THERMODYNAMICS-I PROPERTIES OF PURE SUBSTACES
- 1. LEC - 3 PROPERTIES OF PURE SUBSTANCES
- 2. ME-121 Thermodynamics Department of Mechanical Engineering CECOS University of IT & Emerging Sciences, Peshawar. Semester:II
- 3. Compressed Liquid and Saturated Liquid Consider a piston–cylinder device containing liquid water at 20°C and 1 atm pressure (state 1) as shown in the figure. Under these conditions, water exists in the liquid phase, and it is called a compressed liquid, or a subcooled liquid, meaning that it is not about to vaporize. Heat is now transferred to the water until its temperature rises to, say, 40°C. As the temperature rises, the liquid water expands slightly, and so its specific volume increases. To accommodate this expansion, the piston moves up slightly.
- 4. As more heat is transferred, the temperature keeps rising until it reaches 100°C (state 2) as shown in the figure. At this point water is still a liquid, but any heat addition will A liquid that is about to vaporize is called a saturated liquid.
- 6. Once boiling starts, the temperature stops rising until the liquid is completely That is, the temperature will remain constant during the Midway about the vaporization line (state 3), the cylinder contains equal amounts of
- 8. Any heat loss from this vapor will cause some of the vapor to condense (phase change from vapor to liquid). A vapor that is about to condense is called a saturated vapor. A substance at states between 2 and 4 is referred to as a saturated liquid–vapor mixture since the liquid and vapor phases coexist in equilibrium at these states. As we continue transferring heat, the the last drop of liquid is vaporized (state 4).
- 10. Once the phase-change process is completed, we are back to a single phase region again (this time vapor), and further transfer of heat results in an increase in both the temperature and the specific volume At state 5, the temperature of the vapor is, let us say, 300°C; and if we transfer some heat from the vapor, the temperature may drop somewhat but no condensation will take place as long as the temperature remains above 100°C(for P 1 atm). A vapor that is not about to condense (i.e., not a saturated vapor) is called a superheatedvapor. Therefore, water at state 5 is a
- 12. This constant-pressure phase-change process is illustrated on a T-v diagram in Fig. 3–11.
- 13. If the entire process described here is reversed by cooling the water while maintaining the pressure at the same value, the water will go back to state 1, retracing the same path, and in so doing, the amount of heat released will exactly match the amount of heat added during the heating process.
- 14. At a given pressure, the temperature at which a pure substance changes Likewise, at a given temperature, the pressure at which a pure substance changes phase is called the At a pressure of 101.325 kPa, Tsat is 99.97°C. Conversely, at a temperature of 99.97°C, Psat is 101.325 kPa. (At 100.00°C, Psat is 101.42 kPa
- 15. The amount of energy absorbed or released during a phase- The amount of energy absorbed during vaporization is called the latent heat of vaporization and is equivalent to the energy released during condensation.
- 16. Let us add weights on top of the piston until the pressure inside the cylinder reaches 1 MPa. At this pressure, water has a somewhat smaller specific volume than it does at 1 atm pressure. As heat is transferred to the water at this new pressure, the process follows a path that looks very much like the process path at 1 atm pressure, as shown in Fig. but there are some noticeable differences. First, water starts boiling at a much higher temperature (179.9°C) at this pressure. Second, the specific volume of the saturated liquid is larger and the specific volume That is, the horizontal line that connects the saturated liquid and saturated vapor
- 18. As the pressure is increased further, this saturation line continues to shrink, as shown in Fig. and it becomes a point when the pressure reaches 22.06 MPa for the case of water. This point is called the critical point, and it is defined as the point at which the saturated liquid and saturated vapor states are identical. The temperature, pressure, and specific volume of a substance at the critical point are called, respectively, the critical temperature Tcr, critical pressure Pcr, and critical specific volume vcr.
