Temperature Increase in Pipeline 4

[gator1991]

I have a 16 inches (outside diameter)chilled water pipeline (cast iron) that is 80 feet long. Water is flowing through it with an initial temp of 45 degF. The line is exposed to an air temperature of 55 degF and I would like to know the temperature at the end of the pipeline. The flow rate is 1000 gpm.
I'm trying to generate some kind of pipeline heat gain table using the exposed pipe vs the insulated pipe.
Can anybody helps?

 

[corus]

Thanks dennis but the outside air temperature is 55F and not 50F so I don't know if that will alter your calculation any.

The value of 0.21 you refer to was quoted by 25362 and they are referring to the value of emissivity which has no units.

I'm a little surprised that radiation is so dominant as from my experience radiation only becomes dominant in free surfaces to air when the surface temperature is into the hundreds of degrees celsius, whatever that is in F. However, are you then using that amount of heat flow from the outer surface you have calculated into a similar equation I derived that involves the water flow rate?

corus

 

[denniskb]

Corus,

Sorry my mistake, the last line of temperatures should read

Air 12.77°C 55.00°F

Also I meant to put the 0.21 query to 25362 and when I re-read his post you are right that it is unit free. He actually stated this as absorptivity rather than emissivity.

My calculation could always be wrong though I have tested it against various text book examples and been able to match the results. The texbooks, however, do not usually provide much for combined HT cases.

25362,

If you provide me with some alternative numbers for absorptivity and emissivity I would be happy to re-run the calculation with different values.

I am always looking for additional data to add into the application library and so improve the calculations, though I always try to collect the source reference for each new piece of data. Likewise I can provide much of the data in my library to those interested.

I have been looking for a way to make a pdf printout of the calculation available via Eng-Tips but this does not appear to be possible. I will try to add a new download section to my website this weekend where these things can be accessed.


Dennis Kirk Engineering

http://www.ozemail.com.au/~denniskb

 

[25362]

Denniskb

Holman's table 8.3 compares absorptivities of various surfaces to solar and low-temperature(~25^oC) radiation. A striking difference is given for cast iron: 0.94 and 0.21, respectively.

My estimates are based on a 50 year-old nomograph. This one gives a value for h_r = 0.8 in british units, based on the given temperatures and [ε] = 0.9 for the pipe, to be used in the formula for heat flux:

q_r = h_r*[Δ]t​

According to this graph, q_r = 0.8*10 = 8 Btu/(h*ft²) => 8*3.155 [≈] 25 W/m².

As I said, it is a very old source, and it may be totally wrong.

 

[denniskb]

25362,

The surface finish of the cast iron (like many other materials) can have a dramatic impact on its radiative properties.

For steel I have;
Absorp-Solar Absorp-Ambient Emissivity
Clean 0.45 0.2 0.55
Light Rust 0.8 0.8 0.55
Heavy Rust 0.95 0.95 0.87

There is lots of data about on emissivity (due to the proliferation of hand held IR thermometers) and for Cast Iron the values again vary widely with surface finish.

New (polished?) 100°C 0.05
New unoxidised 100°C 0.21
Oxidised 100°C 0.74
Heavily Oxidised 104°C 0.95

The 0.21 absorptivity number looks likely to be for new clean material (as was much of the older data collected from laboratory tests rather than the condition likely in the field).

Further it is essential that both the absorptivity and emissivity values are taken from the same sample as the two act closely in determining the radiant transfer. I tried the clean steel data above on this pipe and the pipe was a nett rejector of heat rather than an absorber?

I have added a pdf file of my calculation to my website (look for miscellaneous downloads) so you can have a look.


Dennis Kirk Engineering

http://www.ozemail.com.au/~denniskb

 

[25362]

Dennis, how can I reach miscellaneous downloads from your main page ? Thanks.

 

[denniskb]

Go to "Design Calculations"

Fill out the form and "submit" from bottom of page

Last item on page click "here"

Select the file for download.




Dennis Kirk Engineering

http://www.ozemail.com.au/~denniskb

 

[jhartis]

You might want to check out the NAIMA 3E Plus software. Its free and does heat loss calcs for insulated and bare pipes, etc. Did I mention its free?

http://www.pipeinsulation.org/pages/download.html

 

[denniskb]

Yes, it is free and it has some very useful data attached to it. It is however targeted at marketing insulation products and providing comparisons between alternative insulations.

While the application is quite clever in some ways it provides very limited control over the inputs and methods used. It does not deal with solar heating at all. Overall it does not provide a thorough solution which is quite disappointing considering the effort that has gone into it.

