is it possible to download an entire series at once?
if this is possible it would save me a massive amount of time

No it's not.
The best you can do is just open a full page of pictures in new tabs at once, download them, close them, go to the next page, and repeat.
Some series have over 200 pictures so it can be a pain but I guess if you're that dedicated to the series, then why not.

Edit by IGC
Do not do that, if you are caught, you will be temporarily banned!

You cannot, because that will use all our bandwidth and not give us any pageviews... which help out...
Since the site is completely free, users clicking on the ads is the only change we have to recoup any of the money we spend on the server.

Also, if you do what Neotoxiblitz suggested, you might get banned... I usually check the site load, and if I see a user with several processes running at the same time, I usually put a temporary ban on them, so I do not recommend doing that, since you are using up a bunch of resources...

???
I have often open more tabs (3-6) at once (by taging, validating, etc.). It is bed?

It depends on how many you open... 3 to 5 is not that bad, specially if you just open the tabs, but if you are also downloading on each tab, it is 5 tabs + 5 downloads = 10 processes running, specially for big images.

If it is only from time to time, that's alright and I will probably not notice, but I've seen users/bots with 10/20 processes opened at the same time. If you are a registered user, I'm usually more forgiving, but if it is a guest, it usually means a ban with this message: http://www.theanimegallery.com/excessiveUse.html, and that's all you see no matter what page you try to access.