Oil-free screw compressor

1. An oil-free screw compressor comprising:

a screw rotor including a screw and a shaft;

a bearing supporting the shaft;

a first shaft seal device fitted on the shaft, disposed between the screw and the bearing, and including a first seal opposite to the shaft, and a first communication portion communicating between a portion of an inner periphery face of the first shaft seal device and the outer peripheral face of the first shaft seal device on a bearing side of the first seal;

a second shaft seal device fitted on the shaft, disposed between the first shaft seal device and the bearing, and including a second seal opposite to the shaft, and a second communication portion communicating between a portion of an inner periphery face of the second shaft seal device and the outer peripheral face of the second shaft seal device on a screw side of the second seal; and

a casing including a rotor chamber accommodating the screw, and a shaft accommodation space accommodating the shaft, the bearing, the first shaft seal device, and the second shaft seal device,

wherein the casing includes an atmosphere communication portion connected to both of the first communication portion and the second communication portion on the inner peripheral face of the shaft accommodation space, and communicating the first communication portion and the second communication portion with an atmosphere.

2. The oil-free screw compressor according to claim 1 ,

wherein the angular position of the first communication portion with respect to the rotation center of the shaft is different from the angular position of the second communication portion with respect to the rotation center of the shaft, and

wherein the atmosphere communication portion includes a connection space formed in a recess shape in the inner peripheral face of the shaft accommodation space so that the connection space is connected to both of the first communication portion and the second communication portion in which the angular positions are different from each other.

3. The oil-free screw compressor according to claim 2 , wherein the atmosphere communication portion includes an external communication portion communicating the lower portion of the connection space with the atmosphere outside of the casing, and an oil collection portion provided between the external communication portion and the connection space.

4. The oil-free screw compressor according to claim 2 ,

wherein the connection space of the atmosphere communication portion is sectioned into a first sectioning region and a second sectioning region by a partitioning wall,

wherein the first communication portion communicates with the first sectioning region,

wherein the second communication portion communicates with the second sectioning region, and

wherein each of the first sectioning region and the second sectioning region includes an external communication portion communicating with the atmosphere outside of the casing.

5. The oil-free screw compressor according to claim 4 , wherein the external communication portion corresponding to the second sectioning region is located downwardly of the external communication portion corresponding to the first sectioning region.

6. The oil-free screw compressor according to claim 1 , wherein the first communication portion is provided so as to be higher than the second communication portion.

7. The oil-free screw compressor according to claim 1 ,

wherein the second communication portion includes a plurality of second communication portions, and

wherein the angular positions of the plurality of second communication portions with respect to the rotation center of the shaft are different.

8. The oil-free screw compressor according to claim 1 , wherein the flow passage sectional area of the first communication portion is larger than a flow passage sectional area between a portion provided on the bearing side with respect to the first communication portion and on the screw side with respect to the second communication portion and sectioning a space in which the first and second communication portions communicate with each other and the shaft.

9. The oil-free screw compressor according to claim 1 , wherein the first shaft seal device and the second shaft seal device engage with each other so as to partially overlap with each other when seen in a diameter direction of the screw rotor.