When I get start to build a vent in this model, I had some problems of pinching when I smooth the model, but luckily I found the solution after I read the post of kirigoi, thanks for the useful imformation to solve my problems.
this is before I use the new solution
And now I have a new question about distribute the curves of using the new solution. I am not sure if this way will work, but at last I get a better result after smoothing
this is the upper vent
this is the lower one
I will be grateful if any suggestion and advices to improve this model, thank you !!