Steam drum: 

The steam drum divided the steam from the heated water. Water droplets fall to the bottom of the tank to be cycled again and the steam leaves the drum and enters the steam system. The Feedwater enters at the bottom of the drum to begin the heating cycle.

Downcomers - Downcomers are the pipes in that the water from the steam drum travels in sequence to reach the bottom of the boiler whereas the water could enter the distribution headers.

Distribution headers - The distribution headers are huge pipe headers which hold the water from the downcomers to the risers.