| 摘要 |
<p>At least one duct with flexible walls can be blocked by throttling of the wall which is kept in throttling contact with the roller which is progressively moved along the wall to displace fluid. A duct protector of pref. PTFE is between the duct and the roller to reduce wear and stretching of the duct and is flexible enough for the roller to throttle the wall and block the duct. Pref. the roller and wall may also be of PTFE.</p> |
