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It is very common to get complete steam and water properties as output using a number of software with any two of the input parameters like pressure, Temperature, enthalpy, entropy, dryness fraction and specific volume. Fortunately there is only a single solution with two such input parameters for most of the cases and hence one set of output parameters is displayed as output. But we can discuss a special case when more than one solution is possible with two of the input parameters as described earlier. For example, say with input parameters of enthalpy as 2500 kjoules/kg and dryness fraction as 0.9, theoretically there will be solutions as given below:

PropertySolution 1Solution 2

Pressure (bar) 2.5737993 154.8175704
Temperature (Deg C) 128.3748325 344.6966881
Enthalpy (kjoules/kg)2500 2500
Entropy (kj/(kg-K) 6.5001702 5.1234391
Dryness fraction 0.9 0.9
Specific volume (cum/kg) 0.6295390 0.0090151

Almost none of the steam and water properties software, commercially available, provides two solutions. They usually provide one solution as output. The single solution such software provides, in those special cases, is right since it is one of the two solutions. But at the same time the user remains ignorant about the existence of the second solution which also might be of interest. It is true that existence of such dual solutions is limited, but they do exist.

Such special cases can be very clearly understood from Enthalpy-entropy Molliers diagram for steam. Very nature of the dryness fraction line (say for example the line for 0.9 dryness fraction) is such that in Enthalpy- entropy plane it cuts the constant enthalpy line (say for example 2500 kjoules/kg line) in two points. This obviously shows that there will be two solutions. In most of the Mollers diagram generation software, a provision of locating a point on the diagram with two input parameters is generally available. Unfortunately most of them can not show two points simultaneously on the diagram, whereas locating two points on the applicable diagram is very much needed.

You can visit www.steam-water-properties.com where the software they provide caters this feature nicely.

The author is a mechanical engineer from Calcutta University and has more than 30 years experience in power plant engineering. He has been developing engineering software for regular use for the last 15 years. Read his article SOFTWARE FOR STEAM AND WATER PROPERTIES and visit http://www.steam-water-properties.com

posted by gektal  # 6:07 PM