It is very unlikely that the process control network is connected to the Internet. However it is almost certainly connected to the corporate Intranet. Think about all of the metric data available on the process control network - that is needed by engineers for analysis, ERP systems for financials, asset management systems for maintenance etc. With an air gap, you can't do any of that in real-time.
The solution is to run a historian in the DMZ, only the historian can read data from the DCS, and the corporate systems (ERP, BI etc) read data from the historian. And nothing from outside can update the DCS.
