room).

 

[useless]

The calculation:
50% RH at 25 degrees results in the dew point being around 14 degrees.
If the temperature gradient is linear, 14 degrees is located approximately 60% into the insulation.
With 15 mm thick insulation, the dew point is located 9 mm from the inside/6 mm from the outside.

 

[frma71]

Is it really so? In that case, the dew point will always end up in the insulation, and then I can basically have as thin insulation as I want.

Then I'm quite sure that if I (taking an extreme case) for example had 1mm insulation, it would be colder on the surface than the surroundings.

Am I thinking completely crazy?

/Fredrik

On the outside, the insulation will have the same temperature as the room. Condensation problems only occur if the insulation is vapor-permeable so that condensation can occur inside the insulation (except perhaps in the special case with 100% RH in the room).

 

[useless]

Then I am pretty sure that if I had (take an extreme case) for example 1mm insulation, it would be colder on the surface than the surroundings.

That is because the air closest to the insulation will be cooled down. If there is a sufficiently high airflow around the insulation, the temperature will be almost the same as the air. This is the same phenomenon that causes, for example, unprotected skin to feel colder in windy conditions than when it is still outdoors. It is not because the air becomes colder as it moves, but because you have a warmer air layer closest to the skin that becomes thinner the more it blows.

 

[frma71]

I agree that if the air moves a lot, the surface temperature will be roughly the same as the air. In this case, the pipe is in a joist space, so apart from convection, there's basically no movement.

This is because the air closest to the insulation will be cooled off. If there is a sufficiently high air exchange around the insulation, the temperature will almost be the same as the air. It's the same phenomenon that causes, for example, unprotected skin to feel colder when it's windy than when it is calm outside. This isn't because the air gets colder when it moves, but because the warmer air layer nearest to the skin becomes thinner the more it blows.

 

[useless]

In that case, you should consider the stagnant air or insulation in the floor joist as part of the pipe insulation and adjust the temperature gradient accordingly.

 

[frma71]

Hmm, this is really difficult, no wonder the plumber can't answer the question

If you account for a lot of standing air around it, the dew point will always end up outside the pipe insulation. If I embed the pipe in insulation, I move the dew point outward, etc, etc.

The only thing that would make this theoretically safe is to place the pipe inside another pipe and ensure there is air movement inside

In that case, you need to consider the standing air or insulation in the floor structure as part of the pipe insulation and adjust the temperature gradient accordingly.

 

[frma71]

Paroc has a calculation program, wondering how they avoid infinities in the calculations with a wind speed of 0 m/s ?

https://calculus.paroc.com/paroc-calculus/index_se.html#/

 

[useless]

wondering how they avoid infinities in calculations with wind speed 0 m/s?

Even stationary air conducts heat, but not very well.

 

[frma71]

Well, but it doesn't matter. If you look at an extreme case with infinitely much air around the pipe, it always insulates infinitely and moves the dew point infinitely far from my expensive pipe insulation.

My conclusion is that you can't calculate this without making some sort of well-tested simplifications of the surroundings.

Even still air conducts heat, but not particularly well.

 

[Håbbe01]

Sure, but it doesn't matter. If you consider an extreme case with infinitely much air around the pipe, it will always insulate infinitely and move the dew point infinitely far from my expensive pipe insulation.

My conclusion is that you can't calculate this without making some sort of well-tested simplifications of the surroundings.

If you have the surface temperature of the pipe, this should work
https://www.dimensionera.se/fukt/daggpunkt.php

 

[frma71]

Yes, unfortunately I got stuck on the exact surface temperature.

If you have surface temp on the pipe, this should work
[link]