Tray refers to the levels in a distillation tower at which hydrocarbons re-condense resulting in separation based on boiling point.

The trays use bubble caps to allow hydrocarbon flow and to encourage separation. Bubble caps are small, floating caps over holes in the tray that allow vapor to rise through the tray but keep liquid from flowing back down. This results in a pool of liquid on the tray with vapor bubbling through it, resulting in good separation of hydrocarbon based on boiling point.