Air curtains are devices used mainly in doorways to reduce the penetration of heat, moisture or contaminants through the open doorway, while enabling the passage of people, vehicles and other transportation. Usually an air curtain consists of one or two jets blown horizontally or vertically across the door opening. The jet is decreasing the free air movement through the doorway, thus reducing the transportation of heat and mass through the opening.
The technical dimensioning of an air curtain often rests on an insufficient basis when it comes to the influence of the building envelope and the ventilation system. The air curtain has a vital connection to both these elements and this has to be taken into account properly in the dimensioning procedure. This second part of the paper focuses on thermal aspects. A theoretical approach to determine the thermal loss through the doorway, based on defined jet velocity and temperature profiles is presented. A comparison with empirical results is carried out, a simplified equation for the thermal efficiency of the air curtain is derived and a procedure for the technical dimensioning consisting of 10 steps is presented. The thermal loss through a properly working air curtain is due to the turbulence generated entrainment-spill process. The thermal loss is proportional to the thermal loss coefficient and the square root of the jet momentum flux. The jet impact point has a strong effect on the thermal loss. The door dimensions and the neutral pressure level are also major factors influencing the thermal performance of the air curtain. To keep the loss close to the minimum in real conditions where the pressure across the jet is changing, a control for the jet is required. 11 refs.