A screw compressor, comprising: - a first male rotor (200.1) and a second male rotor (200.2), each of the first male rotor (200.1) and the second male rotor (200.2) having convex-helical teeth, the first male rotor (200.1) and the second male rotor (200.2) being rigidly connected together; - a first female rotor (202.1) and a second female rotor (202.2), each of the first female rotor (202.1) and the second female rotor (202.2) having concave-helical teeth, the first female rotor (202.1) and the second female rotor (202.2) being arranged separately from and opposite to each other; wherein the convex-helical teeth of the first male rotor (200.1) are engaged with the concave-helical teeth of the first female rotor (202.1), and the convex-helical teeth of the second male rotor (200.2) are engaged with the concave-helical teeth of the second female rotor (202.2), wherein a first compressing channel is formed between the first male rotor (200.1) and the first female rotor (202.1), the first compressing channel has a first inlet (210.1) and a first outlet (211.1), a first stream of medium flows through the first compressing channel in a first flow direction from the first inlet (210.1) to the first outlet (211.1); wherein a second compressing channel is formed between the second male rotor (200.2) and the second female rotor (202.2), the second compressing channel has a second inlet (210.2) and a second outlet (211.2), a second stream of medium flows through the second compressing channel in a second flow direction from the second inlet (210.2) to the second outlet (211.2); wherein the first flow direction is opposite to the second flow direction; and wherein the first outlet (211.1) and the second outlet (211.2) are combined as a combined outlet, wherein the two ends of the first male rotor (200.1) and the second male rotor (200.2) are amounted on two roller bearings (265.1, 263.1, 265.2, 263.2), respectively; and wherein the two ends of the first female rotor (202.1) and the second female rotor (202.2) are amounted on two roller bearings (261.1, 259.1, 261.2, 259.2), respectively, wherein one of the two ends of the first female rotor (202.1) and the second female rotor (202.2) is amounted on thrust bearings (257.1, 257.2), and wherein the first male rotor (200.1) and the second male rotor (200.2) are not amounted on thrust bearings. - a first male rotor (200.1) and a second male rotor (200.2), each of the first male rotor (200.1) and the second male rotor (200.2) having convex-helical teeth, the first male rotor (200.1) and the second male rotor (200.2) being rigidly connected together; - a first female rotor (202.1) and a second female rotor (202.2), each of the first female rotor (202.1) and the second female rotor (202.2) having concave-helical teeth, the first female rotor (202.1) and the second female rotor (202.2) being arranged separately from and opposite to each other; wherein the convex-helical teeth of the first male rotor (200.1) are engaged with the concave-helical teeth of the first female rotor (202.1), and the convex-helical teeth of the second male rotor (200.2) are engaged with the concave-helical teeth of the second female rotor (202.2), wherein a first compressing channel is formed between the first male rotor (200.1) and the first female rotor (202.1), the first compressing channel has a first inlet (210.1) and a first outlet (211.1), a first stream of medium flows through the first compressing channel in a first flow direction from the first inlet (210.1) to the first outlet (211.1); wherein a second compressing channel is formed between the second male rotor (200.2) and the second female rotor (202.2), the second compressing channel has a second inlet (210.2) and a second outlet (211.2), a second stream of medium flows through the second compressing channel in a second flow direction from the second inlet (210.2) to the second outlet (211.2); wherein the first flow direction is opposite to the second flow direction; and wherein the first outlet (211.1) and the second outlet (211.2) are combined as a combined outlet, wherein the two ends of the first male rotor (200.1) and the second male rotor (200.2) are amounted on two roller bearings (265.1, 263.1, 265.2, 263.2), respectively; and wherein the two ends of the first female rotor (202.1) and the second female rotor (202.2) are amounted on two roller bearings (261.1, 259.1, 261.2, 259.2), respectively, wherein one of the two ends of the first female rotor (202.1) and the second female rotor (202.2) is amounted on thrust bearings (257.1, 257.2), and wherein the first male rotor (200.1) and the second male rotor (200.2) are not amounted on thrust bearings.