This paper describes a simple model-free (i.e., empirical) control strategy for crystal size tuning in a continuously operated tubular crystallizer. The crystallizer is designed for a seeded cooling crystallization process and acetylsalicylic acid crystallization from an ethanol solution was used as model system. Using a crystal size distribution (CSD) analyzer and minor initial studies, we developed a feedback controller that accurately tuned the mean crystal size within the range of 90–140 μm. In addition, we created a cleaning concept for long-term runs based on a consistency study, which demonstrated that the CSD of the products remained robust when process settings were kept constant. Sufficiently small and uniform seed crystals were generated via ultrasound irradiation.