Drying is a process that is widely used in the food processing industry. There are several factors that influence during the drying process, one of which is the speed of air flow in the dryer unit. The air flow velocity inside drying unit can occur by forced airflow and natural airflow. Natural air flow is the flow of air created due to the difference in pressure caused by differences in temperature and air density, so there is no thrust by the blower. This study aims to estimate the natural airflow velocity in modeling system unit. This modeling system unit has a source of heat energy that can be used to adjust the room temperature level of the heat generator, and is equipped with a chimney that has an adjustable height and diameter. This research was conducted experimentally, through empirical mathematical model development using dimensional analysis. The variables measured include: heat generator room temperature (TS), system outside air temperature (TL), chimney diameter (d), chimney height (H), exhaust air flow velocity (V). The results of this study obtained mathematical model of airflow velocity. The mathematical model showed good agreement due to measurement results with average of errors was about 9%.