Metal surface temperature (heat trough a wall)

[Spoonful]

Hi All,

I have a question here, a pipe with inside medium temperature known and outside surrounded by air, pipe dimension also know, eg thickness, diameter, material thermal conductivity. How to find out the metal surface temperature on the outside of the pipe? I would assume the metal temperature on the inside of the pipe is same as the medium?

Could be a very simple question for you experts. or is there to many unknown?

 

[IRstuff]

"I would assume the metal temperature on the inside of the pipe is same as the medium? "

Depends on the thermal conductivity and thickness of the pipe.

As for the surface temperature, by using the continuity relationship, you can equate the conducted heat flow to the convected heat flow from the surface to the air. My last post in thread391-378531 has a similar situation, but with both inner and outer surface temps unknown.

TTFN
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[racookpe1978]

1. How accurate do you need the answer to be?

2. What is the fluid flow rate, pipe ID, and pipe diameter? To calculate anything useful, you need to estimate/know/specify/guess a fluid velocity and the fluid properties to get the actual assumed fluid film coefficient.


3> horizontal or vertical pipe?

read the previous thread on concrete wall temperature. Your problem is the same. But completely different. 8<)

 

[Spoonful]

1. I guess the approximate answer would be both inside and out side metal surface temperature would be almost the same as the fluid temp? But i was hoping to get an answer that the OD surface is actually cooler. but how much cooler

2 let assume 610 diameter, 2mm wall. flow rate unknown, can we assume the inside can keep maintain it hotness regardless of ambient temperature.

3 dose orientation matters?

Q = thermal conductivity * area * delta temperature / thickness
There are two unknown in this equation, Q and dT. What I want to find out is dT.
how to find the heat flow?

Thank you

 

[IRstuff]

That's why you need to use the continuity relationship, i.e., heat flow is conserved.

Nevertheless, 2-mm thickness of most metals like cast iron or carbon steel will only result in about 0.03C drop, so the assumption that the wall temperature is not unreasonable. However, the heat flow per meter of length is more than 500W, so the temperature of the fluid will drop as it travels down the pipe, unless the thermal mass flow is sufficiently high enough to mitigate that.

TTFN
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[corus]

The metal temperature on the inside of the pipe will tend to the medium temperature as the heat transfer coefficient (htc) increases towards infinity. You could consider that the metal temperature is the same as the medium for a worst case scenario for your case. The other case would be that the htc is close to zero and in that case the temperature through the pipe wall will be constant and equal to the outer air temperature. For an accurate solution you'd need to calculate the actual htc based upon the fluid velocity, pipe diameter etc. On the outer wall heat loss would be through natural convection and radiation to the ambient air. This varies with wall temperature but is about 10 W/m^2 K at low temperatures. The value increases with wall temperature. If you can calculate the temperature dependent values then there's a spreadsheet on excelcalcs.com that'll calculate the pipe temperature.

 

[racookpe1978]

Comes back to the original questions: How much fluid flowing at what speed with what physical properties (density, thermal coef's and friction coef's => Those start to get you film coef (Reynolds numbers, Nu and Pa numbers, etc.) starting at what temperature in the pipe?

Does orientation of the pipe matter?

only if you want to get the right answer? How long a pipe running in what direction beginning at what temperature fluid inside the pipe is ESSENTIAL in figuring out how much heat is lost into a room filled with stagnant air when all of the heat energy is going to be lost by natural circulation!

Are you going to assume the room temperature air is going to stay the same? If so, what is the hottest expected room temperature? Or is the problem the maximum heat lost, and you are worried about the lowest expected room temperature?

With no other information, the answer is, obviously, 6.37 Or 1361.2 8<)

 

[Spoonful]

Thanks for all the reply, maybe I should better re-define the question. When designing a piece of the pipeline equipment, all wetted part will be designed to the same temperature of the fluid medium, normally high temp alloys used here. However for the hardware, that is attached to the OD, preferably to use plain carbon steel here. Here is the question, what design temperature should be used here? As partial of the hardware is subject to the heat transferred from the "pipe" OD, while all other surface is surrounded with air.

 

[1gibson]

Interesting start for a question of saving money on bolting. Stop with the questions and just state the problem with all the required information. Is it 500*F or 1000*F?

 

[IRstuff]

THAT'S what you were asking? Given the thickness you specified, then all the metal needs to be designed for the high temperatures. That ignores the safety question of having exposed high temperature surfaces that can cause flash 3rd degree burns.

TTFN
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