Others have pointed out problems with what you are trying to say, since the pallet car doesn't actually do any sintering, but merely coveys the pellets through the sintering process. As much as I personally prefer to use as few words as possible in a sentence, sometimes it's better to be verbose to help retain your intended meaning. After all, language is a tool for conveying information, and not just a nice way to string words together.
So while there are many "good" ways to form this sentence, this comment by TRomano is easily understood:
A pallet car rides on a moving belt which conveys the car and its cargo of pellets to the sintering stage; the car is designed to allow air to sweep through the charge layer as the belt moves along.
But to focus on the question posed in your title: it's not good English style to mix the infinitive and the gerund when forming lists or comparisons of related objects. Consider the following two sentences:
All she enjoys is to dance, to play, and singing.
All she enjoys is dancing, playing, and singing.
The second sentence is good style because all of the elements of the list are in the same form. Similarly the second sentence here is better than the first:
You should check your spelling, grammar, and punctuate properly.
You should check your spelling, grammar, and punctuation.
When describing what a particular mechanism does in a mechanical process, keep this rule in mind, as in the following:
The conveyor belt is designed to catch the outflow from the folding process, deliver the product to the sanitizing spray, and then deposit the sanitized product into the holding bin.