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)
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?):