Embodiments of the invention provide an apparatus or process for devolatilization of flowable materials (such as molten polymers with entrained or dissolved solvent or unreacted monomers or comonomers) using a plate heater having heating channels, the design or operation of which heating channels maintains the flowable material above its bubble point pressure during passage through a larger first zone and then induces flashing in, or downstream of, a smaller second zone of the heating channel. The apparatus enables a higher throughput per heating channel while achieving equivalent or better devolatilization, as compared to current devolatilization apparatus.