How does a trap primer work?

An automatic trap primer works by pumping water into a drain trap. The water is sent from a cold water line in small amounts each time the line runs water. There must be an air gap between the drain water and the fresh water line. The simpler manual trap primer is provided by your usage of the drain. For a seldom used drain pour water into it every few months. A little mineral oil in the drain will slow the evaporation by sealing the water from the room air.