Forced convection in a pipe with thermal resistance

[hamidun]

Hi all,

This is my first post in this forum.

I need help regarding forced convection in a pipe with thermal resistance, i have been trying to solve this problem but i am hesitant about the thermal resistance in the pipe. I don't know how to relate the flow in pipe with the thermal resistance, because we can't just leave the thermal resistance, right?

I have been also trying to search for example of this problem, but all of them always disregard thermal resistance of the pipe.

Any idea for this?
Thanks for the help!!!

 

[IRstuff]

I think you need to describe your problem in more detail. What are you trying to accomplish?

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[LittleInch]

What do you mean by "the thermal resistance in the pipe"?? If you can explain that then you might get some help, but very few pipes I know create "thermal resistance".

More details or a diagram (hand drawn is fine) really really help get across what is in your head into others. Just scan it and upload via the link at the bottom of the posting box - "...or upload your file to ..."

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[AWloo]

Are you trying to calculate the overall heat transfer coefficient by trying to find all thermal resistances?

 

[hamidun]

Thanks for the response.

Ok, so this is the case. I want to calculate heat transfer in pipe, it's PVC which has thickness 15mm. There is water flowing in the pipe 500m3/hr. So, I consider this is forced convection inside the pipe, but as my pipe has thickness, it means the pipe has thermal resistance, am i right?

I already calculate the heat transfer, first assuming no thermal resistance, and surface temp is constant along the pipe, the result is 42 kW with delta T is 3degC (Twater-Tsurface with no thermal resistance which is Tsurface = Tambient). After that, i calculate the thermal resistance, with the same properties, by referring to thermal circuit concept, I got the result which is only 88 W, so it means that from the pipe to the surface can transfer 42 kW, and from surface to ambient only 88 W, but this result is not what i'm hoping. I want to transfer heat as much as possible. Is it possible? or because the pipe is PVC, there is no chance to transfer much heat?

What makes me confused is, how actually internal forced convection works? If there is fluid flowing in the pipe, it's supposed to be internal forced convection, right?

 

[IRstuff]

What are your water and air temperatures? Are you expecting people to guess? What is the area of the pipe?

Your first example should have loss through natural convection into the AIR from the pipe.

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[hamidun]

IRstuff

Water temp = 40 deg C
Air = 35 deg C
Pipe Area = 2.4 m2

But actually, what i want to know is how to calculate internal forced convection in insulated pipe

 

[IRstuff]

again, the internal forced convection is not important. You are completely limited by the thermal conductivity of the PVC and natural convection of the air, coupled with the fact that you only have a 5C temperature delta. I get only 67W transferred with 10 W/m^2-K air convection coefficient and 0.19 W/m-K for the PVC. The temperature drop across the PVT is 2.2C and the drop across the air is 2.8C, so the air is the limiting case.

Your example is suspect, because even with the full 5C delta across the air, the power transferred is only 120W

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[hamidun]

So, we don't have to consider flow rate of water in the pipe? Are you using Steady State Heat Loss calculation? Is that ok?

I'm really confused about this internal forced convection. Like i said, i have been searching this kind of problem, but i couldn't find it. Or internal forced convection is not considered when there is thermal resistance in the system? Is that acceptable?

 

[IRstuff]

If the flow is slow enough, the yes. The total heat flow of the water should be compared to the convective capacity of the outer surface. Your flow rate implies 2.9MW of power, but the air convection is only a maximum of 120W, so the water is not cooled at all, essentially. if your flow rate dropped down to 0.05m^3/hr, then starts to be critical.

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[marty007]

This is a very typical heat transfer problem.

From your description this is a simple series resistance thermal problem. The heat must transfer through three resistances:
1) Convection from bulk fluid to pipe wall.
2) Conduction from inner pipe wall to outer pipe wall.
3) Natural convection from outer pipe wall to the air.

I haven't run the numbers, but from IRstuff's post it appears that the relative resistance of the convection inside the pipe is very low, so has little to no affect on the overall problem solution. The resistance of the pipe wall and natural convection dominate your problem. Essentially the inner pipe wall temperature will equal your bulk fluid temperature in this case.

Cheers,

 

[hamidun]

The flow is 500 m^3/hr, isn't it big enough?
And the pipe conductivity is 0.19 W/m.K.

As I said, I already calculate it, but because of this pipe thermal resistance, the heat transfer is so low, is my calculation right?

And I want to know how internal forced convection work? If it's in my case, how to apply internal forced convection with thermal resistance? I have been searching this problem, but i couldn't find it. I hope someone can help me with this question

 

[IRstuff]

Your questions were already answered.

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[hamidun]

Ok then. Thank you so much IRstuff.

 

[LittleInch]

Hamidun, you've been firing random numbers and figures at us and asking us if it's correct, but no workings or any idea as to what it is you're trying to do. I think the think revolves around your quote "I'm really confused about this internal forced convection. Like i said, i have been searching this kind of problem, but i couldn't find it"

for a flowing pipe full of liquid or gas which is flowing in the turbulent zone, there is no internal convection as you seem to be thinking. Part of the calculation to determine temperature loss or heat "output" will use the thermal conductivity of the pipe and which part is the critical part will vary with each system. describe yours as fully as you can and some more help may appear, but IRstuff has done a good job in answering the questions you have raised.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[hamidun]

LitteInch,

Actually, I'm going to have a loop test which I will recirculate the water for the system.

In the system there will be Open Storage Tank > Pipe > Pump > Pipe > Deaerator > Pipe > Pressure Recovery Vessel > Pipe > Pump > Pipe > Open Storage Tank.

There are three rotating equipment in the system which they will add heat into the water, am i right? Because this is a loop test and will be run non-stop (say) 12 hours, the rotating equipment will continuously heat the water. Besides, these equipment will most likely be placed exposed to the direct sun. (Properties of the system are above).

So, what i want to do is, i want to make the system balance (heat out = heat in) by calculating the heat transfer from the Pipes, Storage Tank and Pressure Recovery Vessel. Is that possible?

Right now, i'm calculating heat transfer from the Pipes. And thanks for the explanation, and now i know the heat transfer in the pipes wouldn't release heat much.

Next, i will calculate heat transfer in Open Storage tank which i expect will release heat much, because the surface water will contact directly to the atmosphere, am i right?

Thanks for the help!

 

[IRstuff]

Not necessarily. The same heat transfer coefficient quoted earlier applies to water to air as well.

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[IRstuff]

I would urge you to find someone who understands heat transfer to work this. You continue to make arbitrary assumptions, and possibly ignoring other factors that might be more important.

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