cleaning a restricted steam coil

zepfan

What is the best way to go about cleaning the internal piping of a steam preheat coil? Pumping acid through it, compressed air,or some other way.We have a job that has a large preheat coil attached to an air handler where the supply pipe is hot to touch,midway through the coil it is stone cold.The coil directly next to it is operating fine.It does not appear any steam is flowing through the coil. There is a trap on the outlet side,and if the union is taken loose on the exit side of the trap,very little if any condensate comes out. The coil is 40 years old,and I don't know what else it could be but over the years when the steam has condensed in the coil,the piping has become restricted.Any suggestions would be much appreciated,thanks to all.