A bubble cap tray is the oldest type of tray. It was widely used in the early years of chemical process industry because it is suitable for small capacity and has high efficiency.

Now, however, Valve trays and sieve trays are common. Bubble cap units come in a variety of types, the most common of which are a slotless bubble cap and a slotted bubble cap. Vapor flows through the riser pipe and the slot area of the cap to contact the liquid.

The operating range of the bubble cap tray is 10:1 which is so wide that it is easy to operate. If this tray is sealed completely, it can be of use for low liquid and vapor flows due to low leakage rates. Compared to the valve tray, however, it costs high and its process capacity is low. In addition, its hydraulic gradient is uneven, so it leads to pressure loss and unstable operating factors. For these reasons, it is little used for normal distillation; rather, it is mostly used for the wash section of a caustic tower, a glycol contactor, etc.