plain tube

例句：

1. 1. The engineer used a plain tube to connect the two pipes.
   工程师使用一根直管将两根管子连接起来。
2. 2.The pipe was made of brass and had a plain tube at the end for easy cleaning.
   这根管子是用黄铜做的，末端有一个直管方便清洗。
3. 3.The plain tube is not suitable for high-pressure applications.
   这种直管不适合高压应用。