Liquid flows across a tray deck toward the outlet weir. The liquid overflows the weir, and drains through the downcomer, to the tray below.
Vapor bubbles up through the sieve holes, or valve caps, on the tray deck, where the vapor comes into intimate contact with the liquid. More precisely, the fluid on the tray is a froth or foam—that is, a mixture of vapor and liquid. In this sense, the function of a tray is to mix the vapor and liquid together to form a foam. This foam separates back into a vapor and a liquid in the downcomer. If the foam cannot drain quickly from a downcomer onto the tray below, then the foamy liquid or froth will back up onto the tray above. This is called flooding.