Low-pressure steam dropwise condensation on durable PFA-coated horizontal tube: Droplet dynamics in active region

•An ‘active droplet region’ on the horizontal tube which is characterized by droplet dynamics is observed and proposed for the first time.•The effect of vapor pressure on the droplet dynamic behaviors in the active region is studied systematically.•Taking the vapor pressure and circumferential angle into consideration, a semi-empirical for the prediction of droplet departure radius is established. The standard deviation of this model is reduced to less than 15%.