Air ingress by buoyancy-driven exchange flow occurs during a standpipe rupture accident in a high-temperature engineering test reactor (HTTR). The exchange flow of helium and air through annular and Round tubes is investigated. The method of mass increment is applied to measure the exchange flow rate. A test cylinder with a small tube on the top is used for the experiment. The following results were obtained: The exchange velocity is largest for the short vertical round tube as compared with the orifice and long tube. In the annular tube, the exchange-velocity or the volumetric exchange flow rate decreases with the equivalent diameter of the annular passage under 6 mm. The annular tube is effective to reduce the air ingress flow rate from the broken standpipe of the HTTR. In the inclined round tube, the inclination angle for the maximum densimetric Froude number decreases with the increase of the length-to-diameter ratio of the tube for the helium-air system. On the other hand, this angle remains almost constant for the water-brine system. Flow visualization results indicate that the exchange flows through the inclined round tubes take place smoothly and stably in the separated passage of the tube. The flow pattern in the vertical annular tube seems to be similar to that in the inclined round tube.