Can i use tubing for drain-water under my sink?
I don't know about Sweden, but here in central U.S. a kitchen sink will be 1 1/2" tube size. This is different than 1 1/2" DWV which is in the wall, but again I don't know how plumbing is run in Sweden. Cost wise, tubing is not much less than the tubing to run a trap, so I would honestly skip the tubing, and run hard pipe. It will be less apt to give to any expansion or contraction with temperature changes, and overall be more reliable in the long run.