The Influence of Air Circulation, Jet Discharge Momentum Flux and Nozzle Design Parameters on the Tightness of an Upwards Blowing Air Curtain

Air curtains are used to eliminate airflows through doorways (doorway tightness). In this paper the impacts of air circulation, jet discharge momentum flux and nozzle design parameters on the tightness of an upwards blowing air curtain are presented. The tightness values were measured using a tracer gas decay method in the laboratory hall. Measurements were undertaken for two nozzle widths, 30 mm and 120 mm, and for two different discharge angles, 10° and 30°. According to the study, the upper limit for the tightness of the upwards blowing air curtain is 80%. The difference in tightness between a recirculated and non-circulated air curtain is significant, being 20%. The selection of the jet design parameters can be undertaken fairly freely as long as the force balance at the doorway is valid. When the jet discharge momentum is optimal, the effect of the nozzle width, discharge angle and discharge velocity on the tightness of the air curtain is small. To keep the tightness level of the air curtain high enough, regardless of outdoor temperature fluctuation, a jet velocity controlling system is recommended. In the typical outdoor temperature range for Finland (-30°C to +10°C), the lack of a jet velocity controlling system reduces the tightness of the air curtain by  20 to 60% depending on the nozzle width and discharge angle. When selecting the jet design parameters by the moment of momentum principle in a relatively tight building (n50-value < 4.2 1/h), it is recommended to use a value of 0.6 - 0.7 times the doorway height for the stack neutral pressure height in order to achieve an airtight air curtain and to avoid the breakthrough phenomenon.