heat transfer for stability chamber

[StephenJcan]

Here's a problem I have at work and I was hoping someone could help me out. I haven't had to do a serious heat transfer problem since college.

A stability chamber that maintains a constant 104F 80% RH room has sprinklers serving it.
The 1" sprinkler pipe extends down into the room by about 18", it goes through a 3" foam insulation metal panel. Let's assume it's 1/4" thick stainless steel sheets sandwiching the 3" foam insulation.
The 1" sprinkler pipe extends 36" above this foam insulated roof and the pipe connects to the 3" fire sprinkler main.

The problem I'd like to try to work out is. If the sprinkler system was just filled with 60F water and the stability chamber is at 104F, what would be the lowest temperature of the pipe located inside the stability chamber?

There is passive convection inside the pipe, conduction in the water and the pipe. Any one have a few suggestions. I was planning on calculating the overall heat transfer from the room into the pipe inside the room versus the heat rejection of the pipe above the room and making a guess on the convective value for the water in the pipe.

Here's a sketch of the problem.

 

[trashcanman]

I think there are too many unknowns. Length of 3" pipe? Surface conductivity of pipe above ceiling? K-factor for the foam insulation? K-factor for the panel? Is the air circulating at the ceiling?

If you are trying to eliminate heat loss to the sprinkler piping, here is what to do:
1. Provide electric heat tracing on the sprinkler piping to maintain 104 Deg.F.
2. Provide insulation over the heat tracing.

With equal temperatures, there will be no heat transfer between your room and the sprinkler piping.

 

[IRstuff]

"what would be the lowest temperature of the pipe located inside the stability chamber?"

At what time? At infinite time, it's basically going to be at the temperature of the chamber, since there is no active flow in the pipe.

TTFN
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[StephenJcan]

Hello guys, sorry for the poor problem scope. Here's a simplified version of the problem. I'm leaning towards a cylindrical fin with an adiabatic tip but would like some feedback as I'm really rusty with heat transfer.
So a bit of background.
The chamber is at 104F and the 3 feet of sprinkler pipe (1 foot in the chamber, 2 feet outside of the chamber) contains air at say 104F.
The sprinkler mains are then filled with 60F water. The 3' of sprinkler pipe is a dry type head such as the ones uses for loading dock areas. So the 3' of pipe is filled with air and the 60F portion is the mains filled with water.
I'm trying to calculate what the lowest surface temperature of the 12" of pipe in the chamber will be once the sprinkler mains are filled.
If someone can show me how to do that I will try to calculate for varying lengths of pipe above the chamber, 3' 4' etc. to see where the breakpoint is if there is one.
Hope that clears it up some. It's not a simple problem at least not for me and it's tough to explain.
Any advice or assistance is appreciated.

 

[IRstuff]

I'll have to retract my earlier comment. The worst case looks to be about 74ºF

http://files.engineering.com/getfil...7bc-4e11-a4cd-634c98bbe708&file=sprinkler.mcd

http://files.engineering.com/getfil...369-d32f37c504f6&file=Mathcad_-_sprinkler.pdf

TTFN
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[keefw1956]

Don't forget that air at a constant 104F 80% RH will have a dew point temperature of 97F.

 

[StephenJcan]

Thanks for running that IRstuff. I'll try to download Mathcad and take a gander.
I'm doing the calcs by hand using my heat transfer text, trying to get a rough estimate.

So from your calculation even though the pipe is a dry head the lowest surface temperature of the pipe in the chamber will get down to 74F?
That seems very unlikely to me but I wanted to make sure I was understanding you right.

keefw1956 The dew point is 97F and that is the problem, at that temperature the pipe starts sweating and they can't have that in the chambers.

I'm going to model the pipe that is outside as a pin fin with a base temperature of 104F and calculate for the maximum length of 30" of dry pipe to see what happens.
I am going to try to calculate the interior temperature of the air in the pipe when/if the pipe surface temperature gets as low as 98F to see if the heat transfer to the pipe inside the chamber overcomes the heat loss of the pipe above the chamber. If it does not I'm going to have to do a calculation on 3/4" elastomeric insulation for the 30" of dry pipe above the chamber to see if that will help.

Thanks again for your advice.

 

[IRstuff]

It surprised me as well. Note that I used copper for the pipe, so that's a big factor.

TTFN
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