A devolatilization apparatus comprising: (a) a pump for supplying a pressurized flowable material comprising at least one molten polymer and at least one dissolved or entrained volatile component, (b) a collection-and-volatile-separation vessel, and (c) a multiplicity of plates defining a plurality of heating channels (12, 112, 212, 312), each channel (12, 112, 212, 312) having a uniform height and having two zones comprising: a first zone (10, 110, 210, 310) having an average hydraulic radius, and an inlet (14, 114, 214, 314) adapted to receive the flowable material from the pump, and a second zone (20, 120, 220, 320) constituting the remainder of each channel (12, 112, 212, 312), which second zone (20, 120, 220, 320) is adapted to receive the flowable material from the first zone (10, 110, 210, 310) and has at least one outlet (26, 126, 127, 226) adapted to discharge the flowable material into the collection-and-volatile-separation vessel, wherein at least a portion of the second zone (20, 120, 220, 320) has a smaller hydraulic radius than the average hydraulic radius of the first zone (10, 110, 210, 310), and wherein the design or operation of at least some of the heating channels (12, 112, 212, 312) is such that the pressure of the flowable material at essentially all positions within the first zone (10, 110, 210, 310) of those heating channels (12, 112, 212, 312) exceeds the bubble point pressure of the flowable material, and (d) a plurality of heating elements adapted to heat at least some of the plates so as to increase the temperature of the flowable material as it flows through the heating channels (12, 112, 212, 312), wherein the ratio of the length of the first zone (10, 110, 210, 310) to the total length of the heating channel (12, 112, 212, 312) is from 0.5:1 to 0.998:1. (a) a pump for supplying a pressurized flowable material comprising at least one molten polymer and at least one dissolved or entrained volatile component, (b) a collection-and-volatile-separation vessel, and (c) a multiplicity of plates defining a plurality of heating channels (12, 112, 212, 312), each channel (12, 112, 212, 312) having a uniform height and having two zones comprising: a first zone (10, 110, 210, 310) having an average hydraulic radius, and an inlet (14, 114, 214, 314) adapted to receive the flowable material from the pump, and a second zone (20, 120, 220, 320) constituting the remainder of each channel (12, 112, 212, 312), which second zone (20, 120, 220, 320) is adapted to receive the flowable material from the first zone (10, 110, 210, 310) and has at least one outlet (26, 126, 127, 226) adapted to discharge the flowable material into the collection-and-volatile-separation vessel, wherein at least a portion of the second zone (20, 120, 220, 320) has a smaller hydraulic radius than the average hydraulic radius of the first zone (10, 110, 210, 310), and wherein the design or operation of at least some of the heating channels (12, 112, 212, 312) is such that the pressure of the flowable material at essentially all positions within the first zone (10, 110, 210, 310) of those heating channels (12, 112, 212, 312) exceeds the bubble point pressure of the flowable material, and a first zone (10, 110, 210, 310) having an average hydraulic radius, and an inlet (14, 114, 214, 314) adapted to receive the flowable material from the pump, and a second zone (20, 120, 220, 320) constituting the remainder of each channel (12, 112, 212, 312), which second zone (20, 120, 220, 320) is adapted to receive the flowable material from the first zone (10, 110, 210, 310) and has at least one outlet (26, 126, 127, 226) adapted to discharge the flowable material into the collection-and-volatile-separation vessel, wherein at least a portion of the second zone (20, 120, 220, 320) has a smaller hydraulic radius than the average hydraulic radius of the first zone (10, 110, 210, 310), and wherein the design or operation of at least some of the heating channels (12, 112, 212, 312) is such that the pressure of the flowable material at essentially all positions within the first zone (10, 110, 210, 310) of those heating channels (12, 112, 212, 312) exceeds the bubble point pressure of the flowable material, and (d) a plurality of heating elements adapted to heat at least some of the plates so as to increase the temperature of the flowable material as it flows through the heating channels (12, 112, 212, 312), wherein the ratio of the length of the first zone (10, 110, 210, 310) to the total length of the heating channel (12, 112, 212, 312) is from 0.5:1 to 0.998:1.