I tried this case using 3E Plus with the following results;
Method 1 48.69 W/m
Method 2 43.35 W/m
My Software 104.38 W/m


Dennis Kirk Engineering

http://www.ozemail.com.au/~denniskb

 

[AndrewLindsay]

Dennis,

What formulas have you used to perform the calculation. I would like to calculate the 'black steel' temperature of a pipe that is exposed to sunlight.

Any assistance would be appreciated.

Regards

Andrew Lindsay

 

[gator1991]

Andrew I got the formulas from this website

http://sti.srs.gov/fulltext/fulltext-1998.htm

The name of the document was Heat Transfer Model of Above and Underground Insulated Piping Systems
I look for it but I couldn't find it you will probably have to ask them about it.

Good luck

 

[AndrewLindsay]

Gator1991,

I've had a look at the paper (managed to find it by typing the title into google).

I want to calculate the temperature of a pipe that is exposed to sun light. Where do I put the heat loading into the equations?

hb (equation 5) appears to be a boltzmann type equation, but I am not sure how to use this. Say I have a 350 W/m2 solar radiation hitting the pipe at 90°. How do I calculate the actual surface temperature of the pipe. I am actually interested in the stagnant case, where there is no flow in the pipe, and could simply assume that the pipe is a solid steel rod.

Any ideas would be appreciated.

best regards

Andrew

 

[25362]

Andrew

Holman's Heat Transfer has a worked exercise (example 8-10) on a similar subject on solar radiation.
I suggest you have a look at this example. Good luck.

 

[denniskb]

Andrew

The temperature of the plate will reach around 50 - 60°C.

The Solar absorption will be around 330 W/m2
The Bacground absorption will be around 300 W/m2

The Surface emmissions will be around 550 W/m2
The Convection loss will be the balance.

I have posted to my website a pdf output from my software for this case Solar_PipeHT_Dev.pdf.

I have also posted an Excel solution which will allow you to solve for other conditions. The solution comes from Incropera and DeWitt Example 12.11.

Enter the pipe length and diameter.
Enter the solar radiation level - range 0 > 1000 W/m2
Enter the sky temperature - range -43 > 0°C
Enter the ambient air temperature
Provide absorptivity for solar - range 0.45 > 0.95 for steel
Provide absorptivity for ambient - range 0.2 > 0.95 for steel
Provide emissivity - range 0.55 > 0.87 for steel

Enter the surface temperature and increase this until the overall heat transfer = 0 (equilibrium)

You can force the convective load to 0 in the blue cell for the case where no air movement is present (e.g. ambient = steel temperature)

The "Heat Transfer Model ...." paper has some useful information but does not deal with either solar heating or radiant heat transfer with the sky. As you can see above these have a major impact on the calculations.

Try running a case with the sky temperature down at -35°C to see how much this matters. (This is the temperature you often see displayed in an aeroplane in flight and on a clear day is quite visible to a pipe on the ground. This is also what make water freeze at night in the desert)

In the center of Australia on a very clear day it is not unusual to have 1000 W/m2 Solar radiation and -43°C sky temperature at the same time. Pipe temperatures in excess of 90°C can be experienced.

Be careful with the absorptivity and emmissivity numbers from the text books as a small layer of dust can significantly affect the real value.


Dennis Kirk Engineering

http://www.ozemail.com.au/~denniskb

 

[AndrewLindsay]

Dennis,

I've looked at your site, but can't seem to find the documents you have described.

Regards

Andrew

 

[corus]

Surely the temperature of the sky is -273C approximately?

corus

 

[TexasHank]

The sky temperature seems to be a great discussion subject. Atmospheric resistances and reflections may be the reason why -273C can not be used. I would be curious to hear what values on the sky temperature has been shown to give reliable results (assume clear sky, and cloudy respectively)?

 

[sterl]

The mechanism of heat transfer from Warm Air to Cold Pipe changes drastically with the Dew Point temperature of the air...and a slight breeze will tend to decrease the corresponding heat gain rather than increase it.

The condensing effect may not be dominant for a 45 deg. F. Pipe in 55 Deg F. Air but will almost certainly be dominant with 65 deg air. For slow moving fluid in a large pipe having considerable potential for radiant gain, there will develop a sharp line of dry surface(top side of horizontal pipe) to wet surface (bottom) which means the actual rate depends on time of day and also means a North-South pipe tranfers heat differently than an East West...

This a phase change, gravity driven mechanism and very dependent on the pipe orientation, for a vertical pipe will actually invert the interior natural convection effect in a still pipe full of cool water.

The overall result remains the same: the heat gain is negligible from a temperature or nominally from the energy standpoint but the destructive and contaminatory effects of the condensation and drippage, even on CI pipe, is usually cause for minimal insulation anyways....unless, of course, the 55 deg air ondition and movement makes condensation impossible.