Treat it like a long, single tube indirect heat exchanger. You will need to know your external air temperature and velocity. If you put the pipe in a ventilated hallway with climate control, you should be OK with a simple calculation.

If you want to put it outside or in a large room with high variability in air flow and temperature, you will have a variable temperature coming out the other end of the pipe. Maybe you can compensate for it with cooling or heating lines on the outside of the pipe and tie into the control system.

If there is just uncertainty and not a lot of variability in the air flow and temperature, you can try to solve the problem with CFD and/or use an experiment during commissioning and insulate until the temperature comes out right. This method would require you to have a reasonable prediction of exposure length in advance and probably quite a bit of buffer length so you can insulate (cheap, low maintenance) rather than install cooling line (more expensive, higher maintenance).

Look at this

http://www.eng-tips.com/viewthread.cfm?qid=100490

This is a simple heat transfer problem. You need to know the following information:

1) The heat transfer coefficient inside the pipe.

2) The thermal conductivity of the material used for the pipe.

3) the wall thickness of the pipe.

4) The air heat transfer coefficient outside the pipe.

From the above information calculate the heat flux leaving the pipe outside surface.

From that heat flux, you can calculate the temperature drop across the pipe.

The heat transfer equation can be obtained from the following textbook:

Heat Transfer: a practical approach, second edition by Y. Cengel, McGraw Hill publisher.

Hope this helps answer your question.

Professor Yehia Khalil

Yale University

USA

I think that your case is a simple air cooling system, but I think that you should know the temperature you want your fluid to be cooled to in addition to what Prof. Yehia said. Then you can use any manual or software for air cooling system calculations to get the area and other specs of this system to satisfy the cooling duty you need 

Your problem will be the effects of wind which leads to a variable convection coefficient at the pipe's outside surface. This could be partly corrected by lateral walls which limit its influence. The other factors are the internal friction which generates heat and the solar energy on the pipe surface which becomes not neglectable if the pipe is long and you are in hot part of the earth. Last could also be corrected by a screen over the pipe. In this case you can assume an "almost" free convection.

In general the thermal resistance of the outside convection is such that the conductive resistance of the pipe wall can be neglected. 

However the internal convection will be temperature dependent ( specific volume does change and accordingly velocity ) which makes the computation much more difficult.

In a 1st approximation you could -in order to estimate the temperature drop magnitude - consider both convections as constant. Result is an exponential drop since with this simplified system you obtain a differential equation of the 1st order. You could repeat with parameters for the mean temperature a couple of times. If not you should make a system simulation which is quite complex.

Your Problem comes under cross flow across cylinder.

You need to calculate Heat Transfer Coefficient , it can be calculate from Nusslet number

Assuming Turbulent flow inside pipe

Nu=0.023RePow(3/4)Prpow(n)=hd/k

From the above equation h can be calculated.this h value is for fluid inside the tube.

For air htc it can be calculate from Churchill and Bernstein equation

0.3+ 0.62 Re1/2 Pr1/3[1  (0.4/Pr)2/3]1/4[1 + Re282,0005/84/5]

From the above htc value of air can be calculated.

Now we are having HTC of fluid inside pipe

HTC of air outside

Properties of air and Fluid inside pipe.

Hope this helps!!