A vapour tower was a device for maintaining the critical temperature of PentoCyleinicMethyldrane at 205 centigrade. On Pluto, the Company used them to maintain their brainwashing of the citizens. There were eight present in Megropolis One, operated from Main Control.
When the Fourth Doctor raised a rebellion, they targeted the control in order to stop the output of PCM. Mandrel, who had experience working in Main Control, deactivated the towers and maintained the temperature at 70 centigrade. (TV: The Sun Makers)