An air curtain is most often used to separate one environment from another. It does this by creating a "curtain" of moving air that is projected over the opening of a doorway. The wall of air acts as an invisible barrier that will not allow air to flow through it.
Typically, the air curtain is mounted over a doorway on the inside, and as a result, the air pulled into the intake of the unit is from the conditioned environment. This air is accelerated and forced through a narrow discharge along the length of the air curtain, creating a laminar airflow.
The discharge angle can be adjusted using the provided turning vanes to achieve optimum performance. As the discharge angle increases, the air has to travel further until it hits the floor. The velocity of the air decreases as it gets further away from the air curtain; if the discharge angle is too large, the air curtain won't be able to stop much wind from entering near the floor. If the discharge angle is too small, the air leaving the unit will not have enough horizontal force to stop a significant gust of wind from entering. Usually, the ideal discharge angle for maximum wind-stopping capability is around 15 degrees.
When the discharged air reaches the floor, it splits, forcing some air outward and some inward. This is why, when a unit is used for climate control, the air curtain should be placed on the opposite side of the doorway from that of the air that is to be kept out.
For example, if it is winter and you are trying to keep cold outside air from entering a building, the air curtain would be placed on the inside so that it is blowing warm air. When the warm air hits the floor, some is leaked to the outside and some is blown back inside. If the unit was on the other side of the opening, some of the cold air that you are trying to keep out would be blown inside, defeating the purpose.
However, if the unit is to be used for insect control, the air curtain can be mounted on the outside of the doorway as long as the discharge is adjusted so the air is blowing back toward the outside. In this situation, it doesn't matter if some of the outside air is blown inside after it splits at the floor. This air should already be free of insects because they will not fit through the intake screen.