A ventilation
system with hardware to direct two independent airflows, wherein an outer
airflow (at least partially) surrounds the an inner
airflow. Preferably the inner and outer airflows come from different sources such that the air of the inner
airflow has higher associated costs than the air of the outer airflow. For example, the inner airflow may be more cleaned and / or more thermally conditioned than the outer airflow. The inner airflow may have a different velocity than the outer airflow. Preferably, the ventilation
system is a PVD, but other applications are also contemplated, such as: (i) spot-cooling; (ii) air curtains (for example for a refrigerated food section or case in a market); (iii) medical devices (for example,
oxygen supply devices, “blankets” of thermally conditioned and / or cleaned air for infants, patients in
surgery or burn victims, shields of cleaned air for protection against microbes for patients with compromised immune systems); (iv) personal
humidifiers; (v) personal dehumidifiers; (vi) thermal and / or
humidity control for
plant or animal enclosures; and / or (vii) any other ventilation
system for controlling at least one type of air characteristic (see Definitions section).