11 are described as follows a slightly concave curtain 72 when an air blowing velocity an air supplying velocity and an air sucking velocity are adjusted to obtain a proper ratio an air isolator formed by the air from the door 12 is slightly concave inward the bsc when the flow is close to the suction slot 141 the air flow is pulled down to be prevented from flowing outside or inside the bsc b straight curtain 71 when the air sucking velocity is smaller than the above one and thus is weak the air isolator formed by the air from the door 12 is straight and not concave with no circulation formed in the cabinet c severely concave curtain 73 when the air sucking velocity is big the air isolator is moved inwardly and is severely concave and thus obvious circulations are formed in the bsc d oscillating curtain 74 obvious circulations are generated inside and outside the bsc with the air isolator swinging in and out of the bsc at different times with the above descriptions concerning the four types of flow fields it is suggested to adjust push pull velocities of air to obtain a slightly concave curtain 72 for operations.