Gas supply blowout nozzle and method of producing flame-proofed fiber and carbon fiber

A method for producing a carbon fiber, satisfying the following (a) to (c):

• (a) introducing a carbon-fiber precursor fiber bundle widened into a sheet shape into a flame-proofing furnace comprising a gas supply blowout nozzle, so as to perform a flame-proofing treatment thereon in a temperature range of 200° C. to 300° C. and introducing a flame-proofed fiber obtained by the flame-proofing treatment into a carbonizing furnace so as to perform a carbonizing treatment thereon in a temperature range of 500° C. to 2500° C.;
• (b) blowing hot air blown out from the gas supply blowout nozzle toward the carbon-fiber precursor fiber bundle horizontally traveling through the flame-proofing furnace;
• and
• (c) in the gas supply blowout nozzle, satisfying the following conditions i and ii:
• i. a difference in pressure between the gas immediately before introduced into the gas supply blowout nozzle and the gas immediately after blown out from the gas supply blowout nozzle is 160 Pa or less;
• and
• ii. a non-uniformity in a wind speed of the gas blown out from the gas supply blowout nozzle is 35% or less,
• wherein the non-uniformity in the wind speed is obtained as follows:
• the wind speed is measured at five points in a direction perpendicular to a yarn traveling direction downstream 2 m from an end surface of the gas outlet port of the gas supply blowout nozzle;
• and
• the non-uniformity in the wind speed is calculated by the following formula (5):
• non-uniformity in the wind speed={(maximum value−minimum value) of wind speed×100}/{(average value of wind speed at five positions)×2} (5),
• further satisfying (d) to (f):
• (d) in the gas supply blowout nozzle, satisfying the following conditions iii to vii:
• iii. the gas supply blowout nozzle comprises a nozzle body and a rectification board, wherein the nozzle body comprises an inclined plate guiding the flow of the gas flowing from a gas inlet port directly in a straight line to the rectification board, and the rectification board is directly attached to the nozzle body so as to rectify the flow of the gas guided by the inclined plate and to blow the gas toward a yarn;
• iv. a gas guiding zone is formed between the inclined plate and the rectification board and one or more guide plates are disposed in the gas guiding zone;
• v. the passage width W1 perpendicular to the gas flowing direction inside a gas passage formed by the inclined plate and the guide plate and a gas passage formed between the guide plates and any downstream passage width W2 thereof satisfy the relation of W1≥W2;
• vi. an opening area A of the gas inlet port and an opening area B of the rectification board satisfy the relation of A≤B;
• vii. a volumetric rate of the gas supplied to the gas inlet port is from 36 m3/min to 115 m3/min;
• (e) dividing the gas supplied to the gas inlet port of the gas supply blowout nozzle into two or more streams by the one or more guide plates near the gas inlet port and guiding the gas to the rectification board by the inclined plate and the one or more guide plates;
• and
• (f) blowing the gas rectified by the rectification board from the gas outlet port in parallel to the yarn traveling direction.