Chlorine Header Redesign

Hi:
I am currently working on a chlorine header redesign project for my design class. I have attached a diagram with all information given to me. I am supposed to move the vaporizers closer to the railcar unloading stations and combine the outlets of the three vaporizers into one stream which is later combined with a recycle stream. I don't understand how to calculate the radiation to the pipe from the sun. I understand that there will also be natural convection which my textbook says can be estimated as 40 W/(m^2*K). How do I calculate the heat due to radiation so that I can figure out what temp the chlorine is inside the pipe. Then, I can use the extended Bernoulli equation to calculate the pressure drop through the pipe. Any help will be greatly appreciated.