Vapor Chambers are thin planar heat pipes, which have the same primary functional areas as tubular heat pipes. These areas are a hermetically sealed hollow vessel, a working fluid, and a closed-loop capillary recirculation system.
Compared to a one-dimensional tubular heat pipe, the width of a two-dimensional heat pipe allows an adequate cross section for heat flow, even with a very thin device.
In the one-dimensional heat pipe the vapor is flowing along the tube center through a constant cross sectional area from the "hot" to the "cold" end.
In the Vapor Chamber the evaporation takes place in the center area and the steam flows to all direction from this hot spot, therefore the cross section area is growing with the distance from the evaporation zone.