- 19. The saturated liquid states in Fig.1 can be connected by a line called the saturated liquid line, and saturated vapor states in the same figure can be connected by another line, called the saturated vapor line. These two lines meet at the critical point, forming a dome as shown in Fig. 2 All the compressed liquid states are located in the region to the left of the saturated liquid line, called the compressed liquid region. All the superheated vapor states are located to the right of the saturated vapor In these two regions, the substance exists in a single phase, a liquid or a vapor. All the states that involve both phases in equilibrium are located under the Fig. 1 Fig. 2
- 21. 3-21 3.4 Generalized compressibility chart Figure 3.3
- 22. Above the critical state, there is no line that separates the compressed liquid region and the superheated vapor region. However, it is customary to refer to the substance as superheated vapor at temperatures above the critical temperature and as compressed liquid at temperatures below the critical temperature.
- 24. PURE SUBSTANCE: Fixed chemical composition, throughout H2O, N2, CO2, Air (even a mixture of ice and water is pure) COMPRESSED LIQUID: NOT about to vaporize (Sub-cooed liquid) e.g., water at 20o C and 1 atmosphere SATURATED LIQUID: About to vaporize e.g., water at 100o C and 1 atmosphere SATURATED VAPOR: About to condense e.g., water vapor (steam) at 100o C and 1 atm. SUPERHEATED VAPOR: NOT about to condense e.g., water vapor (steam) at >100o C and 1 atm. SATURATION TEMPERATURE: The boiling temperature at a given pressure SATURATION PRESSURE: The pressure at which boiling occurs at a given T CRITICAL POINT: The saturated liquid and saturated vapor states are identical No saturated mixture exists - the substance changes directly from the liquid to vapor states.
- 25. The P-v Diagram The general shape of the P-v diagram of a pure substance is very much like the T-v diagram, but the T constant lines on this diagram have a downward trend, as shown in Fig. 3–19. Vaporization - change of phase from liquid to vapor
- 27. The P-v Diagram When a liquid is heated at any one constant pressure there is one fixed temperature at which bubbles of vapour form in the liquid; this phenomenon is known as boiling. The higher the pressure of the liquid then the higher the temperature at which boiling occurs. It is also found that the volume occupied by 1 kg of a boiling liquid at a high pressure is slightly larger than the volume occupied by 1 kg of the same liquid when it is boiling at a low pressure. A series of boiling-points plotted on a p-v diagram will appear as a sloping line as shown in the figure. The points P, Q and R represent the boiling-points of a liquid at pressure pP,pQ and pR respectively.
- 28. When a liquid at boiling-point is heated further at constant pressure the additional heat supplied changes the phase of the substance from liquid to vapour; during this change of phase the pressure and temperature remain constant. The heat supplied is called the specific enthalpy of vapourization. It is found that the higher the pressure then the smaller is the amount of heat There is a definite value of specific volume of the vapour at any one pressure at the point at which vapourization is complete; hence a series of points such P , Q , R can be plotted and joined to form a line as shown in figure.
- 30. When the two curves already drawn are extended to high pressure they form a continuouscurve, thus forming The pressure at which the turning point occurs is called the critical point (Point C). The substance existing at a state point inside the loop consists of a mixture of liquid and dry vapour and is A saturation state is defined as a state at which a change of phase may occur without change of pressure or Hence the boiling-points P,Q and R are saturation states and a series of such boiling-points P,Q and R joined up Similarly the points P ,Q and R at which liquid is completely change into vapour are saturation states and a series of such points joined up is called the saturated vapour line.
- 32. Line of constant temperature, called isothermals, can be plotted on a p-v diagram as The temperature lines become horizontal between the saturated liquid line and the For example, between P and P , Q and Q , R and R . Thus there is a corresponding saturation temperature for each saturation pressure. At pressurepP, the saturation temperature is T1, at pressurepQ the saturation temperature is T2and at pressurepR the saturation temperature is T3. The critical temperature line Tc just touches the top of the loop at the critical point C. When a dry saturated vapour is heated at constant pressure its temperature rises and it becomes superheated. The difference between the actual temperature of the superheated vapour and the