I see this all the time, but I don't know that I've heard it spoken.
For example: インストール中
Is it simply "なか"?
The reading when used as a suffix in this case is ちゅう (the on-yomi of the character 中)
So インストール中 is pronounced インストールちゅう. The meaning is "installing", or "in the process of installation".
More generally, 〜中 used in this way can be thought to mean "in the process of ~"
Edit: There is another possible usage of 〜中 which Hyperworm describes in their answer.
It depends on the meaning.
中 used to mean "during / in the process of" must be pronounced ちゅう (as in ジョン's post)
インストール中【ちゅう】 now installing
読込【よみこみ】中【ちゅう】 now loading
建築【けんちく】中【ちゅう】 under construction
But in other uses, such as "all throughout" or "out of (all the)", it is commonly read じゅう (although it seems that ちゅう is also an option?):
世界【せかい】中【じゅう】の人々 people all around the world
クラス中【じゅう】で一番【いちばん】背【せ】が高【たか】い tallest in the class