Build a nice looking pipe with the arch tool, then:
Create a brush and tie it to a trigger_push entity. If you want to suck the player upwards, you will need to set the angle to up, and the amount of push to around 3000. note that for some reason, if you walk into a trigger push that is facing up, you have to jump to be thrown upwards. For an example look at the fan at the bottom of the "snarc pit" map (A hl multiplayer that comes with the game). To overcome this, the player has to fall into it somehow. So a tiny step-overable ledge should over come that, of if your pipe goes from floor to ceiling, just have it dropping down into the blackness. Another note... don?t ever suck the player down a pipe, or he will fall to his death. Use trigger_gravity at the top of the pipe to lower gravity first, then reset it at the bottom.