Yeah I find they do get burnt. Don't think you can buy replacements since these were never actually made for people to rewick themselves. The idea was you just chuck away the hole atomiser section when its too 'manky' and screw on a new one. I have chucked the sleeve away in one and not had any problems. I would suggest that if you don't want to replace it,rewick it and try it without it, but hang on to it just in case. You can always repeat the process and add it in.
Cheers,
DocVape
I haven't got a nova and haven't rewicked anything but is the wick tube there to help hold more juice near the wick but also to stop the positive wire that runs through the centre from being kinked and touching the atomiser body. Not sure if this is possible as I have been known to talk out of my arse before