Abstract: A horizontal heat treatment device continuously subjects an untreated continuous flat object to heat treatment while horizontally transferring the untreated object within a heat treatment chamber. Seal chambers are interconnected to the untreated-object loading opening and treated-object unloading opening of the heat treatment chamber. A passage is connected to an opening of each of the seal chambers, the opening located on the side opposite the heat treatment chamber. The untreated-object passage loading opening interconnected to the untreated-object seal chamber loading opening and the treated-object passage unloading opening interconnected to the treated-object seal chamber unloading opening are the untreated-object loading opening and treated-object unloading opening of the heat treatment device. A pair of gas ejection nozzles are provided at upper and lower positions of the passages. The nozzles eject gas in specific directions, and the nozzle openings have a specific shape, a direction, and a length.

Abstract: A carbon-fiber-precursor acrylic fiber bundle which can smoothly pass through a flame-resistance impartation step and a carbonization step. The carbon-fiber-precursor acrylic fiber bundle has a high-density part as a portion thereof, wherein the high-density part satisfies the following requirements (A) and (B). Requirement A: The high-density part has a maximum fiber density ?max of 1.33 g/cm3 or higher. Requirement B: The portion extending between an intermediate-density point and a maximum-density-region arrival point has an increase in fiber density of 1.3×10?2 g/cm3 or less per 10 mm of the fiber bundle length.

Abstract: A horizontal heat treatment device continuously subjects an untreated continuous flat object to heat treatment while horizontally transferring the untreated object within a heat treatment chamber. Seal chambers are interconnected to the untreated-object loading opening and treated-object unloading opening of the heat treatment chamber. A passage is connected to an opening of each of the seal chambers, the opening located on the side opposite the heat treatment chamber. The untreated-object passage loading opening interconnected to the untreated-object seal chamber loading opening and the treated-object passage unloading opening interconnected to the treated-object seal chamber unloading opening are the untreated-object loading opening and treated-object unloading opening of the heat treatment device. A pair of gas ejection nozzles are provided at upper and lower positions of the passages. The nozzles eject gas in specific directions, and the nozzle openings have a specific shape, a direction, and a length.

Abstract: Provided is a carbon-fiber-precursor acrylic fiber bundle which can smoothly pass through a flame-resistance impartation step and a carbonization step. The carbon-fiber-precursor acrylic fiber bundle has a high-density part as a portion thereof, wherein the high-density part satisfies the following requirements (A) and (B). Requirement A: The high-density part has a maximum fiber density ?max of 1.33 g/cm3 or higher. Requirement B: The portion extending between an intermediate-density point and a maximum-density-region arrival point has an increase in fiber density of 1.3×10?2 g/cm3 or less per 10 mm of the fiber bundle length. The term “intermediate-density point” means the site which has a density ?m that is intermediate between the fiber density ?o of the non-high-density part and the maximum fiber density ?max. The term “maximum-density-region arrival point” means the site Pr at which the increase in fiber density per 10 mm of the fiber bundle length becomes 1.