Temperature increase from solar radiation on black pipe

[erinsquest]

I'm working on a problem that deals with 132°F slurry flowing through black HDPE piping above ground for 1800ft. The max ambient temperature is 115°F, but the pipe is only useful for fluids up to 140°F. I need to show calculations for a temperature increase due to direct normal solar radiation, but since we always considered it negligible in school, I'm not sure what to do.

 

[saxon]

erinsquest, Check your "Thermotext" under the headings, Black & Grey bodies. See also, Kirchhoff's law and Stefan/Boltzmann's law.

Hope this helps.
saxon

 

[hacksaw]

you cannot ignore the insolation at low velocities.

 

[sailoday28]

Check with ASHRAE. In particular with solar flux variation at your latitude and time of year.

 

[JKEngineer]

The ASHRAE Fundamentals handbook will give some information about the solar loading. (as cited by sailoday28)

I have used it to estimate the thermal history of walls and similar.

Note that you also need to calculate the thermal transport through the pipe, into the liquid, and from the surface of the pipe back to the ambient. This latter term will include convection (you will need wind speed, but can use zero for the worst case), radiation to the sky and surroundings, heat transfer around the circumference of the pipe (may not be significant), and conduction to the ground or supports (may not be significant). For a worst case estimate you could ignore the conduction around the pipe, to the support, and the effects of wind. You should not ignore the radiation from the pipe's surface.

Let me know if I can help.
Jack

Jack M. Kleinfeld, P.E. Kleinfeld Technical Services, Inc.
Infrared Thermography, Finite Element Analysis, Process Engineering

http://www.KleinfeldTechnical.com

 

[erinsquest]

I've found that conduction and convection are almost negligible, and that the solar flux is approximately 240 BTU/ft^2 per day. I'm trying to show that we will need to bury the pipe, so worst case scenario is what I've been looking at (no wind). Thank's for the replies, but I'm still not sure what to do. Please help.

 

[saxon]

erinsquest, Just as an aside and a hint, you're running that line awful close to the max. allowable temp. Have you checked the press./temp. ratings of the pipe? When it comes to polymer materials, press. rating drops off significantly at higher temps. Also, plastic lines just don't have the mechanical strength for the abuse that they see in industrial applications. If I were you, I'd look at something a little more robust. Then there wouldn't be this problem in the first place.

Hope this helps.
saxon

 

[sailoday28]

You stated a solar flux of 240.
Have you taken direction of sun into account? I hope the solar flux is not on the entire surface area?
Have you considered transient effects?
Solar heat flux varies over the day.
Piping and fluid will absorb heat with the varying solar heat flux and take time to heat up.
Radiation from the pipe to the sky?
Have you considered the probabilities of no wind.
Check with weather bureau on the probabilites for the worst conditions which might also include cloud cover effect.
Is there a possible covering/painting that could minimize solar insulation?

 

[sailoday28]

weather bureau
In my last response, please note that cloud cover will help minimize overall impact of incoming solar radiation. But you worst day with regard to ambient temp might have cloud impact help you.

 

[crysta1c1ear]

If I understand you right, you are worried about the pipe overheating at 140°, ie the sun adding another 8° to your 132° slurry.


Just as a guess, your going to have a problem, as 132 and 140 are prety close, and black as a color is a good absorber of heat.

=
I don't think you need to worry about the sun's direction and clouds, etc, since you are calculating for a worst case. The worst case is direct sunlight and no clouds.

I think you are going to need to consider the length of the pipe, its diameter, and the flowrate through it, as well as ask yourself whether the fluid inside is mixing as it goes along and forming a uniform temperature, or if its forming a temperature gradient (hot on the top sunny side and cold at the bottom).

Why worry about flowrate?
If your liquid is slow, pretend it is standing still, the sun bakes the pipe and it gets hot.

Why worry about diameter?
A thin pipe carries less volume for its surface area and so the insides will heat quicker.

Why worry about pipe length?
Like flowrate, its going to be down to how long a bit of slurry in the pipe is warmed by the sun, so a long pipe is going to make things worse.

Warming from the sun can be 1 kilowatt per square metre. Check my figure on solar cell web pages! From the slurry flowrate and pipelength you know how long the slurry is in the sun for. Eg if its in the sun for two hours it will receive 2 KW hours of heat, per square metre of pipe lit by the sun.

From the pipe diameter, you can work out what length of pipe is needed for a square metre of sunlight to hit it (eg if its 1/3 metre in diameter, then three metres in length are needed to make a square metre), and what volume of slurry is in that length of pipe (area is half pi radius squared, and volume: multiply by length). Then with density information, what weight of slurry is in that bit of length of pipe.

Then with the specific heat capacity of the slurry, you could work out how much the sunlight could warm up the slurry, eg could 2 Kilowatt hours in my example warm up the slurry in 3 metres of pipe by your 8 degrees? (140°-132°)

==

You obviously need to use your own figures.

If the liquid is not mixing as it flows, then there is even more chance the stuff at the top could get hot. And you could consider heat radiating off the pipe too.

I'm not really trying to do it all for you, just give pointers about what might need to be considered, how I would approach it, and provide a basis for discussion.

To be honest, I wouldn't use a pipe that can only handle 140° for a 132° liquid if there was the remotest chance it could be hotter or heat up later.

 

[sailoday28]

erinsquest (Chemical) ORIGINALLY WRITES
"I need to show calculations for a temperature increase due to direct normal solar radiation"
crysta1c1ear (Automotive)states

"I don't think you need to worry about the sun's direction and clouds, etc, since you are calculating for a worst case. The worst case is direct sunlight and no clouds."

I believe the calc should be conservative, but need not make worse condition assumptions occuring at the same time.
Weather bureau stats etc. (or ASHRAE) should give a reasonable conservaative design condition.

 

[denniskb]

I have written an excel calculation that solves the simultaneous equations involved and can run some cases for you if it helps. You will need to give me all available information on the fluids, pipe, surroundings and environment.

Fluid
Description, Temperature, Density, Heat Capacity, Conductivity, Viscosity, Flowrate.

Pipe
Description, Material, Diameter, Wall Thickness, Length, Outer Surface Condition

Environment
Air Temperature, Solar Radiation Level, Proximity of any buildings or structures, temperature of surroundings.

If you don't have everything I will guess a value.

My calculation can easily run several cases to test sensitivities to varying conditions. It will find rate of heat transfer and temperature rise as well as the equilibrium temperature.

Don't take too much notice of the black pipe or reflective coatings as they will only work short term. The smallest layer of oxidation or dust will quickly change the performance.

Don't ask for the software though, it took a long time to develop and is not available for free. I will need an email address to forward the results in pdf format as I can't post attachments here.



Dennis Kirk Engineering

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

 

[sailoday28]

denniskb (Mechanical) Writes

Don't take too much notice of the black pipe or reflective coatings as they will only work short term. The smallest layer of oxidation or dust will quickly change the performance.

I agree, however, one must weigh the cost of maintance vs. what is probably huge cost of burying pipe.