A vapor belt is an annular distributor located at a shell nozzle. They have an internal sleeve with a cut-out that typically has a larger area than the nozzle cross-sectional area. The cut-out is usually oriented 90 degrees from the nozzle centerline.
Annular distributors serve to reduce shell-side pressure drop, reduce rho v squared and vibration potential, and improve exchanger end zone cross flow effectiveness.
On fixed tubesheet units, annular distributors may also function as expansion joints.
-Christine