what are some chemical or physical properties that are typically measured in a process stream ?
Typically mass flow, pressure and temperature. You might also measure composition of the process stream itself depending on the exact process.
Similar to the answer given by rock.freak667, the parameters usually measured are temperature, pressure and flow rate. Most properties (such as enthalpy, entropy, etc.) are simply functions of the steam temperature.
One important point to add is that the steam quality (dryness fraction) is an important parameter which should be known but cannot, under most circumstances, be measured directly.
I'm not sure what "dryness fraction" means when characterizing steam quality. I can say, however, that the moisture content in a flowing process is simple to measure using microwaves.
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