A chemical vapor deposition reactor has one or more deposition zones bounded by gas flow virtual walls, within a housing having closed walls. Each deposition zone supports chemical vapor deposition onto a substrate. Virtual walls formed of gas flows laterally surround the deposition zone, including a first gas flow of reactant gas from within the deposition zone and a second gas flow of non-reactant gas from a region laterally external to the deposition zone. The first and second gas flows are mutually pressure balanced to form the virtual walls. The virtual walls are formed by merging of gas flows at the boundary of each deposition zone. The housing has an exhaust valve to prevent pressure differences or pressure build up that would destabilize the virtual walls. Cross-contamination is reduced, between the deposition zones and the closed walls of the housing or an interior region of the housing outside the gas flow virtual walls.