If you place the vent on the top of the radiator, only the top of the radiator will get hot. The steam will rise to the top on the inlet value end and will just flow to the vent at the top. If you place it in the middle the whole radiator will get steam and supply a much higher heating area of the radiator. The air and steam in the radiator are looking for the way out. By placing the vent in the middle the steam will cover the whole radiator.
I am going to install a burnham radiator, the one that is louvered so I can replace my convectors. this is just an experiment to see if one of my rooms gets warmer. anyway, I noticed a provision on both the top and middle of the radiator for a steam vent. I have a one pipe steam system. Where would be the best place to put it. I figure on the top, but why would they put an option to install it in the middle then? Thanks for your help.