Saturation Curve: The saturation curve is the curve showing the volume the steam in the cylinder would occupy, during the expansion stroke if the steam is perfectly dry and saturated at all the points. It is plotted on p–V diagram and the wetness of stream can be seen on it at a glance.
Figure 5.18 shows a calibrated indicator. The total mass of steam in cylinder during expansion stroke = mC + m.
On the expansion curve, consider any point B and read off from the diagram the pressure (pB) and volume (VB) at this point. From steam tables, obtain the specific volume υ of dry steam at pressure pB. The volume the steam at B would occupy if dry saturated = mυ. Let this volume be represented by AC to the volume scale of the p–V diagram. Then, the point C represents the volume the steam at B would occupy if dry saturated.
Following this way, a number of points may be obtained and plotted. The curve passing through these points to known as saturation curve (because all the points on this line represent the condition of steam dry and saturated).
From this saturation curve, the dryness fraction for all points on expansion curve can be obtained. For instance, the dryness fraction at E will be given by
Dryness fraction at 
From Fig. 5.18, it can be seen that the steam is wet at the beginning of the expansion stroke and becomes drier towards the end. This is owing to the fact that high pressure steam in the initial stage of expansion is better than the cylinder walls; this causes the steam to condense. During the expansion stroke the steam pressure falls and towards the end of the stroke the walls will be hotter than the steam; consequently the condensed steam re-evaporates and as a result the dryness fraction is improved. The improvement in dryness fraction will not be there if cylinder walls are not jacketed.
Missing quantity: The missing quantity is the horizontal distance between the actual expansion curve and the saturation curve (Fig. 5.18). At a pressure pD, the missing quantity is represented by EF (m3).
Figure 5.18 Calibrated indicator diagram
The missing quantity is mainly due to condensation of the steam, but a small amount will be due to leakage past the piston. Due to this missing quantity there is a loss of work represented by the area between the expansion curve and saturation curve.
The missing quantity can be reduced in the following ways
- By steam jacketing the cylinder walls efficiently.
- By reducing the temperature range of the steam during the stroke this can be accomplished by compounding the expansion of steam in two cylinders instead of allowing the whole pressure to drop to occur in one cylinder.
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