In a twin-tower desiccant type air dryer using silica gel as an adsorbing agent, what is the procedure for rejuvenating the tower presently on service when the desiccant becomes saturated with moisture?

Correct Answer: Option D — The towers are shifted, and heat is applied to the off-going tower.

Twin-tower desiccant dryers operate on a cycle where one tower adsorbs moisture while the other is regenerated. When the active tower reaches saturation, the flow is shifted to the standby tower. Heat is then applied to the off-going tower to drive off the moisture, preparing it for the next cycle.