Steam is gaseous form of water and ice. When heat applied to ice at 0°C is equal to latent heat of fusion plus sensible heat from 0°C to 100°C plus latent heat of vaporization, ice is transformed into steam. Three variables that are very important are pressure, temperature, and volume. At constant pressure, variation in temperature, and volume can be explained by Figure 4.1.
Figure 4.1 T–V Diagram for Various Phases of Water
Suppose unit mass of ice below freezing point is kept in a cylinder and a constant pressure is applied by a piston with constant load. Now heat is applied. From point 1 and 2 ice gets warm up and temperature increases with volume. Temperature at point 2 is 273 K. From point 2 to 3 temperature remains constant by heat addition due to heat absorbed in phase transition as latent heat of fusion (hfg). At point 3 ice transforms completely into water. On further heating from point 3 to 4, volume decreases up to 4°C (277 K), since water has high density at 4°C. This is sensible heating. From point 4 to point 5 temperature and volume both increase on sensible heating. Again from point 5 to 6, temperature remains constant but volume increases due to phase transition from water to vapour. Heat absorbed in this phase is latent heat of vaporization. At point 6, water transforms completely into vapour. Beyond point 6, vapour is superheated and temperature rises continuously. This temperature rise is known as degree of superheat. If several graphs are plotted between temperature and volume at different pressures, we get a curve which is shown in Figure 4.2.
Figure 4.2 T–V Diagram at Different Pressures
At a pressure of 0.006112 bar, the melting point and boiling point become equal and change of phase ice–water–vapour is shown by a single straight line ABC as shown in Figure 4.2; it is known as triple point line. At this line, all the three phases are in equilibrium (Ptriple = 0.006112 bar, Ttriple = 273.16°C). At very high pressure, latent heat of vaporization becomes zero which is known as critical point (Pc = 221.2 bar, Tc = 647.3°C, Vc = 0.00317 m3/kg).
Similarly, P–V diagram, T–s diagram, h–s diagram, and P–s diagram are shown in Figure 4.3.
Figure 4.3(a) ρ–V Diagram
Figure 4.3(b) T–s Diagram
Figure 4.3(c) h–s Diagram
Figure 4.3(d) ρ–s Diagram
Leave a Reply