Simple Heat Transfer Problem... I think 2

[apieron]

Hey guys,

I have what I think is a simple heat transfer problem. Basically, I have a 4" schedule 40 carbon steel pipe with a 3"x3"x1/4" carbon steel angle iron welded to the top.

- The pipe has 58 psi steam @ 290 degrees F (4 bar @ 144 degrees C)
- It is in an enclosure with 700 CFM of air flow
- Ambient temperature inside the enclosure is 122 degrees F (50 degrees C)
- This is at atmospheric pressure

I am interested in knowing the temperature profile of the system at equilibrium and the time to reach equilibrium.

Please see attached rough sketch. Let me know if anything is missing.

I know this would be simple to model on an Autodesk like program, unfortunately I don't have access to the program I need.

Thanks in advance.

 

[Latexman]

I think you need the length of the pipe, or the CFM/foot of length.

Good luck,
Latexman

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[apieron]

Length of pipe is 10500mm or about 34 feet.

 

[VerneE]

A Quick Approximate Solution

34 feet of 4" pipe has a surface area of about 40 Sq. Ft. As a first guess, assume the 700 CFM of air is heated to 290F (from 122F) which requires the steam to condense enough condensate to heat the air. The heat required for 700 CFM to raise in temp 168F to 290F is 700 x 168 x 1.08 = 127,000 Btu/hr. However, the U value of a 4" steel pipe is only about 8 Btu/hr/SF so there is not enough surface area on the pipe to heat the all the air. The 3" x 3" angle adds about 6" of heat transfer surface to the pipe (several assumptions made here - this is an approximation, I am not prepared to show all the details of this approximation - look at Crane's Engineering for information), the total heat transfer surface area of the piping + angle iron is about 60 sq. ft. so we can say Q = U x A x DelT or 8 x 60 x 168 = 81,000 Btu/hr).

One could suggest that the 3" x 3" angle iron has about 1 SF of surface area per foot of length which would make the total heat transfer area of 80 SF, I won't argue with that, that would just be different assumptions for this quick approximation.

My guess is that the 700 CFM of air will be heated to about 229F and the steam will condense about 90 lbs per hour of condensate (58 psig steam heat of vaporization is about 917 BTU/lb) based on the system surface area. The Steam Piping will remain at about 290F. If a detailed heat transfer analysis is done on the surface of the angle, I suggest it will remain about 290F or at least above 280F, so heat transfer calculations will not change that much based on the angle iron added heat transfer surface.

This is my ballpark guess. The steam pipe will not change temperature (assuming it continues to be supplied with steam). Equilibrium will be reached withing seconds of the 700 CFM of air being supplied.

If the steam is not continuously being supplied to the pipe, then the temperature will decrease in time, use the above calculations to approximate temperature profile over time, take into account the anmount of steam in the pipe, the mass of the piping & angle iron and Cp of steel.

 

[VerneE]

Sorry, I didn't look at the sketch of the pipe & angle iron. This doesn't affect my approximation that much, just that the area of heat transfer surface is about 47 SF instead of 60 SF. All calculations should be adjusted accordingly and are left as an exercise to the reader.

 

[EnergyMix]

apieron

Out of curiosity, does this have anything to do with your previous question:378-339949? Is this an existing steam line that you are trying to repair with a piece of angle iron?

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[zekeman]

Without detailed information about the airflow, you will at best get a wild guess for an answer. Do yourself a favor when you post a problem. Give ALL of the facts and sketches so we understand the problem.

Specifically,tell us how the air flows around the pipe.

 

[apieron]

I made the post because I wanted to learn about the problem, which is why I put "Let me know if anything is missing."